I remember reading that Descartes considered animals as nothing more than automata incapable of experiencing pain because they do not possess "souls" (define that!). Viewing this favorably you could say he was an intellectual living in a rarefied world of his own, simply a product of his age.
Less intelligent people of his time, however, liked, say, dogs and understood that if you kicked them and they howled and ran away then they were experiencing pain. Was there something the matter with Descartes and his view of animals if he couldn't make this simple connection, so clearly cognate with the human experience of pain? I know Hume had problems with causation but surely not in such a painfully obvious empirical manner!