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Metaphysical Dualism Essay

Dualism and Mind

Dualists in the philosophy of mind emphasize the radical difference between mind and matter. They all deny that the mind is the same as the brain, and some deny that the mind is wholly a product of the brain. This article explores the various ways that dualists attempt to explain this radical difference between the mental and the physical world. A wide range of arguments for and against the various dualistic options are discussed.

Substance dualists typically argue that the mind and the body are composed of different substances and that the mind is a thinking thing that lacks the usual attributes of physical objects: size, shape, location, solidity, motion, adherence to the laws of physics, and so on. Substance dualists fall into several camps depending upon how they think mind and body are related. Interactionists believe that minds and bodies causally affect one another. Occasionalists and parallelists, generally motivated by a concern to preserve the integrity of physical science, deny this, ultimately attributing all apparent interaction to God. Epiphenomenalists offer a compromise theory, asserting that bodily events can have mental events as effects while denying that the reverse is true, avoiding any threat to the scientific law of conservation of energy at the expense of the common sense notion that we act for reasons.

Property dualists argue that mental states are irreducible attributes of brain states. For the property dualist, mental phenomena are non-physical properties of physical substances. Consciousness is perhaps the most widely recognized example of a non-physical property of physical substances. Still other dualists argue that mental states, dispositions and episodes are brain states, although the states cannot be conceptualized in exactly the same way without loss of meaning.

Dualists commonly argue for the distinction of mind and matter by employing Leibniz's Law of Identity, according to which two things are identical if, and only if, they simultaneously share exactly the same qualities. The dualist then attempts to identify attributes of mind that are lacked by matter (such as privacy or intentionality) or vice versa (such as having a certain temperature or electrical charge). Opponents typically argue that dualism is (a) inconsistent with known laws or truths of science (such as the aforementioned law of thermodynamics), (b) conceptually incoherent (because immaterial minds could not be individuated or because mind-body interaction is not humanly conceivable), or (c) reducible to absurdity (because it leads to solipsism, the epistemological belief that one's self is the only existence that can be verified and known).

Table of Contents

  1. Dualism
  2. Platonic Dualism in the Phaedo
    1. The Argument From Opposites
    2. The Argument From Recollection
    3. The Argument From Affinity
    4. Criticisms of the Platonic Arguments
  3. Descartes' Dualism
    1. The Argument From Indivisibility
    2. Issues Raised by the Indivisibility Argument
    3. The Argument From Indubitability
    4. The Real Distinction Argument
  4. Other Leibniz's Law Arguments for Dualism
    1. Privacy and First Person Authority
    2. Intentionality
    3. Truth and Meaning
    4. Problems with Leibniz's Law Arguments for Dualism
  5. The Free Will and Moral Arguments
  6. Property Dualism
  7. Objections to Dualism Motivated by Scientific Considerations
    1. Arguments from Human Development
    2. The Conservation of Energy Argument
    3. Problems of Interaction
    4. The Correlation and Dependence Arguments
  8. The Problem of Other Minds
  9. Criticisms of the Mind as a Thinking Thing
  10. References and Further Reading

1. Dualism

The most basic form of dualism is substance dualism, which requires that mind and body be composed of two ontologically distinct substances. The term "substance" may be variously understood, but for our initial purposes we may subscribe to the account of a substance, associated with D. M. Armstrong, as what is logically capable of independent existence. (Armstrong, 1968, p. 7). According to the dualist, the mind (or the soul) is comprised of a non-physical substance, while the body is constituted of the physical substance known as matter. According to most substance dualists, mind and body are capable of causally affecting each other. This form of substance dualism is known as interactionism.

Two other forms of substance dualism are occasionalism and parallelism. These theories are largely relics of history. The occasionalist holds that mind and body do not interact. They may seem to when, for example, we hit our thumb with a hammer and a painful and distressing sensation occurs. Occassionalists, like Malebranche, assert that the sensation is not caused by the hammer and nerves, but instead by God. God uses the occasion of environmental happenings to create appropriate experiences.

According to the parallelist, our mental and physical histories are coordinated so that mental events appear to cause physical events (and vice versa) by virtue of their temporal conjunction, but mind and body no more interact than two clocks that are synchronized so that the one chimes when hands of the other point out the new hour. Since this fantastic series of harmonies could not possibly be due to mere coincidence, a religious explanation is advanced. God does not intervene continuously in creation, as the occasionalist holds, but builds into creation a pre-established harmony that largely eliminates the need for future interference.

Another form of dualism is property dualism. Property dualists claim that mental phenomena are non-physical properties of physical phenomena, but not properties of non-physical substances. Some forms of epiphenomenalism fall into this category. According to epiphenomenalism, bodily events or processes can generate mental events or processes, but mental phenomena do not cause bodily events or processes (or, on some accounts, anything at all, including other mental states). (McLaughlin, p. 277) Whether an epiphenomenalist thinks these mental epiphenomena are properties of the body or properties of a non-physical mental medium determines whether the epiphenomenalist is a property or substance dualist.

Still other dualists hold not that mind and body are distinct ontologically, but our mentalistic vocabulary cannot be reduced to a physicalistic vocabulary. In this sort of dualism, mind and body are conceptually distinct, though the phenomena referred to by mentalistic and physicalistic terminology are coextensive.

The following sections first discuss dualism as expounded by two of its primary defenders, Plato and Descartes. This is followed by additional arguments for and against dualism, with special emphasis on substance dualism, the historically most important and influential version of dualism.

2. Platonic Dualism in the Phaedo

The primary source for Plato's views on the metaphysical status of the soul is the Phaedo, set on the final day of Socrates' life before his self-administered execution. Plato (through the mouth of Socrates, his dramatic persona) likens the body to a prison in which the soul is confined. While imprisoned, the mind is compelled to investigate the truth by means of the body and is incapable (or severely hindered) of acquiring knowledge of the highest, eternal, unchanging, and non-perceptible objects of knowledge, the Forms. Forms are universals and represent the essences of sensible particulars. While encumbered by the body, the soul is forced to seek truth via the organs of perception, but this results in an inability to comprehend that which is most real. We perceive equal things, but not Equality itself. We perceive beautiful things but not Beauty itself. To achieve knowledge or insight into the pure essences of things, the soul must itself become pure through the practice of philosophy or, as Plato has Socrates provocatively put it in the dialogue, through practicing dying while still alive. The soul must struggle to disassociate itself from the body as far as possible and turn its attention toward the contemplation of intelligible but invisible things. Though perfect understanding of the Forms is likely to elude us in this life (if only because the needs of the body and its infirmities are a constant distraction), knowledge is available to pure souls before and after death, which is defined as the separation of the soul from the body.

a. The Argument From Opposites

Plato's Phaedo contains several arguments in support of his contention that the soul can exist without the body. According to the first of the Phaedo's arguments, the Argument from Opposites, things that have an opposite come to be from their opposite. For example, if something comes to be taller, it must come to be taller from having been shorter; if something comes to be heavier, it must come to be so by first having been lighter. These processes can go in either direction. That is, things can become taller, but they also can become shorter; things can become sweeter, but also more bitter. In the Phaedo, Socrates notes that we awaken from having been asleep and go to sleep from having been awake. Similarly, since dying comes from living, living must come from dying. Thus, we must come to life again after we die. During the interim between death and rebirth the soul exists apart from the body and has the opportunity to glimpse the Forms unmingled with matter in their pure and undiluted fullness. Death liberates the soul, greatly increasing its apprehension of truth. As such, the philosophical soul is unafraid to die and indeed looks forward to death as to liberation.

b. The Argument From Recollection

A second argument from the Phaedo is the Argument from Recollection. Socrates argues that the soul must exist prior to birth because we can recollect things that could not have been learned in this life. For example, according to Socrates we realize that equal things can appear to be unequal or can be equal in some respects but not others. People can disagree about whether two sticks are equal. They may disagree about if they are equal in length, weight, color, or even whether they are equally "sticks." The Form of Equality—Equality Itself—can never be or appear unequal. According to Socrates, we recognize that the sticks are unequal and that they are striving to be equal but are nevertheless deficient in terms of their equality. Now, if we can notice that the sticks are unequal, we must comprehend what Equality is. Just as I could not recognize that a portrait was a poor likeness of your grandfather unless I already knew what your grandfather looked like, I cannot reccognize that the sticks are unequal by means of the senses, without an understanding of the Form of Equality. We begin to perceive at birth or shortly thereafter. Hence, the soul must have existed prior to birth. It existed before it acquires a body. (A similar argument is developed in Plato's Meno (81a-86b).

c. The Argument From Affinity

A third argument from the Phaedo is the Argument from Affinity. Socrates claims that things that are composite are more liable to be destroyed than things that are simple. The Forms are true unities and therefore least likely ever to be annihilated. Socrates then posits that invisible things such as Forms are not apt to be disintegrated, whereas visible things, which all consist of parts, are susceptible to decay and corruption. Since the body is visible and composite, it is subject to decomposition. The soul, on the other hand, is invisible. The soul also becomes like the Forms if it is steadfastly devoted to their consideration and purifies itself by having no more association with the body than necessary. Since the invisible things are the durable things, the soul, being invisible, must outlast the body. Further, the philosophical soul, that becomes Form-like, is immortal and survives the death of the body.

d. Criticisms of the Platonic Arguments

Some of these arguments are challenged even in the Phaedo itself by Socrates' friends Simmias and Cebes and the general consensus among modern philosophers is that the arguments fail to establish the immortality of the soul and its independence and separability from the body. (Traces of the Affinity argument in a more refined form will be observed in Descartes below). The Argument from Opposites applies only to things that have an opposite and, as Aristotle notes, substances have no contraries. Further, even if life comes from what is itself not alive, it does not follow that the living human comes from the union of a dead (i.e. separated) soul and a body. The principle that everything comes to be from its opposite via a two-directional process cannot hold up to critical scrutiny. Although one becomes older from having been younger, there is no corresponding reverse process leading the older to become younger. If aging is a uni-directional process, perhaps dying is as well. Cats and dogs come to be from cats and dogs, not from the opposites of these (if they have opposites). The Arguments from Recollection and Affinity, on the other hand, presuppose the existence of Forms and are therefore no more secure than the Forms themselves (as Socrates notes in the Phaedo at 76d-e).

We turn now to Descartes' highly influential defense of dualism in the early modern period.

3. Descartes' Dualism

The most famous philosophical work of René Descartes is the Meditations on First Philosophy (1641). In the Sixth Meditation, Descartes calls the mind a thing that thinks and not an extended thing. He defines the body as an extended thing and not a thing that thinks (1980, p. 93). "But what then am I? A thing that thinks. What is that? A thing that doubts, understands, affirms, denies, wills, refuses, and which also imagines and senses." (1980, p. 63). He expands on the notion of extension in the Fifth Meditation saying, "I enumerate the [extended] thing's various parts. I ascribe to these parts certain sizes, shapes, positions, and movements from place to place; to these movements I ascribe various durations" (1980, p. 85). Bodies, but not minds, are describable by predicates denoting entirely quantifiable qualities and hence bodies are fit objects for scientific study.

Having thus supplied us with the meanings of "mind" and "body," Descartes proceeds to state his doctrine: "I am present to my body not merely in the way a seaman is present to his ship, but . . . I am tightly joined and, so to speak, mingled together with it, so much so that I make up one single thing with it" (1980, p. 94). The place where this "joining" was believed by Descartes to be especially true was the pineal gland—the seat of the soul. "Although the soul is joined to the whole body, there is yet in the body a certain part in which it seems to exercise its functions more specifically than in all the others. . . I seem to find evidence that the part of the body in which the soul exercises its functions immediately is. . . solely the innermost part of the brain, namely, a certain very small gland." (1952, p. 294). When we wish to "move the body in any manner, this volition causes the gland to impel the spirits towards the muscles which bring about this effect" (1952, p. 299). Conversely, the body is also able to influence the soul. Light reflected from the body of an animal and entering through our two eyes "form but one image on the gland, which, acting immediately on the soul, causes it to see the shape of the animal." (1952, p. 295-96).

