Mind and body: the great divide

Descartes’ metaphysics divides all created reality into exactly two substances. A substance, roughly, is a thing that can exist in its own right, and each kind has one defining principal attribute — an essence that all its other properties are modes of. For Descartes there are two such essences, and everything created is one or the other.

Mind — res cogitans (thinking thing). Its whole essence is thought. It is unextended (takes up no space), and therefore indivisible — you cannot cut a mind in half. Its modes are ways of thinking: doubting, willing, imagining, sensing, understanding.

Body — res extensa (extended thing). Its whole essence is extension in length, breadth, and depth. It is unthinking and, being extended, divisible without limit — any body can in principle be cut. Its modes are ways of being extended: shape, size, position, motion. Note what’s absent from the essence of body: no colour, taste, or warmth as the senses present them, and crucially no thought.

The real-distinction argument, Meditation VI. How does Descartes prove mind and body are two substances and not one? The argument (his own summary) runs like this:

1. Whatever I can clearly and distinctly conceive as existing apart, God can create as existing apart (God’s power extends to anything non-contradictory).
2. Therefore, if I can clearly and distinctly conceive A without B, then A and B are really distinct — capable of separate existence.
3. I can clearly and distinctly conceive myself as a thinking, unextended thing, complete without any body; and I can conceive body as an extended, unthinking thing, complete without any mind.
4. Therefore mind and body are really distinct substances, and either could exist without the other.

The interaction problem. Here’s the crack in the edifice. However distinct in essence, mind and body plainly act on each other constantly: I decide to raise my arm and it rises; a pin pierces my skin and I feel pain; wine clouds my judgment. Mental events cause physical ones and physical events cause mental ones, all day long. But if mind is utterly unextended and body is utterly extended, how can they causally interact at all?

This was pressed with unusual force by Princess Elisabeth of Bohemia in her 1643 correspondence with Descartes. She grants his dualism but asks the obvious, deadly question: how can a purely thinking, unextended substance determine the animal spirits to move the body, when causing motion seems to require extension and contact? She notes it would be easier to grant the soul extension than to understand how something unextended could move matter.

Descartes’ unsatisfying reply. He gestures at two things. Anatomically, he locates the point of interaction at the pineal gland — a small, single (unpaired) organ in the brain where, he speculates, the soul exercises its influence on the animal spirits and vice versa. Philosophically, he appeals in The Passions of the Soul and to Elisabeth to a primitive notion of the union of mind and body — insisting the union is a basic fact grasped by ordinary living, not by pure intellect. But naming the place of interaction doesn’t explain the possibility of it, and calling it “primitive” concedes he cannot analyse it. The problem stays open.

The legacy. The interaction problem is not a footnote — it is the engine of the rest of the course. Because Descartes couldn’t explain two substances interacting, his successors re-engineered the metaphysics to dodge the question. Spinoza collapses mind and body into one substance (God-or-Nature) seen under two attributes, so there’s no gap to bridge. Leibniz denies any real interaction at all, replacing it with a pre-established harmony of windowless monads. Later, the occasionalists (Malebranche) make God the only true cause on every occasion. The dead end of Cartesian interaction is exactly what launches the great rationalist systems that follow.