MIND-BODY PROBLEM

In Genesis 3:19, God tells Adam, "dust thou art, and unto dust thou shalt return," reminding Adam that he was fashioned from the dust of the earth. Modern science tells us that the earth was formed from the dust of the sun and that we are composed of materials formed from star dust. We are, however, also possessed of mind: We can think, feel, and exercise our will—as did Eve when she ate the forbidden fruit of the tree of knowledge of good and evil. The ancient mind-body problem is how the mind or soul or spirit is united with the body. It has now been known for several centuries that our minds are related to our bodies via their relation to a certain bodily organ, the brain. The ancient problem led to the mind-brain problem: How are our minds related to our brains? Are they one thing or two? And if two, how are the two united? But the fundamental problem is: What is the place of mental phenomena in nature?

The doctrine that the soul is distinct from the body, existing prior to it and after bodily death, is found in the writings of Plato. (In the Phaedo, one argument of Socrates for immortality is that the soul is not made of parts, and so cannot come apart.) The Platonic idea of a soul independent of the body was embraced by Augustine of Hippo, a major figure in the development of the Christian doctrine of an immaterial, immortal soul. But as to how soul and body are united, Augustine could only marvel: "The manner in which spirits are united to bodies is altogether wonderful and transcends the understanding of men" (On the City of God, XXI, 10 Haldane 1994, p. 335).

René Descartes tried to lay the foundation for a science of nature according to which all bodies are located in a physical realm—a substance, res extensa, which pervades all of space—and all interactions among them are governed by mechanistic laws. But mind (res cogitans ), he argued, lacks spatial extension (and even location at a spatial point) and so is not subject to the mechanistic laws of the physical realm, thus leaving the will free. Minds, moreover, are substances and so capable of existence independently of physical substance; thus, immortality of the mind is possible. Descartes argued that it is certain that he is his mind since doubt itself requires a doubter and thus a thinking subject, an I. And he argued that he is not his body since he can clearly and distinctly conceive of his existing without a body and that it is thus possible for him to exist disembodied.

He nevertheless also acknowledged in Meditations on First Philosophy (1641): "there is nothing nature teaches me more expressly, or more sensibly than that I have a body, which is ill disposed when I feel pain, which needs to eat and drink when I have feelings of hunger and thirst, etc. … I am joined to it very closely and indeed so compounded and intermingled with my body, that I form, as it were, a single whole with it" (Cottingham et. al. 1985, p. 59). On his view, what unites body and mind is causation, from body to mind (as in perception), and from mind to body (as in action), with the pineal gland in the brain being the primary locus of such interaction. In correspondence with Descartes, Princess Elisabeth of Bohemia pressed the issue of how states of, or changes in, a substance not in space could causally affect states of, or changes in, something in space, and declared such causal interaction too incredible to believe. Descartes was never able to provide a satisfactory answer to her how-question, and in a candid moment remarked: "It does not seem to me that the human mind is capable of conceiving quite distinctly and at the same time both the distinction between mind and body, and their union" (Kenny 1970, p. 142).

Nicholas Malebranche denied mind-body causal interaction, maintaining that God is the only causal agent (Nadler 1999). Were a certain a type of brain state B and mental state M to co-occur, then that would be because God, who is continually engaged in acts of creation of the world, only causes an instance of one of them when he causes an instance of the other; B and M would thus co-occur are a result of being dual-effects of God's acts of creation. This brand of parallelism is called occasionalism. Of course, if God is without spatial extension or location, then Elisabeth's how-question will recur for God's causal interaction with the physical world. But it was thought that how-questions come to an end where the ways of God are concerned. Gottlieb Leibniz held a version of parallelism, preestablished harmony, according to which there is no causal interaction among substances, any regularities among them being the result of God's having actualized a world in which those regularities hold. And he held a kind of idealism, according to which all substances are monads, which have only states of perception and appetite (Sleigh 1999). Benedict (Baruch) Spinoza rejected Descartes's claim that the mind is a substance, arguing that only God or Nature (Deus, sive Natura ) is capable of independent existence, and took all mentality and physicality to be different modes of God or Nature. On his view, a kind of pantheism, we are each finite modes of God or Nature, and our mind and body are identical modes though conceived of under two different kinds of attributes: bodily and mental (Garrett 1999). He thus held a kind of dual-aspect theory. Thomas Hobbes, an atheist, held a version of materialism, reminiscent of the ancient atomism of Democritus and Lucretius—Lucretius wrote of atoms moving in an infinite void—according to which all that exists is matter in motion (Gert 1999). He tried to show how mental processes are just mechanical brain processes, maintaining that thinking is just computation, thereby anticipating the computational view of mental processes prevalent in contemporary cognitive science.

