Descartes thought about memory in the distinct contexts of method, metaphysics, medicine, mortality, and morals. Keenly aware of the fallibility and instability of natural corporeal memory, he considered various ways to bypass it or avoid relying on it, but he also came to see its importance in understanding and dealing with the passions and the union of mind and body (see human being). His account of memory influenced Malebranche and associationist traditions but was subject to sharp attack from critics who saw it as dangerously materialist and chaotic.

In the Rules, Descartes worries that the intellect must rely on memory to connect the steps in a reasoning process. But “since memory is weak and unstable, it must be refreshed and strengthened” by a continuous and repeated “movement of thought,” until the thinker can intuit the whole series of operations, passing over them all “so quickly that memory is left with practically no role to play” (AT X 409, CSM I 38) (see deduction). Descartes was not attracted to artificial memory techniques, turning instead to anatomy and physiology to “explain what imagination, memory, etc. consist in” (AT I 263, CSMK 40). In the Treatise on Man, Descartes writes that “the effect of memory that seems to me to be most worthy of consideration here is that, without there being any soul present in this machine, it can naturally be disposed to imitate all the movements” of real humans (AT XI 15, G 157). So there is genuine memory in soul-less bodies: Descartes is consistent in attributing memory to animals (AT IV 310, CSMK 270).

Corporeal ideas are figures traced by animal spirits on the surface of the pineal gland. These occurrent impressions then leave traces “on the internal part of the brain … which is the seat of memory” (AT XI 176–77, CSM I 106; H 86–87; G 149–50). Flowing through the gaps between the tiny fibers of the brain, the spirits enlarge or alter these pores, continually bending and rearranging the fibers and tubes. When the movement of the spirits is stronger, long-lasting, or often repeated, these patterns “are retained there in such a way that by means of them the ideas that existed previously on this gland can be formed again long afterward, without requiring the presence of the objects to which they correspond.