The Mind-Body Problem

Descartes divided the world in two, and did it so effectively that we are still using his categories to argue against him. There is extended substance, which occupies space and works by mechanism, and there is thinking substance, which does not occupy space at all. A headache has no shape. A brain has no opinions. The division matches how the two things feel from the inside, which is a large part of why it has been so durable.

It is also more subtle than the caricature. In the Sixth Meditation he explicitly denies that the mind is in the body as a pilot is in a ship. A pilot notices damage to the vessel by inspecting it; you do not notice a broken leg by inspection, you feel it, immediately and from the inside. Descartes insists the union is far more intimate than mere occupancy. That makes his position more interesting and his central difficulty considerably worse.

The difficulty was put to him in 1643 by Princess Elisabeth of Bohemia, in what is the best short piece of philosophy in the entire correspondence and is usually reduced to a footnote about a clever princess. She grants his whole system and asks one question. On your own account, physical causation works by contact and impulse between things that have surfaces. A mind has no surface. So what does it push with, and what does it push?

He never answers. The replies get progressively weaker across the exchange: first an appeal to a primitive notion we all possess, then a suggestion that the union is best understood by not thinking about it too hard, then an invitation to set the question aside and attend to something else. It is a remarkable document, because you can watch a first-rate mind decline a question it cannot meet.

The interaction problem is what drives everything that follows. Physicalism, the default position in both philosophy and neuroscience, dissolves it at a stroke: if a decision is a physical event, there is no mystery about how it moves an arm, because it is the same kind of thing as the arm. The bill arrives from the other direction. Having identified mental states with physical states, you owe an account of why any physical state should feel like anything at all, and the rest of this book is about that debt.

Before leaving Descartes it is worth noticing where his division is still doing work, because it is not in seminar rooms. Consider a drug that alters mood, motivation or personality. Whether it treats a disease or changes a person is a question that does not arise for a treatment for a fracture, and the difference is not a medical one. It is the residue of the Cartesian split, being applied to somebody's actual life by people who would not describe themselves as holding a metaphysical position at all. That is a reliable test of whether a philosophical problem is live: look for the places where practitioners have to decide it and cannot discuss it.

One methodological note, since it recurs throughout. Research regularly reports the neural correlate of some mental state, and the word correlate is doing careful work. Identity, causation in either direction and a common cause are all consistent with the same finding, and the experiment usually does not discriminate between them. That is not a complaint about the science, which is frequently excellent. It is a note about what a result can carry, and the qualifier is almost always present in the paper and gone by the time it reaches a headline.