Nature also teaches me, through these sensations of pain, hunger, thirst and so on, that I (a thinking thing) am not merely in my body as a sailor is in a ship. Rather, I am closely joined to it—intermingled with it, so to speak—so that it and I form a unit. If this were not so, I wouldn’t feel pain when the body was hurt but would perceive the damage in an intellectual way, like a sailor seeing that his ship needs repairs. And when the body needed food or drink I would intellectually understand this fact instead of (as I do) having confused sensations of hunger and thirst. These sensations are confused mental events that arise from the union—the intermingling, as it were—of the mind with the body.This related passage is curious too:
Again, why should that curious tugging in the stomach that I call ‘hunger’ tell me that I should eat, or a dryness of the throat tell me to drink, and so on? I couldn’t explain any of this, except to say that nature taught me so. For there is no connection (or none that I understand) between the tugging sensation and the decision to eat, or between the sensation of something causing pain and the mental distress that arises from it. It seems that nature taught me to make these judgments about objects of the senses, for I was making them before I had any arguments to support them.The words "(or none that I understand)" are surely crucial here, since there is a connection between hunger and wanting to eat, and another "between the sensation of something causing pain and the mental distress that arises from it" (although I'm really not sure how there can be two things in this case). It seems to me that there is a continuum (perhaps even a discontinuous one, if that makes any sense), from symptoms that one must learn are removable by, say, drinking water to painful knowledge of thirst. So I might cough a little, or feel vaguely like coughing, and diagnose that I should go to the water fountain. A baby would have no clue what was wrong, and perhaps little sense that anything was wrong, if it felt the same sensation in its throat. But at the extreme of thirst the knowledge that one needs water and the feeling of discomfort are, I imagine, indistinguishable. (I think Schopenhauer is good on this sort of thing, the dry throat as thirst embodied. And thirst is for water, so the concept of water is, so to speak, embodied in the throat and tongue.) So we can perceive something like mild thirst or pre-thirst in an intellectual, sailor-in-a-ship kind of way (which I suppose is what enables us to distinguish between the intellectual and the sensory or bodily), but we also sometimes experience "mental events" that are confusions of the intellectual and the bodily.
In the Third Meditation, Descartes says that "nature taught me to believe" means "I have a strong impulse to believe," but if God is no deceiver then, by Descartes's lights, these impulses should be true. So I am an intermingled unit of body and mind. We can conceptually distinguish the bodily from the intellectual, and so God could keep the mind going after the death of the body, but this mind would have no sensations (because sensations are, effectively, proof of the intermingling of mind and body). Nor would it be me in the full sense.
Gordon Baker and Katherine Morris (p. 153) say that Descartes means to say that the connection between the "tugging sensation" and the decision to eat is inexplicable, incapable of being understood, because it involves an apparently causal connection between something physical and something mental. But it only appears impossible when viewed from the sailor's perspective, it seems to me. And I am not a sailor.