If the soul is distinct from the body, how can we explain the fact that there's an ongoing relationship between physiological and psychological life? How does the physical affect the moral, and vice versa? Numerous answers to these questions have been proposed – some metaphysical, others more physiological. Let's study these in turn.

Cudworth, for his part, imagined that there exists between the soul and the body a special substance called the plastic mediator – half body and half spirit. But clearly this theory does nothing but push back the difficulty.

Descartes sought to explain not the relationship between the soul and the body but rather that between the soul and those animal spirits that make the body move. He understood the relationship between thinking and extended substances to be an irreducible fact while at the same time believing that the abyss between them couldn't be bridged.

Malebranche, in his theory of occasional causes, did try to explain the relationship between these two utterly heterogeneous substances. Believing that individual beings are incapable of acting on their own impetus, Malebranche inferred that their movement must come from elsewhere – from God, who alone has real causal power. Indeed, Malebranche says, it would be impious to attribute this divine power to individuals. Human beings and things don't act but are always acted upon. Everything they do is willed by God. So it's in God, so to speak, that extension and thought are brought together.