Beckermann, A. (2009). What is property physicalism? In: B. P. McLaughlin, A. Beckermann, & S. Walter (Eds.), Handbook of Philosophy of Mind (Chapter 8, pp. 152-172).
[Abstract]Since the days of Democritus, Plato, and Aristotle the main concern of philosophers of mind has been whether there is an independent realm of the mental beyond the realm of the physical. This question has mainly been understood as that of whether there are independent mental substances — souls or selves. Most of what has been written in the philosophy of mind during the last seventy to eighty years, however, has instead been concerned with the question of whether there are independent or irreducible mental properties. To many it has seemed obviously true that there are no mental substances, but with properties things are different. The history of answers to questions such as ‘Is being in pain or thinking about Paris really something physical?’ (including the answers associated with logical behaviourism, the identity theory, functionalism, and supervenience theories) has been told time and again. Nonetheless, it is worth taking a new look at it.
[Citing Place (1956)]  
Citing Place (1956) in context (citations start with an asterisk *):
Section 8.1 Property Physicalism as Requiring Identity
* Generally speaking, it turned out to be impossible to define mental predicates in physical language in a non‐circular way.
This finding, however, did not mark the end but rather the beginning of what today is known as ‘identity theory’. At the end of the 1950s Place and Smart argued that statements like ‘The temperature of a gas is identical to the mean kinetic energy of its molecules’, ‘Lightning is an electrical discharge’, and ‘Water is H₂O’ are perfectly true identity statements though ‘temperature’ and ‘mean kinetic energy’ — or for that matter ‘lightning’ and ‘electrical discharge’ or ‘water’ and ‘H₂O’ — are by no means synonymous (see Place 1956; Smart 1959). Nonetheless, ‘temperature’ and ‘mean kinetic energy’ stand for the same property — as physics has shown us. It is therefore at least possible that empirical science will arrive at the result that even ‘pain’ and ‘C‐fibre firing’ stand for the same property, though they are not synonymous. Place and Smart thus held the view that property physicalism amounts to the claim: Each mental property is identical to a physical property even if the corresponding predicates are not synonymous. Or, in other words: property physicalism does not require synonymy, just identity. But if this is the case, we are again confronted with the question: How do we find out that the mental predicate ‘M’ stands for the same property as the physical predicate ‘P’ if it is not by examining whether ‘M’ and ‘P’ are synonymous?