•Part of an intended larger work, World.

•Descartes decided not to publish when learning of the church’s punishment of Galileo.

•In order to write about physiology in the text, Descartes made daily visits to butcher shops to learn about the structure and function of animal bodies. He often took home organs for his own dissections.

Animal Spirits and Material Ideas in Descartes’ Treatise of Man / Traité de l'homme

•Descartes wants to show that the body can function to perceive and react to stimuli, and even form a “material ideas” of them, without the intervention of a soul or mind. Thus we will conceive or bodies as being animated, yet soulless, like machines.

•The extending world of bodies and the non-extending world of mind meet in the human nervous system, which is a mechanism that for most of our behaviors acts independently of the mind but is also able to interact with the mind when necessary.

Will Describe a Model of the Human Body

•Opens with: “These men will be composed, as we are, of a soul and a body; and I must first separately describe for you the body; then also separately, the soul; and finally I must show you how these two natures would have to be joined and united to constitute men resembling us”

•Yet he does not get so far as to describe the soul and its union with the body.

Digestion & Blood

Food in the stomach is broken down chemically into smaller and smaller particles. The smallest of these will be the animal spirits.

Animal Spirits are fine particles in the blood. Tiny holes in brain-arteries ensure that only

the spirits pass into the pineal gland. “[Parts of the blood reaching the brain] produce there a certain very subtle wind, or rather a very lively and very pure flame, which is called the “animal spirits”. […] the arteries that bring blood from the heart, having divided into an infinity of little branches and having composed the little tissues that are stretched like tapestries at the bottom of the concavities of the brain reassemble around a certain little gland [the pineal] […] the arteries in this region have many little holes through which the subtlest parts of the blood can flow into this gland, but which are so narrow that they refuse passage to larger particles.” [19-20]All [the smallest and most agitated particles] enter this gland, which must be imagined as a very full-flowing spring, whence they flow simultaneously in every direction into the cavities of the brain. [20-21]

[The spirits exit the brain through conduits.] “And depending on their entering (or their mere tendency to enter) some nerves rather than others, they are able to change the shapes of the muscles into which these nerves are inserted”

Nerve Channels

“Animal spirits […] enter […] the muscle to inflate and shorten and so pull the eye to which it is attached; […] when they withdraw, the muscle disinflates and elongates again.” [25]

“if fire A is near foot B, the particles of this fire have force enough to displace the area of skin that they touch; and thus pulling the little thread cc […], they simultaneously open the entrance to the pore de where this thread terminates in the brain: just as, pulling on one end of a cord, one simultaneously rings a bell which hangs at the opposite end.” [34]

“if fire A is near foot B, the particles of this fire have force enough to displace the area of skin that they touch; and thus pulling the little thread cc […], they simultaneously open the entrance to the pore de where this thread terminates in the brain: just as, pulling on one end of a cord, one simultaneously rings a bell which hangs at the opposite end.” [34]

“among these figures, it is […] only those traced in spirits on the surface of gland H, where the seat of the imagination and common sense is, that should be taken to be ideas, […] to be the forms or images that the rational soul will consider directly when, being united to this machine, it will imagine or will sense any object”. [86]“I wish to include under the designation Idea all impressions that spirits receive in leaving gland H; and these [a] are all to be attributed to the common sense when they depend on the presence of objects, but [b] can also proceed from several other causes […] and should then be attributed to imagination”. [87]

“Very little [force] is required to cause it to incline and to lean, now more now less, now to this side now to that, and so to dispose the spirits that leave and make their way toward certain regions of the brain rather than toward others”. [91]“not only is the center of gland H a slight distance away from the center of the brain, but also the extremities of the arteries that sustain it are curved in such a way that nearly all the spirits that these arteries bring there proceed through region abc of its surface towards the tubules […] in this way opening pores that face in that direction much more than they do other pores”. [91-92]

“spirits leaving the same point b – by tending toward tubes 4, 4, and 8 – simultaneously turn the two eyes and right arm toward object B”. [93]