It is clear, then, that Descartes held to a form of interactionism, believing that mental events can sometimes cause bodily events and that bodily events can sometimes cause mental events. (This reading of Descartes-as-interactionist has recently been challenged. See Baker and Morris (1996). Also, Daniel Garber suggests that Descartes is a quasi-occasionalist, permitting minds to act on bodies, but invoking God to explain the actions of inanimate bodies on each other and phenomena where bodies act on minds, such as sensation. See Garber, 2001, ch. 10).

a. The Argument From Indivisibility

Descartes' primary metaphysical justification of the distinction of mind and body is the Argument from Indivisibility. He writes, "there is a great difference between a mind and a body, because the body, by its very nature, is something divisible, whereas the mind is plainly indivisible. . . insofar as I am only a thing that thinks, I cannot distinguish any parts in me. . . . Although the whole mind seems to be united to the whole body, nevertheless, were a foot or an arm or any other bodily part amputated, I know that nothing would be taken away from the mind. . ." (1980, p. 97). Decartes argues that the mind is indivisible because it lacks extension. The body, as an object that takes up space, can always be divided (at least conceptually), whereas the mind is simple and non-spatial. Since the mind and body have different attributes, they must not be the same thing, their "unity" notwithstanding.

This Indivisibility Argument makes use of Leibniz's Law of Identity: two things are the same if, and only if, they have all of the same properties at the same time. More formally, x is identical to y if, and only if, for any property p had by x at time t, y also has p at t, and vice versa. Descartes uses Leibniz's Law to show that the mind and body are not identical because they do not have all of the same properties. An illustration (for present purposes a property can be considered anything that may be predicated of a subject): If the man with the martini is the mayor, it must be possible to predicate all and only the same properties of both "the man" and "the mayor," including occupying (or having bodies that occupy) the same exact spatial location at the same time.

Since divisibility may be predicated of bodies (and all of their parts, such as brains) and may not be predicated of minds, Leibniz's Law suggests that minds cannot be identical to bodies or any of their parts or systems. Although it makes sense to speak of the left or right half of the brain, it makes no sense to speak of half of a desire, several pieces of a headache, part of joy, or two-thirds of a belief. What is true of mental states is held to be true of the mind that has the states as well. In the synopsis of the Meditations, Descartes writes, "we cannot conceive of half a soul, as we can in the case of any body, however small." (1980, p. 52). The mind has many ideas, but they are all ideas of one indivisible mind.

b. Issues Raised by the Indivisibility Argument

John Locke argued that awareness is rendered discontinuous by intervals of sleep, anesthesia, or unconsciousness. (Bk.II, ch.I, sect.10). Is awareness then divisible? Locke suggests that the mind cannot exhibit temporal discontinuity and also have thought as its essence. But even if Descartes was wrong to consider the mind an essentially thinking thing, the concept of mind is not reduced to vacuity if some other, positive characteristic can be found by which to define it. But what might that be? (Without some such means of characterizing the mind it would be defined entirely negatively and we would have no idea what it is).

Against Locke, Dualists can argue in several ways. (1) That the mind has both conscious and unconscious thoughts and that Locke's argument shows only that the mind is not always engaged in conscious reflection, though it may be perpetually busy at the unconscious level. Locke argues that such a maneuver creates grave difficulties for personal identity (Bk.II, Ch.I, sect.11), however, and denies that thoughts can exist unperceived. (2) Dualists can argue that the soul always thinks, but that the memory fails to preserve those thoughts when asleep or under anesthesia. (3) Dualists can argue that the Lockean observation is not relevant to the Argument from Indivisibility because the discontinuity Locke identifies in consciousness is not a spatial discontinuity but a temporal one. The Argument from Indivisibility seeks to show that bodies but not minds are spatially divisible and that argument is not rebutted by pointing out that consciousness is temporally divisible. (Indeed, if minds are temporally divisible and bodies are not, we have an argument for dualism of a different sort).

David Hume, on the other hand, questioned of what the unity of consciousness might consist. The Indivisibility Argument suggests that the mind is a simple unity. Hume finds no reason to grant or assume that the diversity of our experiences (whether visual perception, pain or active thinking and mathematical apprehension) constitute a unity rather than a diversity. For Hume, all introspection reveals is the presence of various impressions and ideas, but does not reveal a subject in which those ideas inhere. Accordingly, if observation is to yield knowledge of the self, the self can consist in nothing but a bundle of perceptions. Even talk of a "bundle" is misleading if that suggests an empirically discoverable internal unity. Thus, Descartes' commitment to a res cogitans or thing which thinks is unfounded and substance dualism is undermined. (For a contrary view on what constitutes the unity of the self, see Madell's view that, "What unites all of my experiences…is simply that they all have the irreducible and unanalyzable property of 'mineness,'" in Nagel, 1986, p. 34, n. 5).

Immanuel Kant replied to Hume that we must suppose or posit the unity of the ego (which he called the "transcendental unity of apperception") as a preliminary to all experience since without such a unity the manifold of sense-data (or "sensibility") could not constitute, for example, the experience of seeing a clock. However, Kant agreed that we must not mistake the unity of apperception for the perception of unity—that is, the perception of a unitary thing or substance. Kant also argued that there is little reason to suppose that the mind or ego cannot be destroyed despite its unity since its powers may gradually attenuate to the point where they simply fade away. The mind need not be separated into non-physical granules to be destroyed since it can suffer a kind of death through loss of its powers. Awareness, perception, memory and the like admit of degrees. If the degree of consciousness decreases to zero, then the mind is effectively annihilated. Even if, as Plato and Descartes agree, the mind is not divisible, it does not follow that it survives (or could survive) separation from the body. Additionally, if the mind is neither physical nor identical to its inessential characteristics (1980, p. 53), it is impossible to distinguish one mind from another. Kant argues that two substances that are otherwise identical can be differentiated only by their spatial locations. If minds are not differentiated by their contents and have no spatial positions to distinguish them, there remains no basis for individuating their identities. (On numerically individuating non-physical substances, see Armstrong, 1968, pp. 27-29. For a general discussion of whether the self is a substance, see Shoemaker, 1963, ch. 2).

c. The Argument From Indubitability

Descartes' other major argument for dualism in the Meditations derives from epistemological considerations. After taking up his celebrated method of doubt, which commits him to reject as false anything that is in the slightest degree uncertain, Descartes finds that the entirety of the physical world is uncertain. Perhaps, after all, it is nothing but an elaborate phantasm wrought by an all-powerful and infinitely clever, but deceitful, demon. Still, he cannot doubt his own existence, since he must exist to doubt. Because he thinks, he is. But he cannot be his body, since that identity is doubtful and possibly altogether false. Therefore, he is a non-bodily "thinking thing," or mind. As Richard Rorty puts it: "If we look in Descartes for a common factor which pains, dreams, memory-images, and veridical and hallucinatory perceptions share with concepts of (and judgments about) God, number, and the ultimate constituents of matter, we find no explicit doctrine. . . . The answer I would give to the question 'What did Descartes find?' is 'Indubitability'" (1979 p. 54). In sum, I cannot doubt the existence of my mind, but I can doubt the existence of my body. Since what I cannot doubt cannot be identical to what I can doubt (by Leibniz's Law), mind and body are not identical and dualism is established.

This argument is also featured in Descartes' Discourse on Method part four: "[S]eeing that I could pretend that I had no body and that there was no world nor any place where I was, but that I could not pretend, on that account, that I did not exist; and that, on the contrary, from the very fact that I thought about doubting the truth of other things, it followed very evidently and very certainly that I existed. . . . From this I knew that I was a substance the whole essence or nature of which was merely to think, and which, in order to exist, needed no place and depended on no material things. Thus this 'I,' that is, the soul through which I am what I am, is entirely distinct from the body. . ." (1980, p. 18).

The Argument from Indubitability has been maligned in the philosophical literature from the very beginning. Most famously, Arnauld comments in the objections originally published with the Meditations that, "Just as a man errs in not believing that the equality of the square on its base to the squares on its sides belongs to the nature of that triangle, which he clearly and distinctly knows to be right angled, so why am I perhaps not in the wrong in thinking that nothing else belongs to my nature which I clearly and distinctly know to be something that thinks, except that fact that I am this thinking being? Perhaps it also belongs to my essence to be something extended." (1912, p. 84). Suppose that I cannot doubt whether a given figure is a triangle, but can doubt whether its interior angles add up to two right angles. It does not follow from this that the number of degrees in triangles may be more or less than 180. This is because the doubt concerning the number of degrees in a triangle is a property of me, not of triangles. Similarly, I may doubt that my body is not a property of my body, believing it to be a property of whatever part of me it is that doubts, and that "whatever" may be something extended.

The dualist can reply in two ways. First, he or she may argue that, while doubting the body is not a property of bodies, being doubtable is a property of bodies. Since bodies have the property of being doubtable, and minds do not, by Leibniz's Law the diversity of the two is established. Second, the dualist may reply that it is always possible to doubt whether the figure before me is a triangle. As such, Arnauld's supposedly parallel argument is not parallel at all. Similar objections are open against other, more recent rebuttals to Descartes' argument. Consider, for example, the following parallel argument from Paul Churchland (1988, p. 32): I cannot doubt that Mohammed Ali was a famous heavyweight boxer but can doubt that Cassius Clay was a famous heavyweight boxer. Following Descartes, it ought to be that Ali is not Clay (though in fact Clay was a famous heavyweight and identical to Ali). By way of reply, surely it is possible for an evil demon to deceive me about whether Mohammed Ali was a famous heavyweight boxer. So, the dualist might insist, the case of mind is unique in its immunity from doubt. It is only with reference to our own mental states that we can be said to know incorrigibly.

d. The Real Distinction Argument

A third argument in the Meditations maintains that the mind and body must really be separate because Descartes can conceive of the one without the other. Since he can clearly and distinctly understand the body without the mind and vice versa, God could really have created them separately. But if the mind and body can exist independently, they must really be independent, for nothing can constitute a part of the essence of a thing that can be absent without the thing itself ceasing to be. If the essence of the mind is incorporeal, so must be the mind itself.

4. Other Leibniz's Law Arguments for Dualism

a. Privacy and First Person Authority

As noted earlier, dualists have argued for their position by employing Leibniz's Law in many ingenious ways. The general strategy is to identify some property or feature indisputably had by mental phenomena but not attributable in any meaningful way to bodily or nervous phenomena, or vice versa. For example, some have suggested that mental states are private in the sense that only those who possess them can know them directly. If I desire an apple, I know that I have this desire "introspectively." Others can know of my desire only by means of my verbal or non-verbal behavior or, conceivably, by inspection of my brain. (The latter assumes a correlation, if not an identity, between nervous and mental states or events). My linguistic, bodily and neural activities are public in the sense that anyone suitably placed can observe them. Since mental states are private to their possessors, but brain states are not, mental states cannot be identical to brain states. (Rey pp. 55-56).

A closely related argument emphasizes that my own mental states are knowable without inference; I know them "immediately." (Harman, 1973, pp. 35-37). Others can know my mental states only by making inferences based on my verbal, non-verbal or neurophysiological activity. You may infer that I believe it will rain from the fact that I am carrying an umbrella, but I do not infer that I believe it will rain from noticing that I am carrying an umbrella. I do not need to infer my mental states because I know them immediately. Since mental states are knowable without inference in the first person case, but are knowable (or at least plausibly assigned) only by inference in the third person case, we have an authority or incorrigibility with reference to our own mental states that no one else could have. Since beliefs about the physical world are always subject to revision (our inferences or theories could be mistaken), mental states are not physical states.

b. Intentionality

Some mental states exhibit intentionality. Intentional mental states include, but are not limited to, intendings, such as plans to buy milk at the store. They are states that are about, of, for, or towards things other than themselves. Desires, beliefs, loves, hates, perceptions and memories are common intentional states. For example, I may have a desire for an apple; I may have love for or towards my neighbor; I may have a belief about republicans or academics; or I may have memories of my grandfather.

