The mind-body problem is perhaps the most central issue in the philosophy of mind. The main debate concerns the status of mental phenomena and their relation to physical phenomena. Human beings (and other animals, perhaps) have minds. That is to say, we are "subjects of consciousness" and this is a central part of what it is like to be a human being. We can think and we can have beliefs about the world. This ability to direct thought at the world and to think about things is called "intentionality," a concept associated in modern times with the philosopher and psychologist Franz Brentano (1838-1917). Intentionality and consciousness have come to be referred to as the main "marks of the mental." At the same time, mental phenomena are notoriously difficult to explain or account for, especially in view of the fact that it is equally obvious that human beings have physical bodies. How can consciousness and intentionality occur in a physical body? How can a physical body contain a mind? The mind-body problem deals with the nature of the mind and the body and what their relation to each other may be.
One's views on the mind-body problem relate directly to many other issues in philosophy and cognate areas. One's position in action theory, the field that deals with free will and human agency for example, depends on one's views about the body and the mind. Personal identity is also tied to the mind-body debate. Does personal identity inhere essentially in the mind or soul? Can the mind exist without a body? Can the mind or personal identity continue to exist after the destruction of the body? Is this what immortality would mean? Issues raised by the mind-body problem are also central to the psychological study of human beings. When one studies human beliefs, desires, and other mental phenomena, what exactly is one studying? If one believes, for example, that the mind is nothing more than events and activities of the physical nervous system, one might lean toward the view that what we call "beliefs" or "desires" are really nothing more than folk psychological fictions. There are those who hold that the terms we use to describe mental states serve only a temporary heuristic role that might be supplanted if and when our scientific understanding of the nervous system is able to provide an adequate physical account of these phenomena. Other philosophers hold that a physicalist account of mental phenomena could never be forthcoming, no matter the advances are made in science.
Many different positions on the nature of the mind and the body have been advanced in an attempt to deal with these problems. What follows is a short description of some of the more influential strategies.
Dualism, the position that one's mind and body are distinct, can be traced back to Plato (427-347 BCE) who held the immortal soul and mortal body to be separate. Rene Descartes' (1596-1650) influential arguments in the Meditations gave rise to the problematic nature of the belief in a distinct mind and body. He claimed that the mind and the body are different substances, that the essence of mind is thought, and that the essence of body is space. This view, commonly referred to as "substance dualism;" holds that the body is material or extended substance and the mind is thinking substance.
One of the major difficulties with this view, raised by Princess Elizabeth of Bohemia (1630-1714) in correspondence with Descartes, concerns the manner in which two substances can interact, if at all. Descartes maintained that the pineal gland was the seat, or point of causal interaction, between the distinct substances mind and body. Descartes' position is referred to as "Cartesian interactionism" because Descartes believed the two substances causally interact: mental events can cause physical events (as in voluntary movement) and physical events can cause mental events (in sensation, as when a pin prick causes the experience of pain). The difficulty with dualist interactionism is that it calls for three different types of causal interaction: among thoughts, among physical objects, and between thoughts and physical objects. As physical laws purport to account for all physical motion, the view that some physical events are caused by mental events seems to entail that some physical events are causally overdetermined, that is to say that some physical events have two different sufficient causes. But how, one may ask, can any physical event, such as my arm being raised, be accounted for in terms of a mental cause and a fully sufficient physical cause? Isn't the mental cause superfluous in the explanation of the physical event?
It is also unclear how two distinct substances such as mind and body are capable of interacting in the first place. How exactly can a mental event cause a physical one or vice versa? One solution to the problem of interaction was proposed by Gottfried Leibniz (1646-1716) and Nicolas Malebranche (1638-1715) who developed a variant of non-interactionist Cartesian dualism, called "parallelism." They argued that the two substances, mind and body, in fact do not interact, but merely parallel one another. These views avoid the problems of interactionism by side-stepping it. However, they then must explain how it is that mental events and physicals are so often coordinated. Leibniz and Malebranche rely on implausible theological arguments to explain the coordination.
Another dualistic strategy, "epiphenomenalism," maintains that the causal interaction goes in one direction only, from the physical to the mental. That is, mental events are caused by physical events but physical events are not caused by mental events. This view avoids the problems of overdetermination and the need for a divine will to account for the coordination of physical and mental events. When construed as holding to substance dualism, however, epiphenomenalism still needs to overcome the problem of how an extended substance can interact with an unextended one such as thought. Epiphenomenalism, however, may also be thought to be consistent with substance monism, of which there are many varieties.
