Over on the philosopher's magazine blog, an interesting post appeared concerning Descartes Vegetarianism?! According to Bloodless Revolution, a history of vegetarianism by Tristram Stuart, Descartes was a vegetarian. This is very surprising because Descartes is famous for his view of animals as mere machines.
From the Internet Encyclopedia of Philosophy:
One of the clearest and most forceful denials of animal consciousness is developed by Rene Descartes (1596-1650), who argues that animals are automata that might act as if they are conscious, but really are not so (Regan and Singer, 1989: 13-19). Writing during the time when a mechanistic view of the natural world was replacing the Aristotelian conception, Descartes believed that all of animal behavior could be explained in purely mechanistic terms, and that no reference to conscious episodes was required for such an explanation. Relying on the principle of parsimony in scientific explanation (commonly referred to as Occam's Razor) Descartes preferred to explain animal behavior by relying on the simplest possible explanation of their behavior. Since it is possible to explain animal behavior without reference to inner episodes of awareness, doing so is simpler than relying on the assumption that animals are conscious, and is therefore the preferred explanation.
To be fair, Descartes was a vegetarian for mostly health, not ethical, reasons. However, Stuart suggests that Descartes was disturbed by the problems surrounding sentience. If animals did feel pain, then it would be wrong to kill them for food (see Singer: Animal Liberation 1975).
An easy way around this puzzle is to simply assume that animals are not conscious, feeling no pain. Moreover, this is the simplest argument. However, as Todd pointed out in his post, the simplest argument is not always the right argument. Animal consciousness is a hotly debated topic in animal ethics (see the Stanford Encyclopedia of Ethics entry on Animal Consciousness for a primer). Ockham's razor, much like faith, shouldn't be used as a cognitive default to prevent us from exploring the complexity of the world around us.