The dualist claims that brain states, however, cannot plausibly be ascribed intentionality. How can a pattern of neural firings be of or about or towards anything other than itself? As a purely physical event, an influx of sodium ions through the membrane of a neural cell creating a polarity differential between the inside and outside of the cell wall, and hence an electrical discharge, cannot be of Paris, about my grandfather, or for an apple. [Although Brentano goes further than most contemporary philosophers in regarding all mental phenomena as intentional, he argues that "the reference to something as an object is a distinguishing characteristic of all mental phenomena. No physical phenomena exhibits anything similar." (Brentano, 1874/1973, p. 97, quoted in Rey, 1997, p. 23).] Thus, by Leibniz's Law, if minds are capable of intentional states and bodies are not, minds and bodies must be distinct. (Taylor, pp. 11-12; Rey pp. 57-59).

c. Truth and Meaning

Another attempt to derive dualism by means of Leibniz's Law observes that some mental states, especially beliefs, have truth-values. My belief that it will rain can be either true or false. But, the dualist may urge, as a purely physical event, an electrical or chemical discharge in the brain cannot be true or false. Indeed, it lacks not only truth, but also linguistic meaning. Since mental states such as beliefs possess truth-value and semantics, it seems incoherent to attribute these properties to bodily states. Thus, mental states are not bodily states. Presumably, then, the minds that have these states are also non-physical. (Churchland, 1988, p. 30; Taylor, 1983, p. 12).

d. Problems with Leibniz's Law Arguments for Dualism

Although each of these arguments for dualism may be criticized individually, they are typically thought to share a common flaw: they assume that because some aspect of mental states, such as privacy, intentionality, truth, or meaning cannot be attributed to physical substances, they must be attributable to non-physical substances. But if we do not understand how such states and their properties can be generated by the central nervous system, we are no closer to understanding how they might be produced by minds. (Nagel, 1986, p. 29). The question is not, "How do brains generate mental states that can only be known directly by their possessors?" Rather, the relavent question is "How can any such thing as a substance, of whatever sort, do these things?" The mystery is as great when we posit a mind as the basis of these operations or capacities as when we attribute them to bodies. Dualists cannot explain the mechanisms by which souls generate meaning, truth, intentionality or self-awareness.Thus, dualism creates no explanatory advantage. As such, we should use Ockham's razor to shave off the spiritual substance, because we ought not to multiply entities beyond what is necessary to explain the phenomena. Descartes' prodigious doubt notwithstanding, we have excellent reasons for thinking that bodies exist. If the only reasons for supposing that non-physical minds exist are the phenomena of intentionality, privacy and the like, then dualism unnecessarily complicates the metaphysics of personhood.

On the other hand, dualists commonly argue that it makes no sense to attribute some characteristics of body to mind; that to do so is to commit what Gilbert Ryle called a "category mistake." For example, it makes perfect sense to ask where the hypothalamus is, but not, in ordinary contexts, to ask where my beliefs are. We can ask how much the brain weighs, but not how much the mind weighs. We can ask how many miles per hour my body is moving, but not how many miles per hour my mind is moving. Minds are just not the sorts of things that can have size, shape, weight, location, motion, and the other attributes that Descartes ascribes to extended reality. We literally could not understand someone who informed us that the memories of his last holiday are two inches behind the bridge of his nose or that his perception of the color red is straight back from his left eye. If these claims are correct, then some Leibniz's Law arguments for dualism are not obviously vulnerable to the critique above.

5. The Free Will and Moral Arguments

Another argument for dualism claims that dualism is required for free will. If dualism is false, then presumably materialism, the thesis that humans are entirely physical beings, is true. (We set aside consideration of idealism—the thesis that only minds and ideas exist). If materialism were true, then every motion of bodies should be determined by the laws of physics, which govern the actions and reactions of everything in the universe. But a robust sense of freedom presupposes that we are free, not merely to do as we please, but that we are free to do otherwise than as we do. This, in turn, requires that the cause of our actions not be fixed by natural laws. Since, according to the dualist, the mind is non-physical, there is no need to suppose it bound by the physical laws that govern the body. So, a strong sense of free will is compatible with dualism but incompatible with materialism. Since freedom in just this sense is required for moral appraisal, the dualist can also argue that materialism, but not dualism, is incompatible with ethics. (Taylor, 1983, p. 11; cf. Rey, 1997, pp. 52-53). This, the dualist may claim, creates a strong presumption in favor of their metaphysics.

This argument is sometimes countered by arguing that free will is actually compatible with materialism or that even if the dualistic account of the will is correct, it is irrelevant because no volition on the part of a non-physical substance could alter the course of nature anyway. As Bernard Williams puts it, "Descartes' distinction between two realms, designed to insulate responsible human action from mechanical causation, insulated the world of mechanical causation, that is to say, the whole of the external world, from responsible human action. Man would be free only if there was nothing he could do." (1966, p. 7). Moreover, behaviorist opponents argue that if dualism is true, moral appraisal is meaningless since it is impossible to determine another person's volitions if they are intrinsically private and otherworldly.

6. Property Dualism

Property dualists claim that mental phenomena are non-physical properties of physical phenomena, but not properties of non-physical substances. Property dualists are not committed to the existence of non-physical substances, but are committed to the irreducibility of mental phenomena to physical phenomena.

An argument for property dualism, derived from Thomas Nagel and Saul Kripke, is as follows: We can assert that warmth is identical to mean kinetic molecular energy, despite appearances, by claiming that warmth is how molecular energy is perceived or manifested in consciousness. Minds detect molecular energy by experiencing warmth; warmth "fixes the reference" of heat. ("Heat" is a rigid designator of molecular motion; "the sensation of heat" is a non-rigid designator.) Similarly, color is identical to electromagnetic reflectance efficiencies, inasmuch as color is how electromagnetic wavelengths are processed by human consciousness. In these cases, the appearance can be distinguished from the reality. Heat is molecular motion, though it appears to us as warmth. Other beings, for example, Martians, might well apprehend molecular motion in another fashion. They would grasp the same objective reality, but by correlating it with different experiences. We move toward a more objective understanding of heat when we understand it as molecular energy rather than as warmth. in our case, or as whatever it appears to them to be in theirs. Consciousness itself, however, cannot be reduced to brain activity along analogous lines because we should then need to say that consciousness is how brain activity is perceived in consciousness, leaving consciousness unreduced. Put differently, when it comes to consciousness, the appearance is the reality. Therefore, no reduction is possible. Nagel writes:

Experience . . . does not seem to fit the pattern. The idea of moving from appearance to reality seems to make no sense here. What is the analogue in this case to pursuing a more objective understanding of the same phenomena by abandoning the initial subjective viewpoint toward them in favor of another that is more objective but concerns the same thing? Certainly it appears unlikely that we will get closer to the real nature of human experience by leaving behind the particularity of our human point of view and striving for a description in terms accessible to beings that could not imagine what it was like to be us. (Nagel 1974; reprinted in Block et. al. p. 523).

Consciousness is thus sui generis (of its own kind), and successful reductions elsewhere should give us little confidence when it comes to experience.

Some property dualists, such as Jaegwon Kim, liken "having a mind" to "a property, capacity, or characteristic that humans and some higher animals possess in contrast with things like pencils and rocks. . . . Mentality is a broad and complex property." (Kim, 1996, p. 5). Kim continues: "[Some properties] are physical, like having a certain mass or temperature, being 1 meter long, and being heavier than. Some things—in particular, persons and certain biological organisms—can also instantiate mental properties, like being in pain and liking the taste of avocado." (p. 6). Once we admit the existence of mental properties, we can inquire into the nature of the relationship between mental and physical properties. According to the supervenience thesis, there can be no mental differences without corresponding physical differences. If, for example, I feel a headache, there must be some change not only in my mental state, but also in my body (presumably, in my brain). If Mary is in pain, but Erin is not, then, according to the supervenience thesis, there must be a physical difference between Mary and Erin. For example, Mary's c-fibers are firing and Erin's are not. If this is true, it is possible to argue for a type of property dualism by arguing that some mental states or properties, especially the phenomenal aspects of consciousness, do not "supervene on" physical states or properties in regular, lawlike ways. (Kim, p. 169).

Why deny supervenience? Because it seems entirely conceivable that there could exist a twin Earth where all of the physical properties that characterize the actual world are instantiated and are interrelated as they are here, but where the inhabitants are "zombies" without experience, or where the inhabitants have inverted qualia relative to their true-Earth counterparts. If it is possible to have mental differences without physical differences, then mental properties cannot be identical to or reducible to physical properties. They would exist as facts about the world over and above the purely physical facts. Put differently, it always makes sense to wonder "why we exist and not zombies." (Chalmers, 1996, p. 110). (Kim, 169 and following.; Kripke, 1980, throughout; Chalmers, 1996, throughout, but esp. chs. 3 & 4).

Some have attempted to rebut this "conceivability argument" by noting that the fact that we can ostensibly imagine such a zombie world does not mean that it is possible. Without the actual existence of such a world, the argument that mental properties do not supervene on physical properties fails.

A second rebuttal avers that absent qualia thought experiments (and inverted spectra though experiments) only support property dualism if we can imagine these possibilities obtaining. Perhaps we think we can conceive a zombie world, when we really can't. We may think we can conceive of such a world but attempts to do so do not actually achieve such a conception.

To illustrate, suppose that Goldbach's Conjecture is true. If it is, its truth is necessary. If, then, someone thought that they imagined a proof that the thesis is false, they would be conceiving the falsity of what is in reality a necessary truth. This is implausible. What we should rather say in such a case is that the person was mistaken, and that what they imagined false was not Goldbach's Conjecture after all, or that the "proof" that was imagined was in fact no proof, or that what they were really imagining was something like an excited mathematician shouting, "Eureka! So it's false then!" Perhaps it is likewise when we "conceive" a zombie universe. We may be mistaken about what it is that we are actually "picturing" to ourselves. Against this objection, however, one could argue that there are independent grounds for thinking that the truth-value of Goldbach's theorem is necessary and no independent reasons for thinking that Zombie worlds are impossible; therefore, the dualist deserves the benefit of the doubt.

But perhaps the physicalist can come up with independent reasons for supposing that the dualist has failed to imagine what she claims. The physicalist can point, for example, to successful reductions in other areas of science. On the basis of these cases she can argue the implausibility of supposing that, uniquely, mental phenomena resist reduction to the causal properties of matter. That is, an inductive argument for reduction outweighs a conceivability argument against reduction. And in that case, the dualist must do more than merely insist that she has correctly imagined inverted spectra in isomorphic individuals. (For useful discussions of some of these issues, see Tye 1986 and Horgan 1987.)

