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Type-identity theory

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The theory of mind-brain identity is a form of philosophical materialism concerning the nature of the mind. It is one of the main versions of the theory of psychophysical (mental/cerebral) identity with which it must not be confused.


The theory of mind-brain identity finds one of his first formulations in Thomas Hobbes (1588-1679). Hobbes uses the argument of mental causality to defend this thesis: mental causality is intelligible only if mental causes are identical to physical causes. Hobbes’ main argument in favor of this position is that of the causal closure of bodily movement: since the movement of the body can only be produced by another bodily movement, the psychological cause of the body’s movement (“the movement of the body ‘soul’) is itself a physical cause. Hobbes opposes his theory to the dualism of Descartes’ substances. Today, this argument is the main justification for the theory of identity, given the principle of “causal completeness” of the field of physical states, principle that a physical state can be caused only by another physical state.

Although the theory of identity between mental states and brain states has been defended since Hobbes by the materialistic and naturalistic philosophers of the eighteenth and nineteenth centuries, it was not until the 1950s that philosophers like Ullin Place, Herbert Feigl and John J. C. Smart, presented it as a central thesis of their philosophy, each proposing a different version. In the early 1960s, David M. Armstrong generalized identity: all mental states (including conscious experiences and intentional states) are identical to physical states of the central nervous system. We will then speak of an Australian materialistic school to designate the philosophy of mind that has developed in Australian universities around the theory of mind-brain identity.

Thesis of the type-identity

The theory of mind-brain identity suggests that each “type” (or kind) of mental state is identical to a “type” of brain state. According to this theory, there exists for each type of mental state M a certain type of physical state P to which M is identical, according to a law of nature which remains to be discovered. This is a stronger or more radical thesis than identifying only the occurrences of mental states with occurrences of brain states. It makes it possible to establish a systematic correlation between the psychological characteristics of the mind and those neurophysiological of the brain, whereas the identity of the occurrences only implies that the mental states and the brain states are numerically identical, are realized within a same process in the brain.

A scientific hypothesis

Mind-brain identity does not derive from logical or semantic reasons. Instead of following a priori from an analysis of the content of mental concepts, it is established a posteriori on the basis of empirical research. It is up to the sciences, especially neuroscience and psychology, to discover what types of physical states are the mental states and to establish the psychophysical laws that justify the reduction of the first to the second.

The mind-brain theory can be compared to the theory of the electromagnetic nature of light. Before the theories of electricity developed, light appeared as a mysterious phenomenon. Once the electricity was discovered, the light could be identified with electromagnetic radiation. In this case, it is not a question of asserting that light is caused by such radiation (it would remain enigmatic in itself), but that it is nothing else than this electromagnetic radiation. In the same vein, the goal is not to say that mental states are caused by neurophysiological processes, but that they are nothing more than neurophysiological processes of a certain type. For advocates of this form of physicalism, it is theoretically possible to reduce psychology to physical descriptions of what is happening in the brain by establishing that type of identity as well as the “bridging laws” that connect the statements of the psychology to those of neurophysiology.

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