Type physicalism is a theory in philosophy of mind which asserts that mental events are type-identical to the physical events in the brain with which they are correlated. The thesis of type physicalism is that mental event types such as pain are identical with specific physical event types in the brain.
It is also called type identity in order to distinguish it from a similar but distinct theory called the token identity theory. The type-token distinction is easily illustrated by way of example. In the phrase “yellow is yellow is yellow is yellow”, there are only two types of words (“yellow” and “is”) but there are seven tokens (four “yellow” and three “is” tokens).
According to U.T. Place, one of the popularizers of the idea of type-identity in the 1950s and 1960s, the idea of type-identity physicalism originated in the 1930s with the psychologist E. G. Boring and took nearly a quarter of a century to gain acceptance from the philosophical community.
The barrier to the acceptance of any such vision of the mind was that philosophers and logicians had not yet taken a substantial interest in questions of identity and referential identification in general. The dominant epistemology of the logical positivists at that time was phenomenalism, in the guise of the theory of sense data. Indeed Boring himself subscribed to the phenomenalist creed, attempting to reconcile it with an identity theory and this resulted in a reductio ad absurdum of the identity theory, since brain states would have turned out, on this analysis, to be identical to colors, shapes, tones and other sensory experiences.
The revival of interest in the work of Gottlob Frege and his ideas, along with the discrediting of phenomenalism through the influence of Wittgenstein, led to a more tolerant climate toward physicalistic and realist ideas. Logical behaviorism emerged as a serious contender to take the place of the Cartesian “ghost in the machine” and, although not lasting very long as a dominant position on the mind/body problem, its elimination of the whole realm of internal mental events was strongly influential in the formation and acceptance of the thesis of type identity.