7. Objections to Dualism Motivated by Scientific Considerations

The Ockham's Razor argument creates a strong methodological presumption against dualism, suggesting that the mind-body split multiplies entities unnecessarily in much the way that a demon theory of disease complicates the metaphysics of medicine compared to a germ theory. It is often alleged, more broadly, that dualism is unscientific and renders impossible any genuine science of mind or truly empirical psychology.

a. Arguments from Human Development

Those eager to defend the relevance of science to the study of mind, such as Paul Churchland, have argued that dualism is inconsistent with the facts of human evolution and fetal development. (1988, pp. 27-28; see also Lycan, 1996, p. 168). According to this view, we began as wholly physical beings. This is true of the species and the individual human. No one seriously supposes that newly fertilized ova are imbued with minds or that the original cell in the primordial sea was conscious. But from those entirely physical origins, nothing non-physical was later added. We can explain the evolution from the unicellular stage to present complexities by means of random mutations and natural selection in the species case and through the accretion of matter through nutritional intake in the individual case. But if we, as species or individuals, began as wholly physical beings and nothing nonphysical was later added, then we are still wholly physical creatures. Thus, dualism is false. The above arguments are only as strong as our reasons for thinking that we began as wholly material beings and that nothing non-physical was later added. Some people, particularly the religious, will object that macro-evolution of a species is problematic or that God might well have infused the developing fetus with a soul at some point in the developmental process (traditionally at quickening). Most contemporary philosophers of mind put little value in these rejoinders.

b. The Conservation of Energy Argument

Others argue that dualism is scientifically unacceptable because it violates the well-established principle of the conservation of energy. Interactionists argue that mind and matter causally interact. But if the spiritual realm is continually impinging on the universe and effecting changes, the total level of energy in the cosmos must be increasing or at least fluctuating. This is because it takes physical energy to do physical work. If the will alters states of affairs in the world (such as the state of my brain), then mental energy is somehow converted into physical energy. At the point of conversion, one would anticipate a physically inexplicable increase in the energy present within the system. If it also takes material energy to activate the mind, then "physical energy would have to vanish and reappear inside human brains." (Lycan, 1996, 168).

The dualists' basically have three ways of replying. First, they could deny the sacredness of the principle of the conservation of energy. This would be a desperate measure. The principle is too well established and its denial too ad hoc. Second, the dualist might offer that mind does contribute energy to our world, but that this addition is so slight, in relation to our means of detection, as to be negligible. This is really a re-statement of the first reply above, except that here the principle is valid in so far as it is capable of verification. Science can continue as usual, but it would be unreasonable to extend the law beyond our ability to confirm it experimentally. That would be to step from the empirical to the speculative—the very thing that the materialist objects to in dualism. The third option sidesteps the issue by appealing to another, perhaps equally valid, principle of physics. Keith Campbell (1970) writes:

The indeterminacy of quantum laws means that any one of a range of outcomes of atomic events in the brain is equally compatible with known physical laws. And differences on the quantum scale can accumulate into very great differences in overall brain condition. So there is some room for spiritual activity even within the limits set by physical law. There could be, without violation of physical law, a general spiritual constraint upon what occurs inside the head. (p. 54)

Mind could act upon physical processes by "affecting their course but not breaking in upon them" (1970, p. 54). If this is true, the dualist could maintain the conservation principle but deny a fluctuation in energy because the mind serves to "guide" or control neural events by choosing one set of quantum outcomes rather than another. Further, it should be remembered that the conservation of energy is designed around material interaction; it is mute on how mind might interact with matter. After all, a Cartesian rationalist might insist, if God exists we surely wouldn't say that He couldn't do miracles just because that would violate the first law of thermodynamics, would we?

c. Problems of Interaction

The conservation of energy argument points to a more general complaint often made against dualism: that interaction between mental and physical substances would involve a causal impossibility. Since the mind is, on the Cartesian model, immaterial and unextended, it can have no size, shape, location, mass, motion or solidity. How then can minds act on bodies? What sort of mechanism could convey information of the sort bodily movement requires, between ontologically autonomous realms? To suppose that non-physical minds can move bodies is like supposing that imaginary locomotives can pull real boxcars. Put differently, if mind-body interaction is possible, every voluntary action is akin to the paranormal power of telekinesis, or "mind over matter." If minds can, without spatial location, move bodies, why can my mind move immediately only one particular body and no others? Confronting the conundrum of interaction implicit in his theory, Descartes posited the existence of "animal spirits" somewhat subtler than bodies but thicker than minds. Unfortunately, this expedient proved a dead-end, since it is as incomprehensible how the mind could initiate motion in the animal spirits as in matter itself.

These problems involved in mind-body causality are commonly considered decisive refutations of interactionism. However, many interesting questions arise in this area. We want to ask: "How is mind-body interaction possible? Where does the interaction occur? What is the nature of the interface between mind and matter? How are volitions translated into states of affairs? Aren't minds and bodies insufficiently alike for the one to effect changes in the other?"

It is useful to be reminded, however, that to be bewildered by something is not in itself to present an argument against, or even evidence against, the possibility of that thing being a matter of fact. To ask "How is it possible that . . . ?" is merely to raise a topic for discussion. And if the dualist doesn't know or cannot say how minds and bodies interact, what follows about dualism? Nothing much. It only follows that dualists do not know everything about metaphysics. But so what? Psychologists, physicists, sociologists, and economists don't know everything about their respective disciplines. Why should the dualist be any different? In short, dualists can argue that they should not be put on the defensive by the request for clarification about the nature and possibility of interaction or by the criticism that they have no research strategy for producing this clarification.

The objection that minds and bodies cannot interact can be the expression of two different sorts of view. On the one hand, the detractor may insist that it is physically impossible that minds act on bodies. If this means that minds, being non-physical, cannot physically act on bodies, the claim is true but trivial. If it means that mind-body interaction violates the laws of physics (such as the first law of thermodynamics, discussed above), the dualist can reply that minds clearly do act on bodies and so the violation is only apparent and not real. (After all, if we do things for reasons, our beliefs and desires cause some of our actions). If the materialist insists that we are able to act on our beliefs, desires and perceptions only because they are material and not spiritual, the dualist can turn the tables on his naturalistic opponents and ask how matter, regardless of its organization, can produce conscious thoughts, feelings and perceptions. How, the dualist might ask, by adding complexity to the structure of the brain, do we manage to leap beyond the quantitative into the realm of experience? The relationship between consciousness and brain processes leaves the materialist with a causal mystery perhaps as puzzling as that confronting the dualist.

On the other hand, the materialist may argue that it is a conceptual truth that mind and matter cannot interact. This, however, requires that we embrace the rationalist thesis that causes can be known a priori. Many prefer to assert that causation is a matter for empirical investigation. We cannot, however, rule out mental causes based solely on the logic or grammar of the locutions "mind" and "matter." Furthermore, in order to defeat interactionism by an appeal to causal impossibility, one must first refute the Humean equation of causal connection with regularity of sequence and constant conjunction. Otherwise, anything can be the cause of anything else. If volitions are constantly conjoined with bodily movements and regularly precede them, they are Humean causes. In short, if Hume is correct, we cannot refute dualism a priori by asserting that transactions between minds and bodies involve links where, by definition, none can occur.

Some, such as Ducasse (1961, 88; cf. Dicker pp. 217-224), argue that the interaction problem rests on a failure to distinguish between remote and proximate causes. While it makes sense to ask how depressing the accelerator causes the automobile to speed up, it makes no sense to ask how pressing the accelerator pedal causes the pedal to move. We can sensibly ask how to spell a word in sign language, but not how to move a finger. Proximate causes are "basic" and analysis of them is impossible. There is no "how" to basic actions, which are brute facts. Perhaps the mind's influence on the pineal gland is basic and brute.

One final note: epiphenomenalism, like occasionalism and parallelism, is a dualistic theory of mind designed, in part, to avoid the difficulties involved in mental-physical causation (although occasionalism was also offered by Malebranche as an account of seemingly purely physical causation). According to epiphenomenalism, bodies are able to act on minds, but not the reverse. The causes of behavior are wholly physical. As such, we need not worry about how objects without mass or physical force can alter behavior. Nor need we be concerned with violations of the conservation of energy principle since there is little reason to suppose that physical energy is required to do non-physical work. If bodies affect modifications in the mental medium, that need not be thought to involve a siphoning of energy from the world to the psychic realm. On this view, the mind may be likened to the steam from a train engine; the steam does not affect the workings of the engine but is caused by it. Unfortunately, epiphenomenalism avoids the problem of interaction only at the expense of denying the common-sense view that our states of mind have some bearing on our conduct. For many, epiphenomenalism is therefore not a viable theory of mind. (For a defense of the common-sense claim that beliefs and attitudes and reasons cause behavior, see Donald Davidson.)

d. The Correlation and Dependence Arguments

The correlation and dependence argument against dualism begins by noting that there are clear correlations between certain mental events and neural events (say, between pain and a-fiber or c-fiber stimulation). Moreover, as demonstrated in such phenomena as memory loss due to head trauma or wasting disease, the mind and its capacities seem dependent upon neural function. The simplest and best explanation of this dependence and correlation is that mental states and events are neural states and events and that pain just is c-fiber stimulation. (This would be the argument employed by an identity theorist. A functionalist would argue that the best explanation for the dependence and correlation of mental and physical states is that, in humans, mental states are brain states functionally defined).

Descartes himself anticipated an objection like this and argued that dependence does not strongly support identity. He illustrates by means of the following example: a virtuoso violinist cannot manifest his or her ability if given an instrument in deplorable or broken condition. The manifestation of the musician's ability is thus dependent upon being able to use a well-tuned instrument in proper working order. But from the fact that the exhibition of the maestro's skill is impossible without a functioning instrument, it hardly follows that being skilled at playing the violin amounts to no more than possessing such an instrument. Similarly, the interactionist can claim that the mind uses the brain to manifest it's abilities in the public realm. If, like the violin, the brain is in a severely diseased or injurious state, the mind cannot demonstrate its abilities; they of necessity remain private and unrevealed. However, for all we know, the mind still has its full range of abilities, but is hindered in its capacity to express them. As for correlation, interactionism actually predicts that mental events are caused by brain events and vice versa, so the fact that perceptions are correlated with activity in the visual cortex does not support materialism over this form of dualism. Property dualists agree with the materialists that mental phenomena are dependent upon physical phenomena, since the fomer are (non-physical) attributes of the latter. Materialists are aware of these dualist replies and sometimes invoke Ockham's razor and the importance of metaphysical simplicity in arguments to the best explanation. (See Churchland, 1988, p. 28). Other materialist responses will not be considered here.

8. The Problem of Other Minds

The problem of how we can know other minds has been used as follows to refute dualism. If the mind is not publicly observable, the existence of minds other than our own must be inferred from the behavior of the other person or organism. The reliability of this inference is deeply suspect, however, since we only know that certain mental states cause characteristic behavior from our own case. To extrapolate to the population as a whole from the direct inspection of a single example, our own case, is to make the weakest possible inductive generalization. Hence, if dualism is true, we cannot know that other people have minds at all. But common sense tell us that others do have minds. Since common sense can be trust, dualism is false.

This problem of other minds, to which dualism leads so naturally, is often used to support rival theories such as behaviorism, the mind-brain identity theory, or functionalism (though functionalists sometimes claim that their theory is consistent with dualism). Since the mind, construed along Cartesian lines, leads to solipsism (that is, to the epistemological belief that one's self is the only existence that can be verified and known), it is better to operationalize the mind and define mental states behaviorally, functionally, or physiologically. If mental states are just behavioral states, brain states, or functional states, then we can verify that others have mental states on the basis of publicly observable phenomena, thereby avoiding skepticism about other selves.

Materialist theories are far less vulnerable to the problem of other minds than dualist theories, though even here other versions of the problem stubbornly reappear. Deciding to define mental states behaviorally does not mean that mental states are behavioral, and it is controversial whether attempts to reduce mentality to behavioral, brain, or functional states have been successful. Moreover, the "Absent Qualia" argument claims that it is perfectly imaginable and consistent with everything that we know about physiology that, of two functionally or physiologically isomorphic beings, one might be conscious and the other not. Of two outwardly indistinguishable dopplegangers, one might have experience and the other none. Both would exhibit identical neural activity; both would insist that they can see the flowers in the meadow and deny that they are "blind"; both would be able to obey the request to go fetch a red flower; and yet only one would have experience. The other would be like an automaton. Consequently, it is sometimes argued, even a materialist cannot be wholly sure that other existing minds have experience of a qualitative (whence, "qualia") sort. The problem for the materialist then becomes not the problem of other minds, but the problem of other qualia. The latter seems almost as severe an affront to common sense as the former. (For an interesting related discussion, see Churchland on eliminative materialism, 1988, pp. 43-49.)

