The Problem Isn't a Mental Substance, but a Physical Substance?

The idea of substance is an intelligible idea, right? All the talk of causality is ultimately in the realm of ideas. He's trying to resolve the parent causal problem between a notion of a mental substance and a physical substance. The two things can't interact, and yet somehow they do. We have to explain how it is that the mind can make the arm, you know, move, or how the physical sensations can generate the ideas. So i think that's one of the sorere one of the observations that he's working with.