Dualism

Descartes's case for dualism runs through his method of doubt: he can coherently doubt that his body exists, and even that any physical world exists, but he cannot doubt that he is thinking, since the doubting itself is an act of thought. From this he concluded that mind and body have different essential properties, thought without extension for the one, extension without thought for the other, and things with different essential properties cannot be identical. He located the two in causal contact at the pineal gland, a proposal almost no one since has defended, but the underlying claim, that consciousness cannot be fully described in the vocabulary of physics, has outlived the mechanism he proposed for it.

Plato had already argued a version of the same conclusion on different grounds centuries earlier, holding that the soul, being simple and akin to the eternal Forms, could not be destroyed the way a composite physical body is. Both versions face the same demand from their critics: if mind and body are genuinely distinct substances, how do they interact at all, and why does damage to the brain reliably damage the mind if the mind is not, in the end, the brain. Dualism's sharpest modern critic argued the whole picture rests on a mistake of category rather than of fact: Gilbert Ryle's 1949 The Concept of Mind dismissed the Cartesian mind as a ghost in the machine, arguing that describing thought as a separate inner substance confuses the grammar of mental language with the existence of a hidden mental object.

Outside professional philosophy, dualism is closer to the default position than the exception, because it underwrites the idea that a person survives the death of the body. Christian, Islamic, and Jewish accounts of an immortal soul, Hindu and Buddhist accounts of a subtle body that carries karma between lives, and countless folk traditions of ghosts and possession all assume some version of a mind or self that is not simply identical to its physical organism.

The philosophical criticism, that Descartes never explained how an unextended substance could push against an extended one, remains the position's most durable weak point, and it is precisely the problem that interactionism, occasionalism, and epiphenomenalism each try to solve in different ways. Physicalists regard the whole substance-dualist picture as an artifact of pre-scientific thinking about the brain, resolved once neuroscience is taken seriously; property dualists try to keep something of Descartes's insight, that consciousness is not obviously physical, without committing to two kinds of stuff. The disagreement is not merely academic within the atlas either, since it is the fault line running under every doctrine of an immortal soul recorded here.