9. Criticisms of the Mind as a Thinking Thing

We earlier observed that some philosophers, such as Hume, have objected that supposing that the mind is a thinking thing is not warranted since all we apprehend of the self by introspection is a collection of ideas but never the mind that purportedly has these ideas. All we are therefore left with is a stream of impressions and ideas but no persisting, substantial self to constitute personal identity. If there is no substratum of thought, then substance dualism is false. Kant, too, denied that the mind is a substance. Mind is simply the unifying factor that is the logical preliminary to experience.

The idea that the mind is not a thinking thing was revived in the twentieth century by philosophical behaviorists. According to Gilbert Ryle in his seminal 1949 work The Concept of Mind, "when we describe people as exercising qualities of mind, we are not referring to occult episodes of which their overt acts and utterances are effects; we are referring to those overt acts and utterances themselves." (p. 25). Thus, "When a person is described by one or other of the intelligence epithets such as 'shrewd' or 'silly', 'prudent' or 'imprudent', the description imputes to him not the knowledge, or ignorance, of this or that truth, but the ability, or inability, to do certain sorts of things." (p. 27). For the behaviorist, we say that the clown is clever because he can fall down deliberately yet make it look like an accident We say the student is bright because she can tell us the correct answer to complex, involved equations. Mental events reduce to bodily events or statements about the body. As Ludwig Wittgenstein notes in his Blue Book:

It is misleading then to talk of thinking as of a "mental activity." We may say that thinking is essentially the activity of operating with signs. This activity is performed by the hand, when we think by writing; by the mouth and larynx, when we think by speaking; and if we think by imagining signs or pictures, I can give you no agent that thinks. If then you say that in such cases the mind thinks, I would only draw your attention to the fact that you are using a metaphor. (1958, p. 6)

John Wisdom (1934) explains: "'I believe monkeys detest jaguars' means 'This body is in a state which is liable to result in the group of reactions which is associated with confident utterance of 'Monkeys detest jaguars,' namely keeping 'favorite' monkeys from jaguars and in general acting as if monkeys detested jaguars.'" (p. 56-7).

Philosophical behaviorism as developed by followers of Wittgenstein was supported in part by the Private Language Argument. Anthony Kenny (1963) explains:

Any word purporting to be the name of something observable only by introspection (i.e. a mental event)... would have to acquire its meaning by a purely private and uncheckable performance . . . If the names of the emotions acquire their meaning for each of us by a ceremony from which everyone else is excluded, then none of us can have any idea what anyone else means by the word. Nor can anyone know what he means by the word himself; for to know the meaning of a word is to know how to use it rightly; and where there can be no check on how a man uses a word there is no room to talk of "right" and "wrong" use (p. 13).

Mentalistic terms do not have meaning by virtue of referring to occult phenomena, but by virtue of referring to something public in a certain way. To understand the meaning of words like "mind," "idea," "thought," "love," "fear," "belief," "dream," and so forth, we must attend to how these words are actually learned in the first place. When we do this, the behaviorist is confident that the mind will be demystified.

Although philosophical behaviorism has fallen out of fashion, its recommendations to attend to the importance of the body and language in attempting to understand the mind have remained enduring contributions. Although dualism faces serious challenges, we have seen that many of these difficulties can be identified in its philosophical rivals in slightly different forms.

10. References and Further Reading

  • Aristotle, Categories.
  • Armstrong, D. M.: A Materialist Theory of the Mind (Routledge and Kegan Paul, London 1968) Chapter Two.
  • Baker, Gordon and Morris, Katherine J. Descartes' Dualism (Routledge, London 1996).
  • Block, Ned, Owen Flanagan, and Gueven Guezeldere eds. The Nature of Consciousness: Philosophical Debates (MIT Press, Cambridge 1997).
  • Brentano, Franz: Psychology from an Empirical Standpoint trans. A. Rancurello, D. Terrell, and L. McAlister (Routledge and Kegan Paul, London 1874/1973).
  • Broad, C. D. Mind and Its Place in Nature (Routledge and Kegan Paul, London 1962).
  • Campbell, Keith: Body and Mind (Anchor Books, Doubleday & Co., Garden City NJ 1970).
  • Chalmers, David J.: The Conscious Mind: In Search of a Fundamental Theory (Oxford University Press, Oxford 1996).
  • Churchland, Paul: Matter and Consciousness, Revised Edition (MIT Press, Cambridge MA 1988).
  • Davidson, Donald: "Actions, Reasons and Causes" The Journal of Philosophy 60 (1963) pp. 685- 700, reprinted in The Philosophy of Action, Alan White, ed. (Oxford University Press, Oxford 1973).
  • Descartes, Rene: Discourse on Method and Meditations on First Philosophy, Donald A. Cress trans. (Hackett Publishing Co., Indianapolis 1980).
  • Descartes, Rene: Descartes' Philosophical Writings, selected and translated by Norman Kemp Smith (Macmillan, London 1952).
  • Descartes, Rene: The Philosophical Works of Descartes, vol.2, Elizabeth S. Haldane, trans. (Cambridge University Press, 1912).
  • Dicker, Georges: Descartes: An Analytical and Historical Introduction (Oxford University Press, Oxford 1993).
  • Ducasse, C. J.: "In Defense of Dualism" in Dimensions of Mind, Sydney Hook, ed. (Macmillan, NY 1961).
  • Garber, Daniel: Descartes Embodied: Reading Cartesian Philosophy through Cartesian Science (Cambridge University Press, Cambridge 2001).
  • Harman, Gilbert: Thought (Princeton University Press, Princeton 1973).
  • Hart, William D. "Dualism" in A Companion to the Philosophy of Mind, Samuel Guttenplan, ed. (Basil Blackwell, Oxford 1994) pp. 265-269.
  • Horgan, Terence: "Supervenient Qualia" Philosophical Review 96 (1987) pp. 491-520.
  • Hume, David: An Enquiry Concerning Human Understanding (Hackett Publishing, Indianapolis 1977).
  • Joad, C. E. M.: How Our Minds Work (Philosophical Library, 1947).
  • Kant, Immanuel: Critique of Pure Reason, Norman Kemp Smith, trans. (Macmillan, London 1963).
  • Kenny, Anthony: Action, Emotion and the Will (Routledge & Kegan Paul, London 1963).
  • Kim, Jaegwon: Philosophy of Mind (Westview Press, Boulder 1996).
  • Kripke, Saul: Naming and Necessity (Harvard University Press, Cambridge 1980).
  • Locke, John: Essay Concerning Human Understanding vol. 1, collated and annotated by Alexander Fraser (Dover Publications, NY 1959).
  • Lycan, William: "Philosophy of Mind" in The Blackwell companion to Philosophy, Nicholas Bunnin and E. P. Tsui-James eds. (Blackwell Publishers, Oxford 1996).
  • Madell, G. The Identity of the Self (Edinburgh University Press, 1983).
  • Malcolm, Norman: "Knowledge of Other Minds" in The Philosophy of Mind, V. C. Chappell, ed. (Prentice-Hall , Englewood Cliffs 1962).
  • McLaughlin, Brian P.: "Epiphenomenalism" in A Companion to the Philosophy of Mind, Samuel Guttenplan, ed. (Blackwell, Oxford 1994).
  • Mill, J. S. : An Examination of Sir William Hamilton's Philosophy, 6th ed. (Longman's, London 1889).
  • Nagel, Thomas: "Brain Bisection and the Unity of Consciousness" in Jonathan Glover, ed. The Philosophy of Mind (Oxford University Press, Oxford 1976).
  • Nagel, Thomas: The View From Nowhere (Oxford University Press, Oxford 1986).
  • Nagel, Thomas: "What Is It Like to Be a Bat?" Philosophical Review 83 (1974) 435-450.
  • Plato: Meno.
  • Plato: Phaedo.
  • Rey, Georges: Contemporary Philosophy of Mind (Blackwell Publishers, Cambridge 1997).
  • Rorty, Richard: Philosophy and the Mirror of Nature (Princeton University Press, Princeton NJ 1979).
  • Rorty, Richard: "Mind-Body Identity, Privacy and Categories" in The Mind/Brain Identity Theory, C. V. Borst ed. (Macmillan, London 1970).
  • Ryle, Gilbert: The Concept of Mind, (University Paperbacks, Barnes & Noble, NY 1949).
  • Shoemaker, Sydney: Self-Knowledge and Self-Identity (Cornell University Press, Ithaca 1963).
  • Taylor, Richard: Metaphysics, 3rd edition (Prentice Hall, Englewood Cliffs 1983).
  • Tye, Michael: "The Subjective Qualities of Experience" Mind 95 (Jan. 1986) pp. 1-17.
  • Williams, Bernard: "Freedom and the Will" in Freedom and the Will, D. F. Pears, ed. (Macmillan, London 1966).
  • Wisdom, John: Problems of Mind and Matter, (Cambridge University Press, 1934).
  • Wittgenstein, Ludwig: The Blue and Brown Books (Harper & Row, NY 1958).

Author Information

Scott Calef
Email: swcalef@owu.edu
Ohio Wesleyan University
U. S. A.

Mind–body dualism, or mind–body duality, is a view in the philosophy of mind that mental phenomena are, in some respects, non-physical,[1] or that the mind and body are distinct and separable.[2] Thus, it encompasses a set of views about the relationship between mind and matter, and between subject and object, and is contrasted with other positions, such as physicalism and enactivism, in the mind–body problem.[1][2]

Aristotle shared Plato's view of multiple souls and further elaborated a hierarchical arrangement, corresponding to the distinctive functions of plants, animals, and people: a nutritive soul of growth and metabolism that all three share; a perceptive soul of pain, pleasure, and desire that only people and other animals share; and the faculty of reason that is unique to people only. In this view, a soul is the hylomorphic form of a viable organism, wherein each level of the hierarchy formally supervenes upon the substance of the preceding level. Thus, for Aristotle, all three souls perish when the living organism dies.[3][4] For Plato however, the soul was not dependent on the physical body; he believed in metempsychosis, the migration of the soul to a new physical body.[5]

Dualism is closely associated with the thought of René Descartes (1641), which holds that the mind is a nonphysical—and therefore, non-spatial—substance. Descartes clearly identified the mind with consciousness and self-awareness and distinguished this from the brain as the seat of intelligence.[6] Hence, he was the first to formulate the mind–body problem in the form in which it exists today.[7] Dualism is contrasted with various kinds of monism. Substance dualism is contrasted with all forms of materialism, but property dualism may be considered a form of emergent materialism or non-reductive physicalism in some sense.

