Descartes’s physiology grew and developed as an integral part of his philosophy. Although grounded at fundamental points in transmitted anatomical knowledge and actually performed dissection procedures, it sprang up largely independently of prior physiological developments and depended instead on the articulation of the Cartesian dualist ontology, was entangled with the vagaries of metaphysical theory, and deliberately put into practice Descartes’s precepts on scientific method. Chronologically, too, his physiology grew with his philosophy. Important ideas on animal function occur briefly in the Regulae (1628), form a significant part of the argument in the Discoursde le méthode (1637), and lie behind certain parts of the Principia philosophiae (1644) and all of the Passions de l’âme (1649). Throughout his active philosophical life, physiology formed one of Descartes’s most central and, sometimes, most plaguing concerns.
Descartes hinted at the most fundamental conceptions of his physiology relatively early in his philosophical development. Already in the twelfth regula, he suggested (without, however, elaborating either more rigorously or more fully) that all animal and subrational human movements are controlled solely by unconscious mechanisms. Just as the quill of a pen moves in a physically necessary pattern determined by the motion of the tip, so too do “all the motions of the animals come about” thus one can also explain “how in ourselves all those operations occur which [we] perform without any aid from the reason.” Closely associated in the Regulae with this notion of animal automatism was Descartes’s belief that human sensation is a two-step process consisting, first, of the mechanical conveyance of physical stimuli from the external organs of sense to a common sensorium located somewhere in the body and, second, of the internal perception of these mechanically conveyed stimuli by a higher “spiritual” principle.
Implicit in these two notions and seeming to tie them together is the assumption, evident in broader compass but in as terse a formulation as elsewhere in the Regulae, that all phenomena of the animate and inanimate world, with the sole exception of those directly connected with human will and consciousness, are to be explained in terms of mathematics, matter, configuration, and motion.
The fuller working out of his physiological ideas occupied Descartes in the early 1630’s, when he was concerned generally with the development of his ontological and methodological views. In 1632 he several times referred to physiological themes and projects in his correspondence, and in June he informed Mersenne that he had already completed his work on inanimate bodies but still had to finish off “certain things touching on the nature of man.” The allusion here was to the Traité de l’homme, which with the Traité de lumière was meant to form Le monde. Along with the Traité de lumière, however, the Traité de l’homme was suppressed by Descartes after the condemnation of Galileo in 1633, and although it thus had to await posthumous publication in the 1660’s, his writing of the Traité de l’homme proved extremely important in the further maturation of Descartes’s physiological conceptions.
The Traité de l’homme begins and ends with a proclamation of literary and philosophical license. In the Traité, Descartes writes, we deliberately consider not a real man but a “statue” or machine de terre expressly fashioned by God to approximate real men as closely as possible. Like a real man, the machine de terre will be imagined to possess an immaterial soul and a physical body, and, also like a real man, its physical body will consist of a heart, brain, stomach, vessels, nerves, et al. But since we are considering only an artificial man—a contrivance fashioned more perfectly but on the same principles as a clock or water mill—we will not be tempted to attribute the motions and activities of this man to special sensitive or vegetative souls or principles of life. Nothing more than a contained rational and immaterial soul and “the disposition of organs, no more and no less than in the movements of a clock or any other automaton”will be needed to comprehend the active functioning of this special contrivance formed by God and operated thereafter by the principles of mechanical action. We are to bear in mind, of course, that the man of the Traité is remarkably like men we know, but our literary and philosophical license allows us to hypothesize and analogize freely.
Descartes fully exercises his self-proclaimed license in the rest of the Traité. He first surveys various physiological processes, giving for each of them not the traditional or neoclassical account of such recent physiological writers as Fernel or Riolan but mechanistic details by which the particular function is performed automatically in the homme. Each of Descartes’s explanations borrows something from traditionalist physiological theories, but in each case Descartes wields Ockham’s razor to strip away excess souls, faculties, forces, and innate heats from the corpuscular or chemical core of explanation.
