Descartes couldn't figure out how humans weren't automata either. That's where his explanations start getting supernatural, and not particularly insightful. Any kook can babble on about an invisible pineal gland soul-thread.
edit: I think the interesting thing for people is how insight like Descartes' becomes useless when he tries to distinguish us from automata.
So from that view Descartes argument holds, but the conclusion that such a life has no meaning does not.