Descartes' Dualism: Mind and Body as Distinct Substances
The mind-body problem — the question of how mental states (thoughts, feelings, experiences, intentions) relate to physical states of the brain and body — is one of the oldest and most persistently contested problems in philosophy. René Descartes (1596–1650) gave the problem its canonical modern formulation in his Meditations on First Philosophy (1641), and his solution — substance dualism — defined the terms of the debate for centuries.
Descartes argued through methodological doubt that the one thing he could not doubt was that he was thinking. 'Cogito ergo sum' — 'I think, therefore I am.' This establishes the existence of a thinking thing (res cogitans — 'thinking substance') whose essence is thought. Descartes then argued that he could clearly and distinctly conceive of his mind existing without his body — he could imagine being purely thinking without any bodily extension — and that what can be clearly and distinctly conceived as separate must in fact be distinct substances (by God's power). Physical matter (res extensa — 'extended substance') has spatial extension, location, divisibility, and obeys mechanical laws. The mind has none of these properties — it is unextended, unlocatable, and indivisible.
This is substance dualism: the view that mind and body are two ontologically distinct kinds of substance. Descartes held that they interact — mental states cause physical events (when I decide to raise my arm, my mental decision causes the physical arm movement) and physical events cause mental states (pain in my finger causes the mental experience of pain). He tentatively proposed that this interaction occurs in the pineal gland (the only unpaired brain structure, he noted — suggesting it might be a special meeting point of the two substances).
Substance dualism has intuitive appeal — the first-person experience of consciousness, the feeling of freely choosing, the seeming irreducibility of one's inner life to mechanical physical processes — all seem to support the view that mind is something over and above physical matter. Descartes articulated and gave philosophical rigor to the deep intuition that there is something it is like to be a mind that cannot be captured in purely physical, third-person terms.