Digestion, for example, is for Descartes only a fermentative process in which the particles of food are broken apart and set into agitation by fluids contained in the stomach. Chyle and excremental particles are then separated from one another in a filtration performed merely by a sievelike configuration of the pores and vascular openings in the intestines. Chyle particles go through another filtration and fermentation in the liver, where they thereby—and only thereby—acquire the properties of blood. Blood formed in the liver drips from the vena cava into the right ventricle of the heart, where the purely physical heat implanted there quickly vaporizes the sanguinary mass. The expansion of this sanguinary vapor pushes out the walls of the heart and arteries. Expansion with rarefaction is succeeded by cooling; and, as the vapor condenses, the heart and arteries return to their original size. The heart is fitted with a perfect arrangement of valves, and in addition the homme is served by a perpetual circulation of blood. (Descartes had read William Harvey’s De motu cordis, as is also clear from his prior correspondence, but apparently took seriously only the part on circulatory motion rather than on cardiac action.) Cardiac and arterial pulsation is thus continually and automatically repeated throughout the life of the automaton by mechanical means, not under the control of an active diastolic faculty. And while the sanguinary particles are coursing through the vessels, certain of them separate off into special pores, which accounts for both nutrition and the sievelike production of such secretions as bile and urine.
After this mechanistic survey of general physiology, Descartes moves to the nervous system, which he treats in considerable detail. The nerves are said to be a series of essentially hollow tubes with a filamentous marrow and are similar in operation to the water-filled pipes of those hydraulically controlled puppets and mechanical statues found “in the grottoes and fountains in the gardens of our Kings.” Filling the spaces in the nerves is a fine, material substance, the animal spirits. These spirits are actually the most quickly moving particles of the blood that have traveled through the arteries in the shortest, straightest path from the heart to the brain. Once conveyed to the brain according to the laws of mechanics and then mechanically separated from the coarser parts of the blood, these most agile particles become “a wind or very subtle flame.”
This spiritual wind can flow into the muscles, which are directly connected with the neural tubes. When a particular muscle is inflated by the influx of animal spirits, its belly distends as if by wind billowing a canvas sail, and the ends or insertions of the muscle are necessarily pulled more closely together. The pulling together of the muscular insertions constitutes muscle contraction. Gross movements in the homme (including breathing and swallowing) are produced, therefore, as the necessary mechanical effects of animal spirits discharged to one or another muscle group.
The movement of animal spirits is also controlled, however, by the action of the pineal gland in the brain. The rational soul is itself most closely associated with this gland located centrally in the substance of the cerebral marrow, and by directly causing this gland to move, no matter how slightly, the immaterial, willful soul of man can redirect the animal spirits from one set of nervous channels to another. Redirection of animal spirits results, in turn, in the production of different gross muscle movements. The pineal gland can also be, and often is, directly and unconsciously affected by a whole array of supervening influences, among the most important of which are the sensory. In animals, of course, only the supervening influences operate. Since Descartes has already gotten into a discussion of sensation, he now discusses that subject in great detail. He devotes much attention to the external organs of sense, concentrating to a large degree on the visual apparatus. For his discussion here, Descartes was able to draw upon the prior work of sixteenth-century anatomists and natural philosophers that had culminated in Kepler’s fine account of the eye as a camera-like optical system. Yet Descartes inserts his optical account of the eye into his already developed physiological system; for once the image is formed on the retina, Descartes explains, the nerves—and with them Descartes’s general physiology—take over. Rays of light focused on particular points of the retina cause specific nerves to jiggle slightly. Since the solid, filamentous part of the nerve is continuous from the sense organ to the brain, the externally caused jiggle is immediately transmitted, like the tug on a bell rope, to the interior of the brain as an internal jiggle. Internal jiggles directly control the streaming of animal spirits in the brain, and the rational soul, operating only at the pineal gland, interprets the patterns of the streams as particular sensations. The soul works “in the dark” this way, too, in all other sensations and in the difficult activity of multilevel perception. The soul “reads” various motions produced in the brain by the nerves and spirits, and interprets particular combinations of motions as taste, odor, color, or even distance.
Descartes moves from this complicated discussion of the five external senses of his homme to a consideration of certain internal feelings which it also experiences. The physiology (and psychology) is here, too, based on the manner by which the soul “reads” the messages delivered through the nerves by the spirits, messages which depend, in this instance, upon the internal functioning of the various parts of the body. Thus, when the blood entering the heart is purer and more subtle than usual, it vaporizes very easily in the cardiac chambers, and, as a consequence, it stimulates the nerves placed in the heart in a manner that the soul will associate with joy. All sorts of moods, feelings, and what might be generally labeled chains of somatopsychic effects (Descartes usually calls these “passions”) are schematically accounted for in this same manner—by imagining a passive immaterial soul interpreting the varied motions of the spirits as they stream pass the pineal gland.