Types[edit]

Ontological dualism makes dual commitments about the nature of existence as it relates to mind and matter, and can be divided into three different types:

  1. Substance dualism asserts that mind and matter are fundamentally distinct kinds of foundations.[1]
  2. Property dualism suggests that the ontological distinction lies in the differences between properties of mind and matter (as in emergentism).[1]
  3. Predicate dualism claims the irreducibility of mental predicates to physical predicates.[1]

Substance or Cartesian dualism[edit]

Substance dualism is a type of dualism most famously defended by René Descartes, which states that there are two kinds of foundation: mental and body.[6] This philosophy states that the mental can exist outside of the body, and the body cannot think. Substance dualism is important historically for having given rise to much thought regarding the famous mind–body problem. Substance dualism is a philosophical position compatible with most theologies which claim that immortal souls occupy an independent "realm" of existence distinct from that of the physical world.[1]

Property dualism[edit]

Main article: Property dualism

Property dualism asserts that an ontological distinction lies in the differences between properties of mind and matter, and that consciousness is ontologically irreducible to neurobiology and physics. It asserts that when matter is organized in the appropriate way (i.e., in the way that living human bodies are organized), mental properties emerge. Hence, it is a sub-branch of emergent materialism. What views properly fall under the property dualism rubric is itself a matter of dispute. There are different versions of property dualism, some of which claim independent categorisation.[8]

Non-reductive physicalism is a form of property dualism in which it is asserted that all mental states are causally reducible to physical states. One argument for this has been made in the form of anomalous monism expressed by Donald Davidson, where it is argued that mental events are identical to physical events, and there can be strict law-governed causal relationships. Another argument for this has been expressed by John Searle, who is the advocate of a distinctive form of physicalism he calls biological naturalism. His view is that although mental states are ontologically irreducible to physical states, they are causally reducible (see causality). He has acknowledged that "to many people" his views and those of property dualists look a lot alike. But he thinks the comparison is misleading.[8]

Epiphenomenalism[edit]

Main article: Epiphenomenalism

Epiphenomenalism is a form of Property Dualism, in which it is asserted that one or more mental states do not have any influence on physical states (both ontologically and causally irreducible). It asserts that while material causes give rise to sensations, volitions, ideas, etc., such mental phenomena themselves cause nothing further: they are causal dead-ends. This can be contrasted to interactionism, on the other hand, in which mental causes can produce material effects, and vice versa.[9]

Predicate dualism[edit]

Predicate dualism is a view espoused by nonreductive physicalists such as Donald Davidson and Jerry Fodor, who maintain that while there is only one ontological category of substances and properties of substances (usually physical), the predicates that we use to describe mental events cannot be redescribed in terms of (or reduced to) physical predicates of natural languages.[10][11] If we characterize predicate monism as the view subscribed to by eliminative materialists, who maintain that such intentional predicates as believe, desire, think, feel, etc., will eventually be eliminated from both the language of science and from ordinary language because the entities to which they refer do not exist, then predicate dualism is most easily defined as the negation of this position. Predicate dualists believe that so-called "folk psychology", with all of its propositional attitude ascriptions, is an ineliminable part of the enterprise of describing, explaining and understanding human mental states and behavior.

Davidson, for example, subscribes to Anomalous Monism, according to which there can be no strict psycho-physical laws which connect mental and physical events under their descriptions as mental and physical events. However, all mental events also have physical descriptions. It is in terms of the latter that such events can be connected in law-like relations with other physical events. Mental predicates are irreducibly different in character (rational, holistic and necessary) from physical predicates (contingent, atomic and causal).[10]

Dualist views of mental causation[edit]

This part is about causation between properties and states of the thing under study, not its substances or predicates. Here a state is the set of all properties of what's being studied. Thus each state describes only one point in time.

Interactionism[edit]

Main article: Interactionism (philosophy of mind)

Interactionism is the view that mental states, such as beliefs and desires, causally interact with physical states. This is a position which is very appealing to common-sense intuitions, notwithstanding the fact that it is very difficult to establish its validity or correctness by way of logical argumentation or empirical proof. It seems to appeal to common-sense because we are surrounded by such everyday occurrences as a child's touching a hot stove (physical event) which causes him to feel pain (mental event) and then yell and scream (physical event) which causes his parents to experience a sensation of fear and protectiveness (mental event) and so on.[6]

Non-reductive physicalism[edit]

Main article: Non-reductive physicalism

Non-reductive physicalism is the idea that while mental states are physical they are not reducible to physical properties, in that an ontological distinction lies in the differences between the properties of mind and matter. According to non-reductive physicalism all mental states are causally reducible to physical states where mental properties map to physical properties and vice versa. A prominent form of non-reductive physicalism called anomalous monism was first proposed by Donald Davidson in his 1970 paper Mental events, where it is claimed that mental events are identical with physical events, and that the mental is anomalous, i.e. under their mental descriptions these mental events are not regulated by strict physical laws.

Epiphenomenalism[edit]

Main article: Epiphenomenalism

Epiphenomenalism states that all mental events are caused by a physical event and have no physical consequences, and that one or more mental states do not have any influence on physical states. So, the mental event of deciding to pick up a rock ("M1") is caused by the firing of specific neurons in the brain ("P1"). When the arm and hand move to pick up the rock ("P2") this is not caused by the preceding mental event M1, nor by M1 and P1 together, but only by P1. The physical causes are in principle reducible to fundamental physics, and therefore mental causes are eliminated using this reductionist explanation. If P1 causes both M1 and P2, there is no overdetermination in the explanation for P2.[6]

The idea that even if the animal were conscious nothing would be added to the production of behavior, even in animals of the human type, was first voiced by La Mettrie (1745), and then by Cabanis (1802), and was further explicated by Hodgson (1870) and Huxley (1874).[12]Jackson gave a subjective argument for epiphenomenalism, but later refuted it and embraced physicalism.[13]

Parallelism[edit]

Main article: Parallelism (philosophy)

Psycho-physical parallelism is a very unusual view about the interaction between mental and physical events which was most prominently, and perhaps only truly, advocated by Gottfried Wilhelm von Leibniz. Like Malebranche and others before him, Leibniz recognized the weaknesses of Descartes' account of causal interaction taking place in a physical location in the brain. Malebranche decided that such a material basis of interaction between material and immaterial was impossible and therefore formulated his doctrine of occasionalism, stating that the interactions were really caused by the intervention of God on each individual occasion. Leibniz's idea is that God has created a pre-established harmony such that it only seems as if physical and mental events cause, and are caused by, one another. In reality, mental causes only have mental effects and physical causes only have physical effects. Hence the term parallelism is used to describe this view.[9]

Occasionalism[edit]

Main article: Occasionalism

Occasionalism is a philosophical doctrine about causation which says that created substances cannot be efficient causes of events. Instead, all events are taken to be caused directly by God himself. The theory states that the illusion of efficient causation between mundane events arises out of a constant conjunction that God had instituted, such that every instance where the cause is present will constitute an "occasion" for the effect to occur as an expression of the aforementioned power. This "occasioning" relation, however, falls short of efficient causation. In this view, it is not the case that the first event causes God to cause the second event: rather, God first caused one and then caused the other, but chose to regulate such behaviour in accordance with general laws of nature. Some of its most prominent historical exponents have been Louis de la Forge, Arnold Geulincx, and Nicholas Malebranche.[14]

Kantianism[edit]

Main article: Kantianism

According to Immanuel Kant's philosophy, there is a distinction between actions done by desire and the ones performed by liberty (categorical imperative). Thus, not all physical actions are caused by either matter or freedom. Some actions are purely animal in nature, while others are the result of mental action on matter.

Historical overview[edit]

Plato and Aristotle[edit]

In the dialogue Phaedo, Plato formulated his famous Theory of Forms as distinct and immaterial substances of which the objects and other phenomena that we perceive in the world are nothing more than mere shadows.[5]

Plato makes it clear, in the Phaedo, that the Forms are the universalia ante res, i.e. they are ideal universals, by which we are able to understand the world. In his allegory of the cave Plato likens the achievement of philosophical understanding to emerging into the sun from a dark cave, where only vague shadows of what lies beyond that prison are cast dimly upon the wall. Plato's forms are non-physical and non-mental. They exist nowhere in time or space, but neither do they exist in the mind, nor in the pleroma of matter; rather, matter is said to "participate" in form (μεθεξις methexis). It remained unclear however, even to Aristotle, exactly what Plato intended by that.

Aristotle argued at length against many aspects of Plato's forms, creating his own doctrine of hylomorphism wherein form and matter coexist. Ultimately however, Aristotle's aim was to perfect a theory of forms, rather than to reject it. Although Aristotle strongly rejected the independent existence Plato attributed to forms, his metaphysics do agree with Plato's a priori considerations quite often. For example, Aristotle argues that changeless, eternal substantial form is necessarily immaterial. Because matter provides a stable substratum for a change in form, matter always has the potential to change. Thus, if given an eternity in which to do so, it will, necessarily, exercise that potential.

Part of Aristotle's psychology, the study of the soul, is his account of the ability of humans to reason and the ability of animals to perceive. In both cases, perfect copies of forms are acquired, either by direct impression of environmental forms, in the case of perception, or else by virtue of contemplation, understanding and recollection. He believed the mind can literally assume any form being contemplated or experienced, and it was unique in its ability to become a blank slate, having no essential form. As thoughts of earth are not heavy, any more than thoughts of fire are causally efficient, they provide an immaterial complement for the formless mind.[3]

From Neoplatonism to scholasticism[edit]

In the philosophical school of Neoplatonism, most active in Late Antiquity, claimed that the physical and the spiritual are both emanations of the One. Neoplatonism exerted a considerable influence on Christianity, as did the philosophy of Aristotle via scholasticism.[15]

In the scholastic tradition of Saint Thomas Aquinas, a number of whose doctrines have been incorporated into Roman Catholic dogma, the soul is the substantial form of a human being.[16] Aquinas held the Quaestiones disputate de anima, or "Disputed questions on the soul", at the Roman studium provinciale of the Dominican Order at Santa Sabina, the forerunner of the Pontifical University of Saint Thomas Aquinas, Angelicum during the academic year 1265-66.[17] By 1268 Aquinas had written at least the first book of the Sententia Libri De anima, Aquinas' commentary on Aristotle's De anima, the translation of which from the Greek was completed by Aquinas' Dominican associate at ViterboWilliam of Moerbeke in 1267.[18] Like Aristotle, Aquinas held that the human being was a unified composite substance of two substantial principles: form and matter. The soul is the substantial form and so the first actuality of a material organic body with the potentiality for life.[19] While Aquinas defended the unity of human nature as a composite substance constituted by these two inextricable principles of form and matter, he also argued for the incorruptibility of the intellectual soul,[20] in contrast to the corruptibility of the vegetative and sensitive animation of plants and animals.[21] His argument for the subsistence and incorruptibility of the intellectual soul takes its point of departure from the metaphysical principle that operation follows upon being (agiture sequitur esse), i.e., the activity of a thing reveals the mode of being and existence it depends upon. Since the intellectual soul exercises its own per se intellectual operations without employing material faculties, i.e. intellectual operations are immaterial, the intellect itself and the intellectual soul, must likewise be immaterial and so incorruptible. Even though the intellectual soul of man is able to subsist upon the death of the human being, Aquinas does not hold that the human person is able to remain integrated at death. The separated intellectual soul is neither a man nor a human person. The intellectual soul by itself is not a human person (i.e., an individual supposit of a rational nature).[22] Hence, Aquinas held that "soul of St. Peter pray for us" would be more appropriate than "St. Peter pray for us", because all things connected with his person, including memories, ended with his corporeal life.[23]

The Catholic doctrine of the resurrection of the body does nor subscribe that, sees body and soul as forming a whole and states that at the second coming, the souls of the departed will be reunited with their bodies as a whole person (substance) and witness to the apocalypse. The thorough consistency between dogma and contemporary science was maintained here[24] in part from a serious attendance to the principle that there can be only one truth. Consistency with science, logic, philosophy, and faith remained a high priority for centuries, and a university doctorate in theology generally included the entire science curriculum as a prerequisite. This doctrine is not universally accepted by Christians today. Many believe that one's immortal soul goes directly to Heaven upon death of the body.[25]

Descartes and his disciples[edit]

In his Meditations on First Philosophy, Descartes embarked upon a quest in which he called all his previous beliefs into doubt, in order to find out of what he could be certain.[7] In so doing, he discovered that he could doubt whether he had a body (it could be that he was dreaming of it or that it was an illusion created by an evil demon), but he could not doubt whether he had a mind. This gave Descartes his first inkling that the mind and body were different things. The mind, according to Descartes, was a "thinking thing" (Latin: res cogitans), and an immaterial substance. This "thing" was the essence of himself, that which doubts, believes, hopes, and thinks. The body, "the thing that exists" (Latin: res extensa), regulates normal bodily functions (such as heart and liver). According to Descartes, animals only had a body and not a soul (which distinguishes humans from animals). The distinction between mind and body is argued in Meditation VI as follows: I have a clear and distinct idea of myself as a thinking, non-extended thing, and a clear and distinct idea of body as an extended and non-thinking thing. Whatever I can conceive clearly and distinctly, God can so create.

