The notion “animal spirits” derives from Hellenistic Greek medical theory. Physicians in the school of Alexandria circa 275 B.C.E. postulated a theory that, especially in the version of Claudius Galen (130–216 C.E.), endured into the eighteenth century. Three systems of tubular vessels – the veins, the arteries, and the nerves – were understood as containing, respectively, blood, vital spirit (pneuma zootikon, absorbed from the atmosphere), and psychic spirit (pneuma psychikon). The last, known in Latin as spiritus animalis, was thought to be produced in the brain by filtration or distillation from vital spirit and then distributed throughout the body by the nerves (Smith et al. 2012).

The notion of spirits played a role in Descartes’ philosophy and science from the earliest stages of his investigations into living bodies (ca. 1630–32; see AT XI 505–38). In a 1643 letter, he in fact discusses three kinds of spirits: natural, vital, and animal, differentiated by size and activity (AT III 685–89). But only the animal spirits played a central, psychophysiological role for Descartes. In the Treatise on Man (unpublished, ca. 1633), he explained that animal spirits are gradually filtered from the blood as it leaves the heart. The finest, most subtle and active particles ascend to the brain, in particular to its central chamber (the concavities or ventricles, where the pineal gland is located). From there the spirits can flow into the pores of the brain and into the nerves. He conceives of the nerves as tubes with fibers running down their center and filled with animal spirits. In his famous account of a human being whose naked foot is close to a fire (AT XI 141–42, CSM I 101–2), the heat sets off a motion in the central fibers that is conducted to the periphery of the brain chamber, where the nerve tubes have their orifices; that motion sets a pressure or flow through the ventricular spirits that move the pineal gland; in response, the gland produces countermotions that cause other tube orifices to open or close and thus to admit more or less spirits; this differential motion of spirits is conveyed to the appropriate muscles, which lead to visible motions of the animal (such as moving the foot away from the fire) (Figure 3).