"[S]o long as the soul exists, it is always thinking: The question therefore
arises: what counts as thinking? It would not have been inconceivable for Descartes
to classify digesting, for example, as a kind of thought. But in fact he takes
the view that our thoughts are limited to states of which we are conscious.
Although the processes involved in nutrition and reproduction often go on without
our being aware of them, we cannot, so Descartes claims, sense, perceive, imagine,
remember, doubt, will, understand, or feel passions without being conscious
that we are doing so. These, then, are kinds of thought. And only creatures
that think in these ways have souls."
Susan James, Passion and Action, Oxford, 1997, Clarendon, pp.87,8.