"Descartes, who departs radically from the view that the possession of a soul is what makes a thing alive. Whilst Aristotle and his successors had held that any animate thing capable of nutrition and reproduction must have a soul of sorts, Descartes opts for a narrower definition, arguing that the division between creatures with and without souls lies along the line between those that can and cannot think. Thinking, according to this view, is the essence of the soul."

Susan James, Passion and Action, Oxford, 1997, Clarendon, pp.87.