There is something deeply commonsensical about Descartes's interactionism. It seems that bodily sensations such as aches, pains, itches, and tickles cause us to moan, wince, scratch, or laugh and do so by causing brain states that result in bodily movements. In deliberate action, we act on our desires, motives, and intentions in trying to carry out our purposes; and acting on them seems to involve their causing brain states, which cause our muscles to contract, and so our bodies to move, thereby affecting our environment. Perception of the environment seems to involve physical to mental causal transactions: What we perceive causes us to undergo a sense experience. Thus, when we see the scenes before our eyes, for instance, those scenes cause our visual experiences via their effects on our brains. Descartes's substance dualism, however, seems untenable.

But suppose that minds have not just temporal location but spatial location as well. (It is worthwhile pausing to note that according to the theory of general relativity, nothing can be in time without being in space.) Indeed, suppose that they are located where appropriately biologically functioning brains are but that they are nevertheless neither identical with brains nor composed of material particles, being entirely devoid of matter and lacking physical properties such as mass or charge. The spatiotemporal coincidence of minds and brains would be no violation of the principle that two physical objects cannot occupy exactly the same place at exactly the same time since, by hypothesis, minds are not physical objects. They are entirely disembodied even though they are spatiotemporally coincident with appropriately functioning brains. They are a kind of fundamental energy field coincident with such brains. On this conception might minds causally contribute to the animation of their coincident brains and the brains in turn causally influence them?

This sort of view was a subject of debate in the late nineteenth and early twentieth century, as were debates in biology concerning whether there are wholly immaterial entelechies that are spatiotemporally coincident with organisms and which generate a vital force that causally contributes to guiding the development of organisms and sustaining their integrity (McLaughlin 2003). This view of mentality offers no conception of the nature of minds beyond the negative one that they lack any physical properties save spatiotemporal location and the positive one that they are the seat of mental capacities and abilities, the bearers of mental properties, and what undergo mental change. No hint is offered as to how they could be the seat of mental capacities or abilities—of how such abilities and capacities could be exercised within them. No hint is offered as to what their operations might be, as they are entirely devoid of material constituents. Such matters must be taken as primitive; such how-questions are unanswerable.

Many philosophers have argued that to have a mind is not to bear a relation to an object (physical or otherwise) that is the mind but, rather, to have certain capacities and abilities, such as the capacity to think and to feel and the ability to will. We ourselves have these capacities and abilities. We ourselves are the bearers of mental properties, undergo mental events, and engage in mental activity. Moreover, we are embodied. It does not follow that we are identical with our bodies or some part of them such as our brains. A clay statue may fail to be identical with the lump of clay with which it is spatially coincident. They may fail to be identical because they have different temporal properties (perhaps the lump existed before being shaped into a statue) and because they have different modal properties (the lump can survive being squashed while the statue cannot). Rather, the lump may materially constitute the statue (Pea 1997). On a four-dimensionalist conception of objects, however, the lump and statue are space-time worms that have spatiotemporal segments that are identical (Sider 2001). Perhaps we are materially constituted by our bodies (or brains) but fail to be identical with them since they, unlike us, lack mental properties. They may also have different temporal properties from us. If we could exist in a disembodied form after the death and disintegration of our bodies and their organs, then, of course, we are not identical with our bodies or brains.

But it is also true that we are not identical with our bodies or brains if they can continue to exist after we have ceased to exist. We may cease to exist at brain death; but at brain death, the brain still exists. Albert Einstein's brain was removed from his skull shortly after his death with the hope that it would yield insight into his prodigious intelligence. But if he ceased to exist upon the death of his brain, then he was not his brain; and it was not he who was removed from the skull of his corpse. Einstein with his famous equation E=mc², taught us that mass and energy are interconvertible. (Some contemporary New Age Spiritualists would tell us that Einstein's unique energy was released from the matter of his brain upon the expiration of his body, and so that he continues on decoupled from any body. Why any energy released would be Einstein is left entirely obscure, however; and the question of how his mentality was linked to his brain while it was carrying out its normal biological functions remains unanswered. Suffice it to note here that the study of matter-energy in space-time is the subject of physics. We will return to physics shortly.)

