Mind-Body Identity Theory is the idea that the mind is just a part of the physical body.

Philosophers like to say that "mental states" are "brain states," but we will see that much more than abstract "states," "events," "properties," and "laws" are involved in explaining how the mind emerges from the brain.

A more extreme position is to simply deny the existence of mind (there is only a brain), or to say that mind is at best an epiphenomenon, with no causal influences on the physical world.

Most identity theorists have been materialists who argued for a form of eliminative materialism or reductionism. Ultimately, they regard physics as the foundational science. They expect that molecules are reducible to atoms, biological cells are reducible to molecules, the brain is reducible to its neurons, and the mind is reducible to the brain.

Some philosophers argue that the mind "emerges" from the brain. They see emergence as producing new "laws" at each hierarchical level of "self-organization." Thus, cells have complex biological laws that emerge from simpler molecular laws. On this view, the mind has "states," "events," "properties," and "laws" that are not predictable based on those of the brain.

Some emergentists believe that the new laws in an upper hierarchical level are not reducible to those of the lower levels. They can thus claim to be materialists or physicalists but deny reductionism. Other philosophers describe the relationship between hierarchical levels as one of supervenience. They claim that "mental events" supervene on "physical events."

Many writers over the centuries have simply identified the mind with the brain, noticing the empirical fact that when the brain is damaged, mental properties are also impaired. But others, following René Descartes, have assumed that mind is an immaterial, non-physical substance. Descartes and others simply assumed that the mental world could influence the physical world and vice versa, but the mystery of exactly how this might be possible led to the "mind-body problem" the question how two unlike substances, one material, the other immaterial, can interact. Identity theory is one solution to that problem.

The other solution is dualism and a theory of interactionism (notably the work of Karl Popper and John Eccles).

The first philosophers to argue for an identity of mind (or consciousness) and brain include Ullin T. Place (1956), Herbert Feigl, and J.J.C.Smart (1959).

Place explicitly describes "consciousness as a brain process," specifically as "patterns" of brain activity. He does not trivialize this identity as a succession of individual "mental events and physical events" in some kind of causal chain. He compares this identity to the idea that "lightning is a motion of electrical charges."

("Is Consciousness a Brain Process?", in British Journal of Psychology, 47, pp.44-50 (1956))

Herbert Feigl's work was independent of Place's, but he said that the fundamental idea had been held by many earlier materialist (monist) thinkers. He thought it was stated clearly by Rudolf Carnap in 1925. Feigl describes his own thesis:

The identity thesis which I wish to clarify and to defend asserts that the states of direct experience which conscious beings "live through" and those which we confidently ascribe to some of the higher animals, are identical with certain (presumably configurational) aspects of the neural processes in these organisms.

("The 'Mental' and the 'Physical'", in Concepts, Theories, and the Mind-Body Problem, University of Minnesota Press (1958), p.150)

J.J.C.Smart clarified and extended the identity theory of his colleague U.T.Place

When I say that a sensation is a brain process or that lightning is an electric discharge, I am using "is" in the sense of strict identity. (Just as in the — in this case necessary — proposition "7 is identical with the smallest prime number greater than 5.") When I say that a sensation is a brain process or that lightning is an electric dis- charge I do not mean just that the sensation is somehow spatially or temporally continuous with the brain process or that the lightning is just spatially or temporally continuous with the discharge.

("Sensations and Brain Processes", in Philosophical Review 68 pp. 141-156 (1959))

Smart is a strong materialist. He says "A man is a vast arrangement of physical particles, but there are not, over and above this, sensations or states of consciousness." (Ibid.)

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Information Philosophy rejects the Identity Thesis. The mind is an immaterial and non-physical process going on in the physical and material brain.

The human mind is the most highly evolved form of the biological information processing that goes on in all organisms. Information philosophy sees the mind as a biological information processing system.

Our mind/brain model emphasizes the abstract information content of the mind. Abstract information is neither matter nor energy, yet it needs matter for its concrete embodiment and energy for its communication. Information fits well with the common-sense notion of spirit, or with behaviorist philosopher Gilbert Ryle's derisive "ghost in the machine."

When we are conceived, it is information in our parental DNA (plus the vastly greater information in the human cell) that starts life. When we die, mere matter remains. It is our developmental and experiential information - our life history - that is lost, excepting that we have stored externally in the Sum.

Because it is embodied in the brain, this mind can control the actions of a body. The mind is normally unaffected by its own quantum level uncertainty (excepting when we want to be creative and unpredictable).

Thus our mind/body model explains how a relatively immaterial, "free," unpredictable, and creative mind can exert downward causal control over the adequately determined material body through the self-determinate and responsible actions selected by the will from an agenda of alternative possibilities.

See the Experience-Recorder-Reproducer model of the mind.