Van Gulick, R. (2009). Functionalism In: B. P. McLaughlin, A. Beckermann, & S. Walter (Eds.), Handbook of Philosophy of Mind (Chapter 7, pp. 128-151).
[Abstract]Functionalism at its core is the thesis that minds and mental kinds are to be understood in terms of the roles or functions that specific states and processes play within suitably organized systems. From a functionalist perspective, minds differ from non-minds not in any distinctive substance or fundamental substrate, but in their systemic organization and the roles played by their parts and sub-parts within it. A minded system is simply one that is organized in the right sort of way, though just which ways those are is a difficult and disputed matter. Functionalists classify states or processes largely, if not solely, in terms of the relevant roles or functions that they play in some such system.
[Citing Place (1956)]  
Citing Place (1956) in context (citations start with an asterisk *):
Section 7.1 History
* The psychophysical type–type identity theory aimed to identify mental properties or states with specific neuroscientific properties (Place 1956; Smart 1959). Just as water turned out to be H₂O and heat to be kinetic energy, so too it was supposed that pain might turn out to be simply C‐fibre firing or some other type of event described by neuroscience. Mental properties on such a model find a home in the physical world in the most direct way, by simply being identical with particular physical properties picked out by some non‐mental physical science; most probably, neuroscience. The evidence for such identifications might include observed correlations between first‐person reported mental states and the brain states observed by third‐person empirical means. The simplest explanation of the correlated observations would be that one was observing the very same state or property by two different means.