Our biologically functioning brains serve somehow as the basis of our capacities to think and to feel and of our volition. Another topic of debate in the late nineteenth and early twentieth centuries was whether, despite the nonexistence of any immaterial object that is the mind, the brain serves as only the causal basis of mental capacities and abilities. On this conception, when we exercise our mental capacities and abilities, mental events (and states) occur within our brains. But they are not identical with occurrences of any kinds of nonmental brain events such as physiological ones; and, indeed, mental events are linked to brain occurrences of other kinds only spatiotemporally and causally: They may accompany them and be causes or effects of them. Since, on this conception, mental events occur within the brain, it might be claimed that they thereby count as physical since the brain is a physical object. But that seems merely a verbal issue. The dualist will claim the important point is that types of mental events are not identical with any other types of brains events and that the only (relevant) relations that token mental events bear to tokens of other kinds of brain events are spatiotemporal and causal. The chief concern raised about this view was whether mental events exert any causal influence on other brain events.

Ewald Herring, in his 1870 lecture at the Imperial Academy of Sciences in Vienna, declared that physiologists should make "the unbroken causative continuity of all material processes an axiom of [their] system of investigation" (Butler 1910, pp. 64–65). He took this position on the grounds that, on the evidence, there seem to be no gaps in the physiological processes in the brain to be filled by mental events. The relationship between mental and physiological events, he maintained, should be left as a question for philosophy; brain physiologists can safely bracket it. The fact that there seem to be no gaps in physiological causal chains for mental events to occupy led Thomas Huxley (1874) to maintain that we (and other animals) are conscious automata : conscious events accompany certain physiological brain events as dual effects of other physiological events but are themselves causally inert. Trained as a medical doctor, William James (1890) appropriated the term epiphenomena, a medical term for symptoms of diseases, for mental phenomena that while caused, lack causal efficacy. James Ward (1903) coined the term epiphenomenalism for the view that mental phenomena have no causal effects. The claim that conscious phenomena are epiphenomena is, however, deeply perplexing. If they are, then our belief that we are in pain is never caused by our feeling of pain. And our experience of control over some of our bodily movements cannot give rise to our belief that we are in control of them, for that, too, would require mental causation.

During this period concern was also raised about whether mental causation would violate the law of conservation of energy. (Leibniz had argued earlier that Descartes was committed to minds affecting the motion of material particles in the pineal gland in violation of the conservation laws of momentum and kinetic energy; his mechanics, however, required contact forces, and was eclipsed by Isaac Newton's mechanics, which rejected that requirement [Woolhouse 1985, Papireau 2001].) One response made to the concern about conservation of energy is that causation may very well not require energy transfer; it does not, for instance, on a regularity theory of causation, according to which causation is subsumption under a law of nature, or on a conditional theory of causation, according to which one event causes another if, had the first not occurred, the second would not have occurred either (Broad 1925, ch. III).

Another point made in response was that the conservation of energy principle is silent about the causes of motion, stating only that energy must be conserved within the total system (Broad 1925, ch. III). (Given general relativity, it is mass-energy that is conserved within the total system.) Unlike on Descartes's conception of the mental, on the conception under consideration, mental events occur within the total system of space-time. Indeed, it seems logically possible that certain mental properties are fundamental force-generating properties, just as in classical mechanics the masses of bodies generate the gravitational force, and the electrical charges of bodies generate the electrostatic force.

Perhaps our will involves such a force. There could be a force that is exerted only when matters becomes so configured as to constitute a brain in which certain sorts of mental properties are realized, and that affects the behavior of material particles in ways that causally contribute to bodily behavior that we regard as being under the (partial) control of our volition. Perhaps, further, this configurational force is fundamental, affecting the behavior of bodies in ways unanticipated by laws governing matter at lower-levels of complexity. If so, then in the framework of classical mechanics, there would be a mental force law on a par with the inverse square laws—the law of gravity and Coulomb's law.

