What Is Physicalism?

Crudely, physicalism is the view that there isn't anything other than physical objects in the physical world. The standard contrast is with Cartesian dualism: the physicalist denies that there are souls, ideas, minds in some way that makes them out to be something radically distinct from physical things. Of course, mind isn't the only possibility.

Typically, those who have argued against physicalism have equated physicalism with reduction and argued against reduction, though not so much any more, because of the work of Hellman, Thompson, and also Kim. Some, but by no means all, physicalists have also equated physicalism with reduction, but other physicalists have thought that reduction is implausible. Since the connection between physicalism and reduction seems "obvious" to everyone on first impression, it is an important discovery that reduction does not imply physicalism and that physicalism does not require reduction. That is the point of the Hellman Thompson article. We'll work it out for the physical-mental case, but parallel points apply everywhere.

Strong reduction of the mental to the physical would require that everywhere mental predicate and mental object be extensionally definable in physical terms, and that, using the definitions, every mental law be a consequence of physical laws.

In fact, that doesn't do at all what we thought we wanted. As a trivial example, it doesn't even guarantee that there aren't any mental objects that are not physical: Suppose we have souls, and that our soul is located in the pineal gland. Then we can define the soul in purely physical terms as the thing that is in the pineal gland that isn't physical.

So, we need a replacement. Here is Hellman and Thompson's proposal: every object is an assemblage of positive physical objects. That is, every object is extensionally equivalent to some assemblage of electrons, protons, electric charges, gamma rays, whatever. If a property is thought of extensionally as the set of objects of which it holds, every property is a set of assemblages of positive physical objects. What I am describing is "the principle of physical exhaustion." Note that the principle makes no claims about definability or specifiability in physical terms: the assemblages and collections that count, for all the principle says, may be really random with respect to physical theory. Thus, the principle is unrelated to the idea of reduction.

Exhaustion doesn't do everything Hellman and Thompson want from physicalism: they also want that the physical world is closed: to understand physical things only requires knowledge of physical laws. That will mean, if there only are physical things, that everything that happens can, in some sense, be accounted for using the physical laws. The reductionist wants to capture that by requiring that any psychological laws follow from physical laws. However, if the mental things are physical things that don't have any natural characterization in physical terms, there is zero chance that that can happen. So, we need a different characterization of "determination." We can put the proposed characterization in Lewisian terms, though they don't: The mental is determined by the physical if every possible pair of states of affairs that are physical duplicates are also mental duplicates. The contrapositive: there can be no mental difference without a physical difference. The term of art, following Kim, is that the mental supervenes on the physical.

The Hellman-Thompson characterization of physicalism has nothing to do with reduction. In addition, even when a reduction is available, physicalism may not follow. First, as noted above, there can be nonphysical objects in the presence of reduction. Second, even when a reduction is available (at least in a weak sense) determination may fail. Suppose the mental is epiphenomenal: the mental obeys its laws, the physical obeys its laws, but it just happens to work out that they parallel each other. It might still be possible (in some alternate world) that the mental needn't parallel the physical. In that case, we would have reduction without determination.

Thus, physicalism and reduction are not even related!

-- ShaughanLavine - 05 Mar 2008

Since physicalism and reduction are not even related, it does not follow from physicalism that mental laws are reducible to physical laws, and, should it turn out that mental laws are reducible to physical laws, that does not show that physicalism is true. It seems entirely likely that some mental laws will, as it turns out, be reducible to physical laws, and others won't. In our present state of ignorance, the issue is largely debated by philosphers in the absence of clear evidence one way or the other.

To make our rather abstract discussion more comprehensible, I'll give a few examples on each side.

First, a likely case for reduction of the mental to the physical: spatial rotation.

Second, two arguments that we shouldn't expect any such reduction:

Multiple realizability, functionalism, and anomalous monism all cast doubt on the possibility of reducing the mental to the physical, but all are perfectly compatible with physicalism. That makes the point in pretty clear terms. It doesn't help much in figuring out what we should take the main theses of physicalism to be.

-- ShaughanLavine - 07 Mar 2008