Plate 15 from Nova Reperta
by Johannes Stradanus

Descartes's Body-Machine


René Descartes is best known for having introduced a metaphysical dualism, arguing that mind and matter are two distinct and separable substances. As a result, he is often blamed for various ills of the modern world, most notably the alienation of mind from body, individual from world, and thought from action. Descartes developed his metaphysics in response to competing demands that arose from the lack of clear boundaries between theology, philosophy and science. He aimed to displace the entrenched Aristotelian philosophy of the schools with his vision of a new mathematical science that would explain all natural phenomena by means of a few universal laws of nature. Since what we now call "science" formed part of natural philosophy in his period, Descartes's new science required a philosophical justification. Nor were the questions of natural philosophy clearly separated from theology. Thus Descartes also had to ensure that his new natural philosophy preserved the Christian ideals of free will and the afterlife.

Prior to Descartes, Aristotle's metaphysics, and the account of the soul that it entailed, had prevailed in Western Europe. Aristotle (384-322 BC) accounted for the distinct character of each individual thing in terms of its matter and form. Matter is the material out of which a thing is made, e.g., the iron of an axe or the flesh and bones of a person. But matter always has to have a form, an organizing principle that makes it one kind of material rather than another. The iron of an axe has to have the form of iron before it can become the material for the axe, for if it had the form of rubber, it would not be capable of taking on the form of an axe (although it would be suited to become a tire). Flesh and bones likewise have distinct forms. The yielding nature of flesh is due to its form, and this makes it capable of constituting different organs. By contrast the form of bone makes it unyielding and thus suited for the skeleton. In nature, as in art, everything in Aristotle's universe has a distinct structure and purpose. Matter without any shape, size, texture, structure, color, temperature—in short, matter without any form at all—simply could not exist. Therefore, even if we break a thing down to its most basic elements (for Aristotle, earth, air, fire, and water) they will still be a combination of matter and form.

Thus matter, for Aristotle and his followers, is defined relative to form. It is, in effect, the capacity to take on form. At each level of complexity, we find materials with certain forms, making them capable of taking on some additional forms, but not others. Thus flesh and bones can acquire the form of an organism, but iron cannot. Aristotle calls the form that makes a body a living body the soul (psyche). As a form, the soul is nothing more than the actualization of the potentials of the matter it informs, and is inseparable from the mortal body. The form of a living thing is identical to its life functions, which for plants includes nutritive and reproductive functions (the nutritive soul), for animals, the latter plus locomotive and sensitive functions (the sensitive soul), and for humans, in addition to all of the above, the intellective functions (the intellective soul). The intellective soul is the only one that is not dependent on the body for some of its actions.

Christian medieval philosophers, like St. Thomas Aquinas (1224-1274), accepted Aristotle's definition of the soul but sought to reconcile it with the Christian belief in the immorality of the soul. Since personal immortality requires that individual souls (and their memories) be separable from their bodies, adjustments had to be made to Aristotle's conception of the soul to show that it could survive the body.

In constructing a philosophical foundation for his new science, Descartes rejected Aristotle's metaphysics. Instead of defining matter relative to form, he defined matter as a distinct, passive substance, possessing only the geometrical attributes of extension, i.e., size, shape, number, and motion. Certain forms are built into matter, as it were, and matter can only take on variations of these basic forms, i.e., the parts of material substance can take on particular sizes, shapes, numbers and motions (what Descartes calls modes of matter) but not colors, sounds, temperatures, etc. This view accounts well for the relations between inanimate bits of matter, but Descartes was hard pressed to explain how particles, possessing only these modes, could give rise to a living body that not only breathes, digests, and reproduces, but sees colors, hears sounds, remembers, responds emotionally, and engages in abstract thinking. For Aristotelian philosophers, all the latter functions, except pure intellectual thought, are realizations of potentials inherent to the living human body. But Descartes's geometrical conception of matter did not allow for such a complex view of the body.

Descartes addresses this problem in the Meditations on First Philosophy by creating a division between mind and body. For example, in Meditation Six he explains the causes of bodily sensations—such as pain, pleasure, thirst, and hunger—in terms of the motions of material particles and the mechanisms by which these motions are transmitted via the nerves to the brain. But the sensations themselves are located in a separate substance—the thinking thing—which Descartes describes as that which senses, imagines, remembers, doubts, thinks abstractly, and wills freely. By making the mind and body two distinct and separable substances, Descartes was able to redefine matter so that all its properties could be described mathematically while preserving personal immortality and freedom of the will for the human soul. However, it was not clear how these two distinct substances could form one unified human being and interact with each other through the pineal gland in the brain.

