Res cogitans

Descartes famously doubts those attributes of his soul that are body-dependent (nutrition, movement, and sense-perception). But notice the different kinds of dependence: Nutrition and movement are mere ‘fabrications’ if body does not exist, whereas sense-perception does not occur without a body, and so (presumably), though it need not include body within what it perceives, it must be illusory if the bodily state it would occur in does not in fact obtain.

If that is so, it does not really matter what the object of sense-perception is to the Cartesian skeptic. All that is required to support his (negative) worry is that the bodily state required for the sensory state fail to be realized.

If that is so, then doesn’t the dream argument become superfluous? Descartes even suggests as much, mentioning it as an aside:

“Sense-perception? This surely does not occur without a body, and besides, when asleep I have appeared to perceive through the senses many things which I afterwards realized I did not perceive through the senses at all.”

But more importantly, I think, it makes the argument much more transcendental than I realized. I always think of the Cartesian skeptic as preoccupied with the objects of thought—I think I see a tomato, but that’s not really a tomato, because this sensory information has a faulty object. But Descartes seems to be arguing about the conditions for the possibility of a sensory experience (surprising since Kant usually gets the credit for that), and—very surprising—the conditions for that possibility seem to be this or that bodily state.

That carries through to the res cogitans notion: “I am, I exist—that is certain. But for how long? For as long as I am thinking.” Whereas the cogito itself is an objective account (it answers the question, what is the object of thought, and what makes it true) the res cogitans argument is a transcendental account (it answers the question, what are the conditions for the possibility of a thinking experience).