As a logical extension of his consideration of the “passions”, Descartes turns, finally, to a discussion of sleep, dreams, memory, and imagination—other consequences of the interaction of the soul with internal neurophysiology. All these latter psychological phenomena are said to depend on the special motion of animal spirits, sometimes through favored pathways created by habitual or normal daytime activity.
With this discussion (and with his restatement of literary and philosophical license) Descartes terminates the Traité. It was obviously written as a full working out of the physiological hints included in the Regulae and elaborated in the light of his own philosophical development. A clearly stated dualist ontology runs through the Traité, while mechanistic details analogous to those of the Traité de lumière are evident at almost every turn.
But the Traité de l’homme served not only to clarify and develop Descartes’s physiological views; it also quickly became a rich fund of ideas upon which he drew throughout the rest of his intellectual life. In 1637, for example, Descartes published two important works: Discours de la méthode and Dioptrique. In both he uses physiological ideas from the unpublished Traité at important points in his argument. Specifically, in part V of the Discours, Descartes employs a summary of his cardiovascular physiology to illustrate how the newly discovered laws by which God orders his universe are sufficient to explicate certain of the most important human functions, and the Dioptrique includes a summary of his general theory of sensation as a preliminary to a detailed study of image formation and visual perception. In the 1640’s, too, Descartes drew heavily upon the unpublished Traité. The complicated arguments of the Passions de l’âme, published near the end of that decade, rest firmly on the extensive survey of basic Cartesian physiology incorporated in part I, while Descartes’s unruffled assertiveness in his correspondence and later philosophical writings on the “beast-machine” makes full sense only against a background provided by the Traité’s automaton.
Descartes, however, had left one major physiological problem untreated in the Traité: the reproductive generation of animals and men. He had insisted for reasons of methodological circumspection that the homme of the Traité was directly contrived by God. The Cartesian program, of course, was to explain all but rational, deliberately willful, or selfconscious behavior in terms of mere mechanism. He had eliminated the souls, principles, faculties, and innate heats of traditional physiology and had systematically replaced them with hypotheses and analogies of purely physical nature. But generation had escaped, and its explanation in mechanistic terms was clearly needed for the logical completion of his system. Recently proposed theories of animal generation had left the subject replete with Galenic faculties and Aristotelian souls, and even William Harvey was soon to show himself content with innate principles and plastic forces as the controlling agents of embryonic development. Descartes, to be consistent, could not accept these explanatory devices and had, therefore, to formulate some alternative.
His correspondence and certain manuscript remains show that Descartes had actually been deeply concerned with the problem of animal generation for a considerable period of time. Earliest references in the former occur in 1629, and snippets of the latter reveal fitful grapplings with the problem, some of them even leading to direct anatomical investigations undertaken, apparently, as a means for providing clues to the processes involved. Descartes’s ideas on the subject really seem to have crystallized, however, in the late 1640’s, when he triumphantly announced his “solution” to the long-plaguing problem in a series of enthusiastic letters to Princess Elizabeth. The ideas alluded to in these letters appear to be those published as the De la formation du foetus, which Descartes completed not long before his death.
First published by Clerselier in 1664, the Formation is a curious essay. Unlike the Traité de l’homme, which much preceded it in date of composition, the Formation consists mainly of bald assertions and only the vaguest mechanisms. Generation commences when the male and female seeds come together and mutually induce a corpuscular fermentation. The motion of certain of the fermenting particles forms the heart, that of others the lungs. Streaming of particles as the process continues furrows out the blood vessels; later, membranes and fibers are formed which ultimately weave together to construct the solid parts. The formation of the bodily parts is described in these vague terms (no mechanism of organ or vascular development is ever made more precise than this), yet Descartes apparently felt satisfied with his results. For by describing generation in chemical and corpuscular, rather than vital or teleological, terms Descartes had, at least in his own mind, completed the mechanistic program for physiology. Everything in the animal’s life, from its first formation to its final decay, now had an automatic, mechanical explanation.
