Interaction Between The Two is Impossible
Nor can we entirely pass over the difficulty of the relations between mind and matter, even though we have been warned in advance that the theory does not undertake to solve this problem.
For it does, as commonly held, make certain statements about their relations. It holds that mind can know matter, that it can move matter by an act of will, and that it is somehow connected with a piece of matter known as the body of that particular mind; also that matter by its motions can produce certain effects in mind, for instance, pleasure and pain, derangement and death.
These are merely examples; it matters little what examples we choose.
But is it really so easy to conceive how two things, defined in the way in which we have defined matter and mind, can act on each other ?
Matter can only operate in one way, namely, by moving; and all motion in matter is caused either by impact or by attraction or repulsion; influences exerted in either case by another piece of matter. If therefore mind influences matter, that is to say, moves it, it can only do so by impinging on it or attracting it. But we do not associate these powers with mind as ordinarily conceived.
They can (we should say) only belong to a thing which is spacial, possesses mass, and is capable of motion. Therefore mind cannot affect matter in any way in which matter can be affected, unless mind has properties characteristic of matter itself. That is to say, only matter can affect matter : mind can only affect matter if mind is itself material.
Can matter then influence mind ? clearly not; for its influence consists in causing motion, and this it can only do in something capable of motion, something spacial; that is, in matter. The two halves of the universe go each its own way, each alike uninfluenced by the other. Mind cannot, by an act of will, move a piece of matter as I imagine that I am moving my pen; and no change in the position of a material body can disturb, still less annihilate, the activity of a mind.
The difficulty is not merely that the dualistic theory omits to explain how these things happen, or that it offers an unsatisfactory account of them; it definitely implies that they cannot happen at all.