"Descartes … 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. ... [S]ince only humans possess souls, only humans are immortal"
Susan James, Passion and Action, Oxford, 1997, Clarendon, p.87,8.