In Descartes’s view, the mind was made out of an ethereal substance, a fluid, that was stored in a receptacle in the brain. He called the fluid res cogitans. Mental substance. When he dissected the brain looking for the receptacle of the soul, he noticed that almost every brain structure came in pairs, one on each side. In his view, the human soul was a single, unified entity, and therefore it could not possibly be divided up and stored in two places.