In the framework of nonrelativistic quantum mechanics, mental energy would contribute to determining the value of the Hamiltonian of Erwin Schröedinger's equation. Since mechanics is a branch of physics, it might be claimed that if mechanics has to take mental properties into account, then the properties would thereby count as physical. But the dualist would regard that as a merely verbal point and note that the important point is that mental properties would be fundamental, irreducible force-generating properties. It should be noted, however, that while such configurational forces could be accommodated within Newtonian mechanics and are compatible with Schröedinger's equation, the role of mental properties would by no means be straightforward on the view in question. By hypothesis, the configurational forces would be exerted only when certain enormously complex microstructural properties were realized by minute physical structures of portions of the brain. On the dualist hypothesis in question, mental properties are distinct from any microstructural properties—at most, accompanying them as a matter of fundamental law. But, then, mechanics would, arguably, have to advert only to the microstructural properties in question, taking them to be the configurational force-generating properties (McLaughlin 1992).

Another view discussed during the period in question is that every mental event is a physiological event but that mental properties are not physiological properties (Lewes 1985, Alexander 1920, Broad 1925). If mental events are physiological events, then they have causal effects. And the mistake made by theorists who found no gaps to be filled by mental events would be that they failed to realize that certain physiological events are mental events in that they fall under mental event types. This view faces the following issue: What is it about a physiological event in virtue of which it falls under a mental event type (or exemplifies a mental property)? Suppose that physiological event P falls under mental event type M and that physiological event P* does not. It seems, then, that there must be some difference between P and P* in virtue of which P is and P* is not an event of type M. The issue is what that difference is. George Henry Lewes (1875) seems to have anticipated a functionalist answer of a kind sometimes given today (See Lewis 1966): He spoke of the role of the physiological event in the organism. But the most widely discussed answer during the period in question was that there are fundamental, irreducible laws of nature linking physiological properties with mental properties (Alexander 1920, Broad 1925).

Thus, the reason P is and P* is not an instance of M is that P is an instance of a physiological event type that is linked via a fundamental noncausal law of nature to M while P* falls under no such physiological type. Charles Dunbar Broad (1925–) called this view emergent materialism, and he called such laws of nature transordinal laws. (Transordinal laws were later denigrated as nomological danglers [Feigl 1950].) The guiding idea was that through the course of evolution, complex structures are formed that have genuinely new kinds of properties that are fundamental and thus irreducible. The emergent properties of wholes are linked to properties of their parts and relations among their parts only by fundamental laws. Emergent materialism is thus a kind of dual-aspect theory according to which the mental and physiological aspects of events are linked only by fundamental laws. On this view mental events are causes. But Broad raised the issue of whether they enter into causal relations only in virtue of their physiological properties and so not in virtue of their mental properties (Broad 1925, p. 473). If so, then emergent materialism is committed to a kind of property or type epiphenomenalism (McLaughlin 1989).

In the twentieth century science made truly momentous advances. The atomic theory of matter was vindicated, a quantum mechanical explanation of chemical bonding was provided—dispelling the idea that there are fundamental chemical forces—and organic chemistry and molecular biology made giant strides leading to the demise of any form of vitalism. There seem to be no fundamental mental forces of nature, no mental energy on par with electromagnetic energy, no mental force fields. At least mechanics has as yet no need of such hypotheses. It is now thought that all the fundamental forces are ones that are exerted below the level of the atom: the gravitational force, the electromagnetic force, the weak force, and the strong force. There is some hope for unification, but no role is envisioned for the mental. Of course, current microphysics may well be false; there is at present no quantum theory of gravity. It is, moreover, at least logically possible that our current physics is profoundly mistaken and that the physics in fact true of our world is a kind of Cartesian physics in which mentality plays a fundamental role. But that seems just a fantasy. It is fairly widely assumed that whatever revisions lie ahead for physics, they will not substantially change the dialectic as concerns the mind-body problem.

The mind-body problem is fundamentally the problem of the place of mental phenomena in nature. Contemporary philosophical discussions of the mind-body problem typically proceed under the (often) tacit assumptions that: We are wholly constituted by atoms and more fundamental physical particles, all of which are ingredients of beings entirely devoid of mentality; any fundamental forces at work in us are also at work in many such beings; and that for any (caused) microphysical event P, there is a distinct microphysical event P* that causally determines the objective probability of P (if determinism is true, that probability will be 1). The last—which, unlike the others, is often explicitly stated—is sometimes called the closure of the microphysical though it goes under other names as well.

