Mental properties and union dualism

Suppose union dualism is true (I am a composite consisting of soul and body) and I am looking at a red cube. Then I am consciously aware of a red cube in virtue of my soul having some property instance M. Now, this property instance M makes me have a perception of a red cube, but it doesn’t make my soul have a perception as a red cube, because otherwise two things have that perception, and we have the too many thinkers problem. Thus considered as a property of my soul, we shouldn’t describe M as “the property (instance) of perceiving a red cube”. Instead, it is the soul’s property (instance) of “being an x such that x’s possessor perceives a red cube”.

Is M also a property (instance) of me, or just of my soul? Indeed, can the same thing be a property (instance) of two things? Maybe. It sure seems that a watermelon is partly red because its flesh is red, and yet we shouldn’t multiply instances or tropes of red, so there is a single instance of red—which makes the part red, thereby making the whole partly red. Thus, the redness can be thought of as both a property of the flesh and of the watermelon. It is an immediate property of the flesh and a mediate property of the watermelon. As a property of the flesh, it is the property of being red. As a property of the watermelon, it is the property of being partly red. Perhaps, though, this is a merely semantic question. We might say that “property possession” is immediate property possession or that it is mediate or immediate property possession.

In any case, insofar as M is a property of my soul, there is a sense in which it is a mental property and a sense in which it is not. As a property of the soul, it is not a property of thinking, perceiving, believing or of any other of the usually named mental activities, because the soul engages in none of these, since these are activities of me, not of my soul. But at the same time, it is a property that makes me perceive, and hence can be considered mental in that sense. Perhaps we might want to say this: M is a proto-mental property, and to have a mental property is to have a soul that has the requisite proto-mental property.

In any case, I think the above considerations show that arguing for Cartesian dualism—the view that I am a soul—on the grounds that the soul has mental properties and yet there are not two thinkers is too quick (Merricks argues like that here). In one sense, the soul has mental properties, but these properties do not turn the soul, only its possessor, into a thinker. In another sense, the soul has only proto-mental properties.

Maybe it’s a bit like this. Water isn’t moist, but it makes other things moist. Water does this by having some property—a property the physical chemist will have more to say about—in virtue of which anything that has the right association with the water is moist.