The central claim of what is often called Cartesian dualism, in honor of Descartes, is that the immaterial mind and the material body, while being ontologically distinct substances, causally interact. This is an idea that continues to feature prominently in many non-European philosophies. Mental events cause physical events, and vice versa. But this leads to a substantial problem for Cartesian dualism: How can an immaterial mind cause anything in a material body, and vice versa? This has often been called the "problem of interactionism."

Descartes himself struggled to come up with a feasible answer to this problem. In his letter to Elisabeth of Bohemia, Princess Palatine, he suggested that spirits interacted with the body through the pineal gland, a small gland in the centre of the brain, between the two hemispheres.[7] The term "Cartesian dualism" is also often associated with this more specific notion of causal interaction through the pineal gland. However, this explanation was not satisfactory: how can an immaterial mind interact with the physical pineal gland? Because Descartes' was such a difficult theory to defend, some of his disciples, such as Arnold Geulincx and Nicholas Malebranche, proposed a different explanation: That all mind–body interactions required the direct intervention of God. According to these philosophers, the appropriate states of mind and body were only the occasions for such intervention, not real causes. These occasionalists maintained the strong thesis that all causation was directly dependent on God, instead of holding that all causation was natural except for that between mind and body.[14]

Recent formulations[edit]

During the 19th and 20th centuries, materialistic monism became the norm.[26] Still, in addition to already discussed theories of dualism (particularly the Christian and Cartesian models) there are new theories in the defense of dualism. Naturalistic dualism comes from Australian philosopher, David Chalmers (born 1966) who argues there is an explanatory gap between objective and subjective experience that cannot be bridged by reductionism because consciousness is, at least, logically autonomous of the physical properties upon which it supervenes. According to Chalmers, a naturalistic account of property dualism requires a new fundamental category of properties described by new laws of supervenience; the challenge being analogous to that of understanding electricity based on the mechanistic and Newtonian models of materialism prior to Maxwell's equations.

A similar defense comes from Australian philosopher Frank Jackson (born 1943) who revived the theory of epiphenomenalism which argues that mental states do not play a role in physical states. Jackson argues that there are two kinds of dualism. The first is substance dualism that assumes there is second, non-corporeal form of reality. In this form, body and soul are two different substances. The second form is property dualism that says that body and soul are different properties of the same body. He claims that functions of the mind/soul are internal, very private experiences that are not accessible to observation by others, and therefore not accessible by science (at least not yet). We can know everything, for example, about a bat's facility for echolocation, but we will never know how the bat experiences that phenomenon.

Arguments for dualism[edit]

The subjective argument[edit]

An important fact is that minds perceive intramental states differently from sensory phenomena,[27] and this cognitive difference results in mental and physical phenomena having seemingly disparate properties. The subjective argument holds that these properties are irreconcilable under a physical mind.

Mental events have a certain subjective quality to them, whereas physical seem not to. So, for example, one may ask what a burned finger feels like, or what the blueness of the sky looks like, or what nice music sounds like.[28] Philosophers of mind call the subjective aspects of mental events qualia. There is something that it's like to feel pain, to see a familiar shade of blue, and so on. There are qualia involved in these mental events. And the claim is that qualia cannot be reduced to anything physical.[1]

Thomas Nagel first characterized the problem of qualia for physicalistic monism in his article, "What Is It Like to Be a Bat?". Nagel argued that even if we knew everything there was to know from a third-person, scientific perspective about a bat's sonar system, we still wouldn't know what it is like to be a bat. However, others argue that qualia are consequent of the same neurological processes that engender the bat's mind, and will be fully understood as the science develops.[29]

Frank Jackson formulated his well-known knowledge argument based upon similar considerations. In this thought experiment, known as Mary's room, he asks us to consider a neuroscientist, Mary, who was born, and has lived all of her life, in a black and white room with a black and white television and computer monitor where she collects all the scientific data she possibly can on the nature of colours. Jackson asserts that as soon as Mary leaves the room, she will come to have new knowledge which she did not possess before: the knowledge of the experience of colours (i.e., what they are like). Although Mary knows everything there is to know about colours from an objective, third-person perspective, she has never known, according to Jackson, what it was like to see red, orange, or green. If Mary really learns something new, it must be knowledge of something non-physical, since she already knew everything about the physical aspects of colour.[30]

However, Jackson later rejected his argument and embraced physicalism.[31] He notes that Mary obtains knowledge not of color, but of a new intramental state, seeing color.[13] Also, he notes that Mary might say "wow," and as a mental state affecting the physical, this clashed with his former view of epiphenomenalism. David Lewis' response to this argument, now known as the ability argument, is that what Mary really came to know was simply the ability to recognize and identify color sensations to which she had previously not been exposed.[32]Daniel Dennett and others also provide arguments against this notion (see Objections).

The zombie argument[edit]

Main article: Philosophical zombie

The zombie argument is based on a thought experiment proposed by David Chalmers. The basic idea is that one can imagine, and, therefore, conceive the existence of, an apparently functioning human being/body without any conscious states being associated with it.

Chalmers' argument is that it seems plausible that such a being could exist because all that is needed is that all and only the things that the physical sciences describe and observe about a human being must be true of the zombie. None of the concepts involved in these sciences make reference to consciousness or other mental phenomena, and any physical entity can be described scientifically via physics whether it is conscious or not. The mere logical possibility of a p-zombie demonstrates that consciousness is a natural phenomenon beyond the current unsatisfactory explanations. Chalmers states that one probably could not build a living p-zombie because living things seem to require a level of consciousness. However (unconscious?) robots built to simulate humans may become the first real p-zombies. Hence Chalmers half-joking calls for the need to build a "consciousness meter" to ascertain if any given entity, human or robot, is conscious or not.[33][34]

Others such as Dennett have argued that the notion of a philosophical zombie is an incoherent,[35] or unlikely,[36] concept. In particular, nothing proves that an entity (e.g., a computer or robot) which would perfectly mimic human beings, and especially perfectly mimic expressions of feelings (like joy, fear, anger, ...), would not indeed experience them, thus having similar states of consciousness to what a real human would have. It is argued that under physicalism, one must either believe that anyone including oneself might be a zombie, or that no one can be a zombie—following from the assertion that one's own conviction about being (or not being) a zombie is a product of the physical world and is therefore no different from anyone else's.

Special sciences argument[edit]

Robinson argues that, if predicate dualism is correct, then there are "special sciences" that are irreducible to physics. These allegedly irreducible subjects, which contain irreducible predicates, differ from hard sciences in that they are interest-relative. Here, interest-relative fields depend on the existence of minds that can have interested perspectives.[9]Psychology is one such science; it completely depends on and presupposes the existence of the mind.

Physics is the general analysis of nature, conducted in order to understand how the universe behaves. On the other hand, the study of meteorological weather patterns or human behavior is only of interest to humans themselves. The point is that having a perspective on the world is a psychological state. Therefore, the special sciences presuppose the existence of minds which can have these states. If one is to avoid ontological dualism, then the mind that has a perspective must be part of the physical reality to which it applies its perspective. If this is the case, then in order to perceive the physical world as psychological, the mind must have a perspective on the physical. This, in turn, presupposes the existence of mind.[9]

However, cognitive science[37] and psychology[38] do not require the mind to be irreducible, and operate on the assumption that it has physical basis. In fact, it is common in science to presuppose a complex system;[39] while fields such as chemistry,[40]biology,[41] or geology[42] could be verbosely expressed in terms of quantum field theory, it is convenient to use levels of abstraction like molecules, cells, or the mantle. It is often difficult to decompose these levels without heavy analysis[43] and computation.[44] Sober has also advanced philosophical arguments against the notion of irreducibility.[45]

Argument from personal identity[edit]

This argument concerns the differences between the applicability of counterfactual conditionals to physical objects, on the one hand, and to conscious, personal agents on the other.[46] In the case of any material object, e.g. a printer, we can formulate a series of counterfactuals in the following manner:

  1. This printer could have been made of straw.
  2. This printer could have been made of some other kind of plastics and vacuum-tube transistors.
  3. This printer could have been made of 95% of what it is actually made of and 5% vacuum-tube transistors, etc..

Somewhere along the way from the printer's being made up exactly of the parts and materials which actually constitute it to the printer's being made up of some different matter at, say, 20%, the question of whether this printer is the same printer becomes a matter of arbitrary convention.

Imagine the case of a person, Frederick, who has a counterpart born from the same egg and a slightly genetically modified sperm. Imagine a series of counterfactual cases corresponding to the examples applied to the printer. Somewhere along the way, one is no longer sure about the identity of Frederick. In this latter case, it has been claimed, overlap of constitution cannot be applied to the identity of mind. As Madell puts it:

"But while my present body can thus have its partial counterpart in some possible world, my present consciousness cannot. Any present state of consciousness that I can imagine either is or is not mine. There is no question of degree here."[46]

If the counterpart of Frederick, Frederickus, is 70% constituted of the same physical substance as Frederick, does this mean that it is also 70% mentally identical with Frederick? Does it make sense to say that something is mentally 70% Frederick?[47] A possible solution to this dilemma is that of open individualism.

Argument from reason[edit]

Main article: Argument from reason

Philosophers and scientists such as Victor Reppert, William Hasker, and Alvin Plantinga have developed an argument for dualism dubbed the "argument from reason". They credit C.S. Lewis with first bringing the argument to light in his book Miracles; Lewis called the argument "The Cardinal Difficulty of Naturalism", which was the title of chapter three of Miracles.[48]

The argument postulates that if, as naturalism entails, all of our thoughts are the effect of a physical cause, then we have no reason for assuming that they are also the consequent of a reasonable ground. However, knowledge is apprehended by reasoning from ground to consequent. Therefore, if naturalism were true, there would be no way of knowing it (or anything else), except by a fluke.[48]

Through this logic, the statement "I have reason to believe naturalism is valid" is inconsistent in the same manner as "I never tell the truth."[49] That is, to conclude its truth would eliminate the grounds from which to reach it. To summarize the argument in the book, Lewis quotes J. B. S. Haldane, who appeals to a similar line of reasoning:[50]

If my mental processes are determined wholly by the motions of atoms in my brain, I have no reason to suppose that my beliefs are true ... and hence I have no reason for supposing my brain to be composed of atoms.

— J. B. S. Haldane, Possible Worlds, page 209

In his essay "Is Theology Poetry?", Lewis himself summarises the argument in a similar fashion when he writes:

If minds are wholly dependent on brains, and brains on biochemistry, and biochemistry (in the long run) on the meaningless flux of the atoms, I cannot understand how the thought of those minds should have any more significance than the sound of the wind in the trees.

— C. S. Lewis, The Weight of Glory and Other Addresses, page 139

But Lewis later agreed with Elizabeth Anscombe's response to his Miracles argument.[51] She showed that an argument could be valid and ground-consequent even if its propositions were generated via physical cause and effect by non-rational factors.[52] Similar to Anscombe, Richard Carrier and John Beversluis have written extensive objections to the argument from reason on the untenability of its first postulate.[53]

Arguments against dualism[edit]

Causal interaction[edit]

One argument against Dualism is with regard to causal interaction. If consciousness (the mind) can exist independently of physical reality (the brain), one must explain how physical memories are created concerning consciousness. Dualism must therefore explain how consciousness affects physical reality. One of the main objections to dualistic interactionism is lack of explanation of how the material and immaterial are able to interact. Varieties of dualism according to which an immaterial mind causally affects the material body and vice versa have come under strenuous attack from different quarters, especially in the 20th century. Critics of dualism have often asked how something totally immaterial can affect something totally material—this is the basic problem of causal interaction.

First, it is not clear where the interaction would take place. For example, burning one's finger causes pain. Apparently there is some chain of events, leading from the burning of skin, to the stimulation of nerve endings, to something happening in the peripheral nerves of one's body that lead to one's brain, to something happening in a particular part of one's brain, and finally resulting in the sensation of pain. But pain is not supposed to be spatially locatable. It might be responded that the pain "takes place in the brain." But evidently, the pain is in the finger. This may not be a devastating criticism.

