Levin, J. (2023). Functionalism. In E. N. Zalta, & U. Nodelman (Eds.), The Stanford Encyclopedia of Philosophy (Summer 2023 Edition). plato.stanford.edu/archives/sum2023/entries/functionalism/
[Abstract]Functionalism in the philosophy of mind is the doctrine that what makes something a mental state of a particular type does not depend on its internal constitution, but rather on the way it functions, or the role it plays, in the system of which it is a part. This doctrine is rooted in Aristotle’s conception of the soul, and has antecedents in Hobbes’s conception of the mind as a “calculating machine”, but it has become fully articulated (and popularly endorsed) only in the last third of the 20th century. Though the term ‘functionalism’ is used to designate a variety of positions in a variety of other disciplines, including psychology, sociology, economics, and architecture, this entry focuses exclusively on functionalism as a philosophical thesis about the nature of mental states.
[Citing Place (1956)]  
Citing Place (1956) in context (citations start with an asterisk *):
Section 1. What is Functionalism?
* [T]hough functionalism is officially neutral between materialism and dualism, it has been particularly attractive to materialists, since many materialists believe (or argue; see Lewis, 1966) that it is overwhelmingly likely that any states capable of playing the roles in question will be physical states. If so, then functionalism can stand as a materialistic alternative to the Psycho-Physical Identity Thesis (introduced in Place 1956, Feigl 1958, and Smart 1959, and defended more recently in Hill 1991, and Polger 2011), which holds that each type of mental state is identical with a particular type of neural state. This thesis seems to entail that no creatures with brains unlike ours can share our sensations, beliefs, and desires, no matter how similar their behavior and internal organization may be to our own, and thus functionalism, with its claim that mental states can be multiply realized, has been regarded as providing a more inclusive, less “(species-) chauvinistic” (Block 1980b) – theory of the mind that is compatible with materialism. (More recently, however, some philosophers have contended that the identity thesis may be more inclusive than functionalists assume; see Section 6 for further discussion.)
Section 6. The Future of Functionalism
* [R]ecently [...] there has been a resurgence of interest in the psycho-physical (type-) identity thesis. This is fueled in part by the observation that in the actual practice of neuroscience, neural states are type-individuated more coarsely than early identity theorists such as Place, Feigl, and Smart assumed, and the contention that, contrary to the claims of early defenders of functionalism (e.g. Putnam), there are relatively few extant creatures that are physically unlike humans but share our functional organization. If this is so, then it may well be that many of our genuine functional equivalents that differ from us in their fine-grained neurophysiological make-up can nonetheless share our neural, as well as mental, states, and thus that the psycho-physical identity thesis can claim some of the scope once thought to be exclusive to functionalism. (See Bechtel 2012, Bickle 2012, McCauley 2012, Shapiro & Polger 2012, and Polger & Shapiro 2016.)