Of course, one way of responding to the question of the place of an alleged mental phenomenon in nature is by denying that there actually is any such phenomenon. One can be an eliminativist about it. Most contemporary philosophers are eliminativists concerning not only nonspatial, immaterial minds, but also spatiotemporally located immaterial minds: They deny that there are any such things. And they do so for much the same reasons mentioned earlier. Moreover, most contemporary philosophers deny that there are sense data, essentially private mental objects of which only the subject can be aware. Nevertheless, most hold that there are mental properties, capacities, abilities, states, events, and processes. And discussion mainly focuses on their place in nature.

There are many unresolved questions. One central issue concerns the manner in which biologically functioning brains serve as a basis for our capacities to think and to feel and our ability to will: Are they merely a causal basis, or are they rather a constitutive basis? Other issues include whether freedom of the will is compatible with the manner in which they are, such a basis, and with the closure of the microphysical; whether there could be other kinds of material bases for mental capacities and abilities (e.g., silicon-based brains); and what the conditions for personal identity are given the fact of our material embodiment. And there are, as well, theological questions such as whether immortality may somehow be possible despite the fact of our material embodiment. (Might it be possible through the resurrection of the body?)

Among our mental capacities is the capacity to reflect on our own mental lives. Indeed, it is because we have such a capacity that we are able to formulate the mind-body problem. We are not only conscious (as are most kinds of animals), but self-conscious as well. The place in nature of our capacities for self-consciousness must be found. Engagement with the mind-body problem, moreover, requires theoretical reasoning. We form beliefs on the basis of others that provide reasons for them. And we engage in practical reasoning when we deliberate about courses of action (e.g., whether to finish reading the present article). Our capacities for theoretical and practical reasoning must also be located in nature.

The exercise of mental capacities and abilities involves mental states and events (including mental acts). The fundamental problem of the place of mental states and events in nature is that, on the one hand, they have or are instantiations of properties that seem sui generis, and on the other hand, they occur in space-time (arguably, within our skulls) and seem to enter into causal relations with other states and events, including microphysical ones (as, for example, when we deliberately move our bodies across the room with the result that physical particles in our bodies come to be on the other side of the room). The apparently sui generis properties primarily include those of intentionality and phenomenal consciousness.

Properties of intentionality divide into two broad kinds: modes of representation and representational contents. Beliefs, desires, hopes, and intentions, for example—so called propositional attitudes —are representational. They have an intentional (representational) mode—belief, desire, hope, intention—and they have an intentional (representational) content, a content that is (semantically) satisfied or not, depending on the way the world is. States of phenomenal consciousness have phenomenal characters (qualia ): It is like something for the subject of such a state to be in the state (Nagel 1974). States of phenomenal consciousness include bodily sensations, sense experiences, acts of mental imagery, felt emotions, and occurrent thoughts. Thus, for instance, it is like something for a subject to feel pain, or to visually experience red, or to visualize a sunset. Emotions such as fearing that P and being joyous that P have contents, and their characteristic manifestations in phenomenal conscious—feelings of fear and feelings of joy—have phenomenal characters. An occurrent thought such as thinking to oneself that it will rain tomorrow will have a representational content and a phenomenal character as well (even if not a distinctive, characteristic one). (Suffice it to note that the relationship between intentionality and phenomenal consciousness and whether one is primary are highly controversial issues.)

Many contemporary philosophers of mind are engaged in the project of trying to naturalize either intentional properties or phenomenal characters—that is to say, to locate them in nature conceived as fundamentally microphysical. It has been argued that such naturalization projects are doomed to failure where intentional properties are concerned because such properties are identifiable only by their place in a network of normative, rational relations and are thus irreducible, having no echo in the physical sciences (Davidson 1970). But even some philosophers who are optimistic about the prospects of naturalizing intentional properties maintain that the attempt to naturalize phenomenal consciousness may face insuperable difficulties. Huxley mused: "How it is that anything so remarkable as a state of consciousness comes about as a result of irritating nervous tissue, is just as unaccountable as the appearance of the Djinn, when Aladdin rubbed his lamp" (Huxley 1986, p.193). Indeed, it has been claimed that "consciousness is what makes the mind-body problem really intractable" (Nagel 1974, p. 435); that it is "the hard nut of the mind-body problem" (McGinn 1989, p. 394); that it is "the hard part of the mind-body problem" (Strawson 1994, p. 93); and that phenomenal character poses "the hard problem" of consciousness (Chalmers 1996, p. xiii).

