Despite his famous claim that mind and body are really distinct substances, Descartes repeatedly points out that a human being is not a mere composite of two entities. For example, he calls the human being “a true ens per se” and asserts that the mind “is united in a real and substantial manner to the body” (AT III 493, CSMK 206). If we intend to give a full account of a human being, we need notions not just of mind and of body but also of the union of mind and body – the latter notion being as primitive as the other two and therefore irreducible to them (AT III 665, 691; CSMK 218, 226). Only this third notion enables us to describe and explain the features that are distinctive of a living human being – namely, sensory perceptions and passions (see primitive notion).

Given this thesis, it is hardly surprising that Descartes does not conceive of himself as a purely immaterial mind once he has completed an analysis of mind and body. Nor does he claim that his body is only accidentally connected to his mind, serving as a mere instrument. In the Sixth Meditation, he unmistakably holds that his mind is not in his body as a sailor is present in a ship. It is rather “very closely joined” to the body and, as it were, “intermingled with it” (AT VII 81, CSM II 56). Should there be no intermingling, no sensations of hunger, thirst, and pain could arise, because it is neither the mind alone nor the body alone that has these states, but the union of mind and body.