What does it mean to say that the mental is nothing over and above the physical? In other words, what exactly is the thesis of physicalism about the mental? The question has not received the philosophical attention it deserves. If that sounds woefully uninformed, it's probably because you are mistaking my restricted thesis of physicalism about the mental for the unrestricted thesis of physicalism simpliciter. Physicalism simpliciter is the doctrine that everything is physical; equivalently, that there is nothing over and above the physical. Physicalism simpliciter has been the subject of intense philosophical scrutiny. There are ongoing debates over how 'the physical' should be defined for the purpose of formulating the thesis, over how the 'nothing over and above' clause should be understood, and over various other matters yet.1
To be sure, philosophers discussing physicalism simpliciter often focus on the mental as opposed to, say, the chemical or geological. Still, the two theses are distinct. Imagine central state materialism is the correct [End Page 167] mind-body theory but that, much to our astonishment, water turns out to be not H2O but instead the spooky, non-physical stuff ectoplasm. In functional role terms: further empirical investigation reveals that ectoplasm rather than H2O is what actually occupies the water role, and so is what fills the lakes and rivers, falls from the sky when it rains, quenches our thirst, and so on. Then physicalism about the mental would be true while physicalism simpliciter would be false. The mental would be nothing over and above the physical, but something over and above the physical would exist.2 What is it that would be true in this scenario? That is my question: What is physicalism about the mental?
The discussion is divided into three sections. In §I, I present a content externalist argument showing that the most obvious way of trying to characterize physicalism about the mental does not work. In §2 I explore the metaphysics of wide mental states to defend a crucial assumption made by the argument in §1. Finally, in §3 I offer my own favored approach to characterizing the thesis of physicalism about the mental, and then use my approach to argue for a new view of physicalism simpliciter. The question of how to formulate physicalism about the mental is interesting in its own right, but it is also interesting for the light it sheds on physicalism simpliciter. [End Page 168]
Here is the obvious thought. To formulate physicalism about the mental, start with the canonical formulation of physicalism simpliciter, whatever that happens to be. Presumably it will involve some sort of unrestricted universal quantification since intuitively physicalism simpliciter is the thesis everything is physical.3 Take that universal quantifier and restrict its domain from everything to just the mental. The resulting proposition is the thesis of physicalism about the mental. So, if physicalism simpliciter is the thesis that everything supervenes on the physical, physicalism about the mental is the thesis that the mental supervenes on the physical. If physicalism simpliciter is the thesis that everything is physical or physically realized, physicalism about the mental is the thesis that the mental is physical or physically realized. Call this the straightforward approach to formulating physicalism about the mental.
In the end I will defend a version of the straightforward approach. First though I want to argue that the approach cannot work if we assume physicalism simpliciter is best formulated in terms of some relation at least as strong as supervenience. This assumption is almost universally accepted. There are various critics of supervenience-based formulations of physicalism simpliciter, but these critics almost always contend that the problem is that supervenience is too weak of a relation. Something stronger is needed, like realization, which entails supervenience.4 These critics thus accept our assumption that at least supervenience is needed.
What I am going to argue is that physicalism about the mental could be true even if psychophysical supervenience fails to obtain, and thus even if any psychophysical relation stronger than supervenience fails to obtain. The argument assumes standard mental content externalism, although this assumption can be treated as provisional. Whatever...