The impact of the Cartesian physiological program, once it was publicly known, was enormous. In two ways—philosophically and physiologically—Descartes transformed long-standing beliefs about animals and men. Philosophically, of course, his notions of mind-body dualism and animal automatism had extremely important implications that were not lost on Henry More, Malebranche, Spinoza, and Leibniz, along with many others in the seventeenth century. The “beast-machine” idea also had continuing ramifications in the eighteenth century, leading, at least according to Aram Vartanian, directly to La Mettrie’s L’homme machine. Also, according to Vartanian, Descartes’s posthumously published views on human function and animal generation exerted important philosophical influence, contributing greatly to the eighteenth-century concern with these biological subjects by many of the philosophes. But physiologically, too, Descartes’s conceptions had an impact that in many ways was even more impressive than the philosophical influence, because it affected the actual course of contemporary science.
Almost as soon as Descartes published his Discours de la méthode, a few professors of medicine began to react to specific Cartesian physiological ideas and to the general Cartesian program. Plempius at Louvain and Regius at Utrecht were among the first. Although Plempius proved relatively hostile to Cartesian ideas (his objections were not unlike those William Harvey was to raise a few years later), Regius became so enthusiastic that for a time he entered into something of a student-teacher relationship with Descartes. As their correspondence for 1641 makes clear, Regius would send his students’ theses to Descartes for comment and correction. Descartes would then return them with such specific excisions of classical residues as “In the first line of the Thesis I would get rid of these words: vivifying heat.”
The intimate contact between Descartes and Regius marked the beginning of the direct influence of Cartesian ideas and modes of thought on seventeenthcentury physiology. That influence was deliberately continued later, even more vigorously after Descartes’s death, by such influential figures as Thomas Bartholin and Nicholas Steno. These men, especially Steno, tried to wed the Cartesian method of mechanistic explanation to careful anatomical investigation. Steno was particularly highly regarded by contemporaries for his perfection of the mechanical theory of muscular contraction in his Elementorum myologiae specimen (1667) and for his defense of the Cartesian physiological methodology in his anatomically sound Discours sur l’anatomie du cerveau (1669).
Many other prominent seventeenth-century physiological writers were influenced by the Cartesian program, either directly by reading Descartes’s writings or indirectly through such followers as Steno. Among those deeply influenced by Cartesian physiology were Robert Hooke, Thomas Willis, Jan Swammerdam, and Giovanni Alfonso Borelli. These men saw in Cartesian physiology exactly what Descartes had intended it to be: a method of mechanistic formulation by which traditional categories of physiological explanation could be circumvented. Without Descartes, the seventeenth-century mechanization of physiological conceptions would have been inconceivable.
The main primary sources—letters, MSS, and published works—are all handsomely printed in the Adam-Tannery Oeuvres de Descartes; vol. XI contains the largest sample of relevant works.
Useful secondary studies of Descartes’s philosophy which seriously consider his physiological writings range from Étienne Gilson’s Études sur le rôle de la pensée médiévale dans la formation du système Cartésien (Paris, 1930) to Norman Kemp Smith’s New Studies in the Philosophy of Descartes (New York, 1966). Two older monographic studies are also useful: Bertrand de Saint-Germain’s Descartes considéré comme physiologiste (Paris, 1869) and Auguste-Georges Berthier’s “Le mécanisme Cartésien et la physiologie au XVIIe siècle,” in Isis, 2 (1914), 37–89, and 3 (1920),21–58. Some of the background to Descartes’s treatment of vision is made clear in A. C. Crombie, “The Mechanistic Hypothesis and the Scientific Study of Vision,” in Proceedings of the Royal Microscopical Society, 2 (1907), 3–112; fundamental aspects of his mechanistic philosophy are discussed in Georges Canguilhem, La formation du concept de réflexe aux XVIIe et XVIIIe siècles (Paris, 1955); while the seventeenth-century impact of Cartesian ideas is studied by Berthier (op. cit.) and in two helpful general works, Paul Mouy, Le développement de la physique Cartésienne (Paris,1934) and vol. I of Thomas S. Hall, Ideas of Life and Matter(Chicago, 1969).
A sense of the influence of Cartesian ideas and methods can also be gleaned from Michael Foster, Lectures on the History of Physiology (Cambridge, 1901); and Gustav Scherz’s various studies of Nicholas Steno. Finally, see Aram Vartanian, Diderot and Descartes (Princeton, 1953) and La Mettrie’s “L’homme machine” (Princeton, 1960)
Theodore M. Brown