Substance dualism takes there to be two fundamental sorts of thing in the universe - physical stuff and minds. (A substance for philosophers is not some icky, gooey stuff but a technical term for whatever counts as a real, genuine thing.) Materialists just accept the physical, so they have no problem of how the two interact. [Similarly,
Dualists often do not give a mechanism to explain this interaction. They usually just take it to happen, treating it as a mystery. There are mysteries in the universe, and our ignorance of the mechanism doesn't make it false that there is one. This isn't super-satisfactory, but the objection isn't devastating. After all, materialists don't have a similar, well-developed explanation about how physical matter leads to thinking. The real problem involves a principle of physics - the law of conservation of matter and energy. This law states that matter and energy can interact and be converted into each other, but the total of all of it doesn't change. If minds can cause things in the physical realm, and vice-versa, then the physical events leading up to a mental event somehow must get the mental event to happen. Does that expend energy? If so, then the energy is somehow transferred into the mental realm. Something similar would happen for the other way around. Doesn't that violate the law of conservation?
The dualist has a fairly easy reply, one overlooked in most of the literature. Not too long ago, there were two conservation laws. Physicists thought the realm of matter and the realm of energy were constant and separate. They believed in no interaction between the two, no conversion from one to the other. They were wrong. Perhaps the objector is making the same mistake. Perhaps we need to be willing to consider the possibility that the correct law of conservation includes mental stuff in it along with matter and energy. If so, then the objection isn't anywhere near as powerful, though materialists still might not be satisfied by this response. There is textual evidence that such a reply is very much in the spirit of Descartes.
The next post will look at a different kind of dualist view that responds to this criticism in a very different way.