"Monism" in general is the view that mind and body are but one substance. Baruch Spinoza (1632-1677) developed a monistic view that has come to be referred to as a "dual aspect." Spinoza that there is only one substance which has two aspects, or properties: thought and extension. In one its contemporary forms the theory is referred to as "property dualism." George Berkeley's (1685-1753) idealism is a monist, dual aspect theory according to which everything is mental. On Berkeley's view, both mental and physical phenomena are perceptions in the mind of God. David Hume (1711-1776), Ernst Mach (1838-1916), and Bertrand Russell (1872-1970) argued for "neutral monism." This view holds that the mental and the physical are really just bundles of neutral entities. Russell named his version of these neutral entities'sensibilia' and maintained that mental and physical phenomena are logical constructions of sensibilia. "Phenomenalism" is another monist position according to which observational statements about the mental are taken to refer to conscious, phenomenal appearances. This position was advocated by A.J. Ayer (1910-1989) in the 20th century and is generally either endorsed as a variety of neutral monism or idealism.
"Materialism" is a monist view that holds there is only one type of substance and it only has one type of property, physical properties. Advances in the physical sciences popularized this position during the first half of the 20th century. Gilbert Ryle (1900-1976) famously labeled the Cartesian mind as a'ghost in the machine,' arguing that the belief that the term'mind' refers to a distinct entity other than the body is a category error. Mental states such as beliefs or desires, Ryle maintained, describe certain capacities or dispositions to behave, as opposed to certain states of a separate entity. To say "Sally believes it is raining" is only to describe a certain dispositional state of Sally to behave in overt ways, and nothing more. Ryle's view is therefore a version of "behavioralism. "Logical behaviorism" is the view that all our psychological vocabulary (e.g., belief, pain, etc.) can be translated into actual or potential dispositions to behave. The behavioralist position, which was popular during the middle of the twentieth century, has its own difficulties, however. One can be in pain, for example, without manifesting any behavior or without being disposed to behave in a certain way. Similarly, one can hold a belief and fail to manifest it in behavior, as in cases of deception.
Another variety of materialism is the "identity theory," advocated by Herbert Feigl (1902-1988), J.J.C. Smart (1920-), Ullin Place (1924-2000) and others. This view claims that mental states are identical to brain states and that sensations are identical to physical processes. On this view mental-physical identity statements thought to be contingent and are empirically discoverable. The statement "a pain just is the firing of a c-fiber" seen to be analogous to other scientifically verifiable statements such as, "heat is molecular motion" or "lightning is electrical discharge." Critics of identity theory argue that it fails to account for the qualitative nature of consciousness and intentionality. It also fails to take into account relations between persons and their environments. Hilary Putnam, in his famous "twin earth" thought experiment, argued that two people might be identical in terms of brain states but differ in what they believe in virtue of differences in the environment around them.
"Eliminative materialism" is another version of physicalism that seeks to solve the mind-body problem by arguing that mental states and properties simply do not exist. This is the view that statements ascribing mental states to individuals are strictly false. This theory holds that the vocabulary of folk psychology ("beliefs," "desires," etc.) is non-referring, akin to words that attribute supernatural powers to things.
Instead of maintaining that psychological vocabulary is non-referring, Donald Davidson (1917-2003) developed a version of monist materialism called "anomalous monism." Davidson argued that mental events are identical with physical events but that mental events are not reducible to physical events. The view, which addresses certain ontological characteristics of mental and physical events, is designed to preserve 3 intuitions: (1) that mental events causally interact with physical events and vice versa, (2) that events are causally related, but (3) that the mental is "anomalous" in the sense that that no laws can be formulated to explain or predict mental events.
As against the idea that mental events (such as pain) can be causally linked to distinct physical events such as states of the brain, some philosophers, such as Hilary Putnam (1926-) argue that mental events are "multiply realizable" across various kinds of physical states. On this view, mental events are constituted by various aspects of their functional role rather than by any identity with physical states or events. The view, an early version of which was advanced by George Lewes (1817-1878), serves to explain how it is that entities (animals, computers, and aliens, for example) with physical make-ups different from human beings might be thought to be capable of mental events such as sensation. The view is not committed to monism or dualism, but many of its advocates hold that functionalism is compatible with monistic materialism.
Materialist views, which have received considerable attention in the last several decades, are not without their critics. Materialist views are sometimes charged with a fatal flaw: the difficulty of not being able to account for so-called qualitative phenomena such as "what it is like to see red," a point famously argued by Frank Jackson (1943-). This kind of criticism raises the possibility that, even if our understanding of the physical make-up and activities of the brain and associated structures were to be dramatically improved, the status of the mind as a non-physical substance, will remain a major area of philosophical dispute.