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.