Mind-brain identity theory is a philosophy that purports the mind and brain are the same. In other words, the state of mind is the same as brain processes; that mental state is the same as the physical state of the brain.
British philosopher and psychologist U.T. Place, one of the developers of the identity theory of mind, wrote in his 1954 paper "Is Consciousness a Brain Process?" this assertion on the topic: "The view that there exists a separate class of events, mental events, that cannot be described in terms of the concepts employed by the physical sciences no longer commands the universal and unquestioning acceptance among philosophers and psychologists that it once did."
In the most simplistic terms, mind-brain identity theory purports that the mind is simply a part of the physical body.
Like all ideas and theories on the state of being, this philosophy of mind seeks to explain the nature of human consciousness and to address the mind-body problem -- a philosophical conundrum over the relationship between the mind (with its thoughts, beliefs and emotions) and the physical body.
Origins of identity theory
Mind-brain identity theory arose in the mid-20th century when it was promoted in ideas set forward by several philosophers and academics (namely Place, Herbert Feigl and J.J.C. Smart).
Building on the work happening at the time in psychology and the physical sciences, these identity theorists advanced the core idea put forth in materialism (a philosophy of mind asserting that all aspects of being exist as matter).
Physicalism is often used synonymously with materialism. Some also equate mind-brain identity theory with reductive materialism, another related philosophical response to the mind-body problem.
Mind-brain identity theory vs. functionalism
Mind-brain identity theory is also often compared and contrasted with functionalism, which is another philosophy addressing the mind-body relationship question.
Functionalism borrows from the world of computers and information processing, saying that mental states exist as what they do or, in other words, they are tied to their functions.
There are differing opinions about how functionalism relates to mind-brain identity theory. Some argue that functionalism is an extension of or improvement to identity theory, others say it's a route to identity theory; and some others say it conflicts with ideas established in identity theory.
Arguments for/against mind-brain identity theory
Mind-brain identity theory contrasts with long-held beliefs attributed to Rene Descartes, a seventeenth-century French philosopher, mathematician and scientist who is considered the father of modern Western philosophy. The ideas put forth by Descartes and other philosophers over the past three centuries assert that the mind is both immaterial and non-physical.
In fact, the mind-brain identity theory stands in contrast to all other philosophies in the category of mind-body dualism, which contends that the mind and body are distinct and different substances.
Meanwhile, other philosophies have been developed in the years after the introduction of mind-brain identity theory.
For example, American philosopher, mathematician, and computer scientist Hilary Putnam in the 1960s published a series of papers that introduced "multiple realizability" -- the idea that, in very simple terms, the same mental state can be realized in different physical states. It is seen as one of the strongest arguments against identity theory, although discussion and debate on the mind-body relationship continues in philosophy and academic spheres.