The Treatise on Man, published posthumously, is Descartes's main work involving traditional Aristotelian themes. There, Descartes seems to focus more on the relationship between the physical and the mental. He gives, for instance, an account of the physiological processes involved in perception and memory. At the beginning of this work, Descartes invites us to imagine the human body as "an earthen machine" (or an automaton) and implies that though more complex, the movements of this machine can be explained in the same way as the movements of clocks, fountains, or mills. This is a radically new conception of a living body and yet, as John Sutton argues in Philosophy and Memory Traces, when it comes to giving specific explanations, Descartes retains several traditional principles. For example, Descartes's conception of bodily functions in terms of a kind of fluid dynamics is reminiscent of ancient and medieval humoral theories that attributed the health and proper functioning of the body to a balance among four basic bodily fluids (humors). Descartes's animal spirits, like those posited by the ancient physician Galen (129/30-199/200), are concocted from blood and responsible for moving muscles and communicating motions to the seat of perception in the brain. While physical, these animal spirits seem to play the role of intermediary between mind and body.

A diagram, which appears in Part I of the Click Here for a Larger ViewFrench edition of the Treatise on Man, illustrates how the blood is distributed to different parts of the body, thus indicating where the animal spirits are produced. Blood is forced out of the heart (labeled A) through the aperture at B upwards towards C (the cavities of the brain). Descartes invokes his second law of motion, namely, that all moving bodies move rectilinearly if they are not impeded. However, the aperture at B being small, only the liveliest, strongest, and most subtle parts of the blood will proceed to C. The coarser, more sluggish ones will reach D and then descend towards E rather than F or G because E lies in a straight line from D. The blood that reaches the brain serves to nourish and maintain it, and also produces the animal spirits, which Descartes describes as "a certain very subtle wind or rather, a very lively and pure flame." Descartes compares the tissue surrounding the pineal gland to stretched-out tapestries and observes that it has very small veins. Since only the subtlest blood particles can pass through these tiny openings, the brain tissue acts as a sieve, further purifying the blood in order to separate out the most subtle and active parts of matter known as the animal spirits.

A pair of images found in Part V of the French edition represent cross sections of the brain, and Click Here for a Larger Viewdepict the brain tissues and apertures surrounding the pineal gland (labeled H). They serve to illustrate the physiological states of waking and sleeping. In two treatises, On Memory and On Dreams, Aristotle explains the occurrence of memories and dreams in terms of physiological changes in the body. Memories are formed by movements that stamp an impression of the percept on matter of the right degree of moistness (On Memory, lines 450a30-450b1). Dreams are caused by movements that Aristotle compares to little eddies in rivers. They are eclipsed by greater disturbances in the fluids of our bodies while we are awake but are borne down to the seat of sense perception (the heart) when the greater motions subside during our sleep (lines 461a1-14). Descartes compares the brain matter of a person who is awake to the sails of a ship filled out by the wind. Just as the wind causes a tightening of the ropes holding the sail in place, the brain tapestry that is pushed outwards by the animal spirits (here seen as radiating outwards in straight lines from the pineal gland at H) causes the nerve fibers (ending in D) to become tight and taut. This is illustrated in the first diagram where the animal spirits have dilated the part of the brain at A and caused an opening of the pores, thus facilitating the passing of animal spirits into the nerves to D. The second diagram illustrates the brain of a sleeping person, which Descartes compares to the state of a sail on a calm day. Here the nerve fibers are slack and relaxed and dreams are comparable to the occasional breeze filling out part of the sail. Dreams occur during sleep because occasionally some animal spirits push one part of the brain outward (as represented by the dotted lines from H to A) thus causing part of the brain to be responsive to the passage of animal spirits.

While it is clear that Descartes drew on ancient and medieval theories to flesh out the details of his account of bodily processes, his novel conception of the body as a machine that performs basic functions independently of the soul paved the way for more recent attempts to reduce higher cognitive functions to physiological changes in the brain.

Helen Hattab
Southern Illinois University, Carbondale

Suggested Reading

René Descartes, Meditations on First Philosophy, in The Writings of Descartes, trans. John Cottingham, Robert Stoothoff, Dugald Murdoch. Vol. 2. (Cambridge: Cambridge University Press, 1984).

René Descartes, Letters to Elizabeth, The Correspondence, in The Writings of Descartes, Vol.III, trans. John Cottingham, Robert Stoothoff, Dugald Murdoch and Anthony Kenny. Cambridge: Cambridge University Press, 1991.

The World and Other Writings, trans. Stephen Gaukroger. Cambridge: Cambridge University Press, 1992.

Dennis Des Chene, Life's Form and Spirits and Clocks. Ithaca: Cornell University Press, 2000.

Reason, Will and Sensation, edited by John Cottingham. Oxford: Clarendon Press, 1994.

John Sutton, Philosophy and Memory Traces: Descartes to Connectionism. New York: Cambridge University Press, 1998.