Descartes argues that many powers which are accompanied by thinking
in humans are in animals purely mechanical. When a dog welcomes its owner, or
a horse finds its way home, no thinking occurs. These actions are due entirely
to the motions of the body, of which neither animal has any consciousness. Only
humans, then, are capable of thinking, and only humans possess souls."
Susan James, Passion and Action, 1997, Oxford, Clarendon, pp.87,8.