  In the Treatise on Man and Passions, Descartes described purely mechanical processes in the sense organs, brain, and muscles, that were to account for the functions of the sensitive soul. These processes involved “animal spirits,” or subtle matter, as distilled out of the blood at the base of the brain and distributed down the nerves to cause muscle motions in accordance with brain structures and current sensory stimulation. The brain structures that mediate behavior may be innate or acquired. Descartes ascribed some things that animals do to instinct; other aspects of their behavior he explained through a kind of mechanistic associative memory. He held that human physiology is similar to nonhuman animal physiology, as regards both vegetative and (some) sensitive functions—those sensitive functions that do not involve consciousness or intelligence: