René Descartes may just be the Thinking Man’s thinking man. More than any other modern philosopher, he is identified with the view that the soul is separate from the body and superior to it—in fact, we refer to this position as Cartesian dualism. The synonymy is so overwhelming, one can imagine him subjected to some hackneyed literary or television treatment wherein he is brought forcibly into the present, only to find success as an advertising executive with his slogan for the Winterman sneaker account that promises “mind over matter.” (For the women’s line: I pink therefore I am.)

Any dualistic theory encounters what is known in philosophy as the mind-body problem: how is it possible for two entirely discrete substances to act in concert and produce what we conceive of as unitary being? Curiously enough, Descartes’ lifelong passion for experimental physiology—which, for him, was just rationalistic epistemology by other means—influenced his answers. He was an avid practitioner of dissection on both human and animal bodies. (Because he believed animals were mindless machines and could not feel pain, he often dissected them while they remained alive.) In his search to discover the differences that distinguish humans and animals from one another as res intelligens and res extensa—that is, intelligent beings and “machines,” respectively—he hit upon the pineal gland, which he found present only in the human brain.

The pineal gland, located near the center of the brain, had been the subject of philosophical and scientific speculation since the days of Galen in Greek Antiquity. In most theories, it plays the part of a regulator, coordinating the flow of “spirits” (vaguely analogous to contemporary “nerves”) between different parts of the brain. Descartes came to the conclusion that pineal gland was the threshold over which pure mind and outward matter were connected. His observations as an anatomist were largely responsible for such a view. In response to questions arising from his 1639 Treatise of Man, he wrote:

“Since it is the only solid part in the whole brain which is single [microscopic research has proved this unity false], it must necessarily be the seat of the common sense, i.e., of thought, and consequently of the soul; for one cannot be separated from the other. The only alternative is to say that the soul is not joined immediately to any solid part of the body, but only to the animal spirits which are in its concavities, and which enter it and leave it continually like the water of river. That would certainly be thought too absurd” (24 December 1640, AT III:264, CSMK 162).

A decade later, in The Passions of the Soul, Descartes further elaborated on how the pineal gland worked:

“the activity of the soul consists entirely in the fact that simply by willing something it brings it about that the little gland to which it is closely joined moves in the manner required to produce the [bodily] effect corresponding to this volition” (AT XI:359, CSM I:343).

A unique hypothesis to be sure, but Descartes never satisfactorily explained, even in his own mind, just how the will caused the pineal gland to move in its particular fashion. His speculations about it were abandoned by his followers and detractors alike. In the 20th C, after several centuries of relative neglect, Yale scholar Aaron Lerner and his colleagues discovered that the pineal gland is the body’s major producer of melatonin, a hormone that is involved in sexual development, sleep rhythms, metabolic functions, and seasonal breeding in animals.

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