One of Descartes’ arguments for dualism relies on the claim that whereas mind is by its nature indivisible, body is by its nature divisible (AT VII 86, CSM II 59). It is tempting to see this argument as affirming the simplicity of the human mind, but Descartes never calls the mind “simple.” In this argument, he writes that “the whole mind seems to be united to the whole body” and that when a part of the body is removed, no part of the mind is removed. These claims imply the view he states explicitly elsewhere, namely, that the mind is whole in the whole body and whole in its parts (see holenmerism). He also denies that the faculties of the mind are parts of it; this claim is intended to deny a common Scholastic view, that the faculties of the soul are really distinct from it (see distinction [real, modal, and rational]). Descartes’ statement of the indivisibility of the soul is not meant to deny all complexity in the soul: the soul does have modes that change over time. But he means to deny that there are any really distinct entities within the soul (Rozemond 2010).

It is natural to expect that Descartes uses this difference between mind and body as his ground for saying that the mind, unlike the body, is immortal, since this is a common line in the history of philosophy. But when he sketches an argument for immortality in the Synopsis to the Meditations, he does not argue this way (Rozemond 2010, Fowler 1999) (see soul, immortality of the). Instead, he argues that both mind and body are immortal in virtue of their status as substances:

So body is divisible, but it is not corruptible. For Descartes, something is divisible if it can be divided into parts that are of the same kind as each other and the whole.