However, there is a second problem about the interaction. Namely, the question of how the interaction takes place, where in dualism "the mind" is assumed to be non-physical and by definition outside of the realm of science. The mechanism which explains the connection between the mental and the physical would therefore be a philosophical proposition as compared to a scientific theory. For example, compare such a mechanism to a physical mechanism that is well understood. Take a very simple causal relation, such as when a cue ball strikes an eight ball and causes it to go into the pocket. What happens in this case is that the cue ball has a certain amount of momentum as its mass moves across the pool table with a certain velocity, and then that momentum is transferred to the eight ball, which then heads toward the pocket. Compare this to the situation in the brain, where one wants to say that a decision causes some neurons to fire and thus causes a body to move across the room. The intention to "cross the room now" is a mental event and, as such, it does not have physical properties such as force. If it has no force, then it would seem that it could not possibly cause any neuron to fire. However, with Dualism, an explanation is required of how something without any physical properties has physical effects.[54]

Replies to the argument[edit]

Alfred North Whitehead and, later, David Ray Griffin framed a new ontology (process philosophy) seeking precisely to avoid the pitfalls of ontological dualism.[55]

The explanation given by Arnold Geulincx and Nicholas Malebranche is that of occasionalism, where all mind–body interactions require the direct intervention of God.

At the time C. S. Lewis wrote Miracles,[56]quantum mechanics (and physical indeterminism) was only in the initial stages of acceptance, but still Lewis stated the logical possibility that, if the physical world was proved to be indeterministic, this would provide an entry (interaction) point into the traditionally viewed closed system, where a scientifically described physically probable/improbable event could be philosophically described as an action of a non-physical entity on physical reality. He states, however, that none of the arguments in his book will rely on this. It should also be noted that, while some interpretations of quantum mechanics consider wave function collapse to be indeterminate,[57] in others this event is defined and deterministic.[57]

Argument from physics[edit]

The argument from physics is closely related to the argument from causal interaction. Many physicists and consciousness researchers have argued that any action of a nonphysical mind on the brain would entail the violation of physical laws, such as the conservation of energy.[58][59][60][61]

By assuming a deterministic physical universe, the objection can be formulated more precisely. When a person decides to walk across a room, it is generally understood that the decision to do so, a mental event, immediately causes a group of neurons in that person's brain to fire, a physical event, which ultimately results in his walking across the room. The problem is that if there is something totally nonphysical causing a bunch of neurons to fire, then there is no physical event which causes the firing. This means that some physical energy is required to be generated against the physical laws of the deterministic universe—this is by definition a miracle and there can be no scientific explanation of (repeatable experiment performed regarding) where the physical energy for the firing came from.[62] Such interactions would violate the fundamental laws of physics. In particular, if some external source of energy is responsible for the interactions, then this would violate the law of the conservation of energy.[63] Dualistic interactionism has therefore been criticized for violating a general heuristic principle of science: the causal closure of the physical world.

Replies to the argument from physics[edit]

The Stanford Encyclopedia of Philosophy[6] and the New Catholic Encyclopedia[64] give two possible replies to the above objections. The first reply is that the mind may influence the distribution of energy, without altering its quantity. The second possibility is to deny that the human body is causally closed, as the conservation of energy applies only to closed systems. However, physicalists object that no evidence exists for the causal non-closure of the human body.[65] Robin Collins responds[66] that energy conservation objections misunderstand the role of energy conservation in physics. Well understood scenarios in general relativity violate energy conservation and quantum mechanics provides precedent for causal interactions, or correlation without energy or momentum exchange.[67] However, this does not mean the mind spends energy and, despite that, it still not exclude the supernatural.

Another reply is akin to parallelism—Mills holds that behavioral events are causally overdetermined, and can be explained by either physical or mental causes alone.[68] An overdetermined event is fully accounted for by multiple causes at once.[69] However, J. J. C. Smart and Paul Churchland have pointed out that if physical phenomena fully determine behavioral events, then by Occam's razor an unphysical mind is unnecessary.[70]

Robinson suggests that the interaction may involve dark energy, dark matter or some other currently unknown scientific process.[9] However, such processes would necessarily be physical, and in this case dualism is replaced with physicalism, or the interaction point is left for study at a later time when these physical processes are understood.[citation needed]

Another reply is that the interaction taking place in the human body may not be described by "billiard ball" classical mechanics. If a nondeterministic interpretation of quantum mechanics is correct then microscopic events are indeterminate, where the degree of determinism increases with the scale of the system (see Quantum decoherence). Philosophers Karl Popper and John Eccles and physicist Henry Stapp have theorized that such indeterminacy may apply at the macroscopic scale.[71] However, Max Tegmark has argued that classical and quantum calculations show that quantum decoherence effects do not play a role in brain activity.[72] Indeed, macroscopic quantum states have only ever been observed in superconductors near absolute zero.[citation needed]

Yet another reply to the interaction problem is to note that it doesn't seem that there is an interaction problem for all forms of substance dualism. For instance, Thomistic dualism (the dualism of Thomas Aquinas) doesn't obviously face any issue with regards to interaction.[73]

Thomas Breuer in 1994 had proven[74] that physical theories valid for the whole universe are impossible. Any theory will be wrong when applied to a system which contains the observer himself due to self-reference.[dubious– discuss] This proves that the observer's own body does not follow the same physical laws as the rest of the universe. But other people from the observer's point of view will obey the usual physical laws, so conducting experiments on them would not indicate any divergence from the physical predictions.

Argument from brain damage[edit]

This argument has been formulated by Paul Churchland, among others. The point is that, in instances of some sort of brain damage (e.g. caused by automobile accidents, drug abuse, pathological diseases, etc.), it is always the case that the mental substance and/or properties of the person are significantly changed or compromised. If the mind were a completely separate substance from the brain, how could it be possible that every single time the brain is injured, the mind is also injured? Indeed, it is very frequently the case that one can even predict and explain the kind of mental or psychological deterioration or change that human beings will undergo when specific parts of their brains are damaged. So the question for the dualist to try to confront is how can all of this be explained if the mind is a separate and immaterial substance from, or if its properties are ontologically independent of, the brain.[75]

Property dualism and William Hasker's "emergent dualism"[76] seek to avoid this problem. They assert that the mind is a property or substance that emerges from the appropriate arrangement of physical matter, and therefore could be affected by any rearrangement of matter.

Phineas Gage, who suffered destruction of one or both frontal lobes by a projectile iron rod, is often cited as an example illustrating that the brain causes mind. Gage certainly exhibited some mental changes after his accident. This physical event, the destruction of part of his brain, therefore caused some kind of change in his mind, suggesting a correlation between brain states and mental states. Similar examples abound; neuroscientist David Eagleman describes the case of another individual who exhibited escalating pedophilic tendencies at two different times, and in each case was found to have tumors growing in a particular part of his brain.[77][78]

Case studies aside, modern experiments have demonstrated that the relation between brain and mind is much more than simple correlation. By damaging, or manipulating, specific areas of the brain repeatedly under controlled conditions (e.g. in monkeys) and reliably obtaining the same results in measures of mental state and abilities, neuroscientists have shown that the relation between damage to the brain and mental deterioration is likely causal. This conclusion is further supported by data from the effects of neuro-active chemicals (such as those affecting neurotransmitters) on mental functions,[79] but also from research on neurostimulation (direct electrical stimulation of the brain, including transcranial magnetic stimulation).[80]

Argument from biological development[edit]

Another common argument against dualism consists in the idea that since human beings (both phylogenetically and ontogenetically) begin their existence as entirely physical or material entities and since nothing outside of the domain of the physical is added later on in the course of development, then we must necessarily end up being fully developed material beings. There is nothing non-material or mentalistic involved in conception, the formation of the blastula, the gastrula, and so on.[81] The postulation of a non-physical mind would seem superfluous.[citation needed]

Argument from neuroscience[edit]

See also: Neurophilosophy, Neuroscience of free will, Neuroscience of free will § Criticisms, and Thought identification

In some contexts, the decisions that a person makes can be detected up to 10 seconds in advance by means of scanning their brain activity.[82] Furthermore, subjective experiences and covert attitudes can be detected,[83] as can mental imagery.[84] This is strong empirical evidence that cognitive processes have a physical basis in the brain.[85][86]

Argument from simplicity[edit]

The argument from simplicity is probably the simplest and also the most common form of argument against dualism of the mental. The dualist is always faced with the question of why anyone should find it necessary to believe in the existence of two, ontologically distinct, entities (mind and brain), when it seems possible and would make for a simpler thesis to test against scientific evidence, to explain the same events and properties in terms of one. It is a heuristic principle in science and philosophy not to assume the existence of more entities than is necessary for clear explanation and prediction (see Occam's razor).

This argument was criticized by Peter Glassen in a debate with J. J. C. Smart in the pages of Philosophy in the late 1970s and early 1980s.[87][88][89] Glassen argued that, because it is not a physical entity, Occam's Razor cannot consistently be appealed to by a physicalist or materialist as a justification of mental states or events, such as the belief that dualism is false. The idea is that Occam's razor may not be as "unrestricted" as it is normally described (applying to all qualitative postulates, even abstract ones) but instead concrete (only applies to physical objects). If one applies Occam's Razor unrestrictedly, then it recommends monism until pluralism either receives more support or is disproved. If one applies Occam's Razor only concretely, then it may not be used on abstract concepts (this route, however, has serious consequences for selecting between hypotheses about the abstract).[90]

See also[edit]

References[edit]

  1. ^ abcdefgHart, W.D. (1996) "Dualism", in A Companion to the Philosophy of Mind, ed. Samuel Guttenplan, Oxford: Blackwell, pp. 265-7.
  2. ^ abCrane, Tim; Patterson, Sarah (2001). "Introduction". History of the Mind-Body Problem. pp. 1–2.  
  3. ^ abAristotle (c. mid 4th century BC) On the Soul (De anima), ed. R.D. Hicks, Cambridge: Cambridge University Press, 1907; Books II-III trans. D.W. Hamlyn, Clarendon Aristotle Series, Oxford: Oxford University Press, 1968.
  4. ^Aristotle (c. mid 4th century BC) Metaphysics (Metaphysica), ed. W.D. Ross, Oxford: Oxford University Press, 1924, 2 vols; Books IV-VI, trans. C.A. Kirwan, Clarendon Aristotle Series, Oxford: Oxford University Press, 1971; Books VII-VIII trans. D. Bostock, Clarendon Aristotle Series, Oxford: Oxford University Press, 1994; Books XIII-XIV trans. J. Annas, Clarendon Aristotle Series, Oxford: Oxford University Press, 1976.
  5. ^ abPlato (390s-347 BC) Platonis Opera, vol. 1, Euthyphro, Apologia Socratis, Crito, Phaedo, Cratylus, Theaetetus, Sophistes, Politicus, ed. E.A. Duke, W.F. Hicken, W.S.M. Nicoll, D.B. Robinson and J.C.G. Strachan, Oxford: Clarendon Press, 1995.
  6. ^ abcdeRobinson, Howard, "Dualism", The Stanford Encyclopedia of Philosophy (Fall 2003 Edition), ed. Edward N. Zalta, http://plato.stanford.edu/archives/fall2003/entries/dualism/.
  7. ^ abcDescartes, R. (1641) Meditations on First Philosophy, in The Philosophical Writings of René Descartes
René Descartes's illustration of dualism. Inputs are passed on by the sensory organs to the epiphysis in the brain and from there to the immaterial spirit.
Four varieties of dualist causal interaction. The arrows indicate the direction of causations. Mental and physical states are shown in red and blue, respectively.
Another one of Descartes' illustrations. The fire displaces the skin, which pulls a tiny thread, which opens a pore in the ventricle (F) allowing the "animal spirit" to flow through a hollow tube, which inflates the muscle of the leg, causing the foot to withdraw.
Cartesian dualism compared to three forms of monism.

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