Some philosophers have maintained that the link between phenomenal characters and physicality is so mysterious that it is reasonable to hypothesize that the particles—the star dust—from which we are composed must have as yet undiscovered protomental properties, which, though their mode of combination somehow constitute phenomenal characters (James 1890; Nagel 1979). Physics, however, has as yet found no need of this panpsychism hypothesis. Moreover, if the protomental properties are not themselves phenomenal characters and are objective in nature, then the concern arises that their link with phenomenal characters would also be mysterious. In any case, so mysterious has the connection between phenomenal character and physicality seemed that some philosophers have maintained that we are cognitively closed to the sorts of concepts required for understanding the place of phenomenal characters in nature and thus that the matter transcends human understanding (McGinn 1989).

There are a variety of different naturalizing projects, and some are incompatible with others. However, there have been attempts to state a commitment shared by them all. One leading formulation of such a shared commitment is the following global supervenience thesis: Any minimal physical duplicate of our world is a duplicate simpliciter of it (Jackson 1998). A physical duplicate of our world (the actual world) is any possible world that is exactly like our world in every microphysical respect, in respect to its world-wide pattern of distribution of microphysical properties and relations, its world-wide pattern of distribution of microphysical objects, its microphysical laws of nature, and so on. A minimal physical duplicate of our world is any physical duplicate of it that contains nothing other than what is metaphysically required to be a physical duplicate of it. Proponents of different naturalizations programs will offer different explanations of why mental phenomena do not yield a counterexample to the supervenience thesis.

Philosophers, however, who maintain that mental properties of certain sorts are emergent properties, fundamental constituents of nature, linked to other properties only by contingent fundamental laws of nature, will deny the supervenience thesis. Since the laws in question (Broad's transordinal laws ) are contingent and fundamental, it is possible for them to fail to hold even though all of the actual microphysical laws of our world hold. Such philosophers are committed to there being a possible world that is a minimal physical duplicate of our world yet not a duplicate simpliciter of it because the world is devoid of the mental properties in question (or instantiations of them). For example, someone who holds that phenomenal characters are fundamental in nature will claim there is a possible world that is a minimal physical duplicate of our world yet, unlike our world, is devoid of phenomenal consciousness—a zombie world (Chalmers 1996)—and thus not a duplicate simpliciter of our world. And, similarly, someone who held that intentional properties are fundamental will be committed to their being a possible world that is a minimal physical duplicate of our world but which fails to be a duplicate simpliciter of our world since it is devoid of intentionality.

Any world that is a minimal physical duplicate of our world will be one in which exactly the same microphysical causal transactions occur as do in our world. If either normative intentional properties or phenomenal characters yield counterexamples to the supervenience thesis, then such properties make no difference to what microphysical causal transactions occur in our world. And they could make a difference to whether certain causal transactions occur in our world only if those transactions fail to be implemented by microphysical ones. Such, it seems, are the facts of our world.

Whether intentionality and phenomenal consciousness can be naturalized—whether they can be located in nature conceived of as fundamentally microphysical—are the fundamental issues of the contemporary mind-body problem. These are issues of intensive, ongoing debate.

See also Augustine, St.; Broad, Charlie Dunbar; Cartesianism; Computationalism; Descartes, René; Dualism in the Philosophy of Mind; Einstein, Albert; Elisabeth, Princess of Bohemia; Functionalism; Hobbes, Thomas; Huxley, Thomas Henry; James, William; Leibniz, Gottfried Wilhelm; Leucippus and Democritus; Lucretius; Malebranche, Nicholus; Mental Causation; Newton, Isaac; Plato; Reductionism in Philosophy of Mind; Self-knowledge; Socrates; Spinoza, Benedict (Baruch) de; Supervenience.

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Brian P. McLaughlin (2005)