By the time Descartes went to school, Aristotelian physics was facing challenges. Galileo's discovery of four moons of the planet Jupiter was celebrated at La Flèche in 1610. More generally, Copernicus had, in the previous century, offered a forceful argument for believing that the sun, not the earth, is at the center of the solar system. Early in the seventeenth century, Johannes Kepler announced new results in optics, concerning the formation of images, the theory of lenses, and the fact that the retinal image plays a central role in vision. By the early 1630s, Descartes was aware of William Harvey's claim that the blood circulates in the body.
Descartes himself contributed some specific new results to the mathematical description of nature, as co-discoverer of the sine law of refraction and as developer of an accurate model of the rainbow. Nonetheless, as significant as these results are, his primary contribution to the “new science” lay in the way in which he described a general vision of a mechanistic approach to nature and sketched in the details of that vision to provide a comprehensive alternative to the dominant Aristotelian physics.
In the textbooks of Aristotelian physics of Descartes' day, it was common to divide physics into “general” and “special.” General physics pertained to the basic Aristotelian principles for analyzing natural substances: form, matter, privation, cause, place, time, motion. Special physics concerned actually existing natural entities, divided into inanimate and animate. Inanimate physics further divided into celestial and terrestrial, in accordance with the Aristotelian belief that the earth was at the center of the universe, and that the earth was of a different nature than the heavens (including the moon, and everything beyond it). Inanimate terrestrial physics first covered the four elements (earth, air, fire, and water), then the “mixed” bodies composed from them, including the various mineral kinds. Animate terrestrial physics concerned the various powers that Aristotelians ascribed to ensouled beings, where the soul is considered as a principle of life (possessing vital as well as mental or cognitive powers). In the simplest textbooks, the powers of the soul were divided into three groups: vegetative (including nutrition, growth, and reproduction), which pertained to both plants and animals; sensitive (including external senses, internal senses, appetite, and motion), which pertain to animals alone; and rational powers, pertaining to human beings alone. All the bodies in both inanimate and animate terrestrial physics were governed by a “form” or active principle.
Descartes' ambition was to provide replacements for all the main parts of Aristotelian physics. In his physics, there is only one matter and it has no active forms. Thus, he dissolved the boundary that had made the celestial and the terrestrial differ in kind. His one matter had only the properties of size, shape, position, and motion. The matter is infinitely divisible and it constitutes space; there is no void, hence no spatial container distinct from matter. The motions of matter are governed by three laws of motion, including a precursor to Newton's law of inertia (but without the notion of vector forces) and a law of impact. Descartes' matter possessed no “force” or active agency; the laws of motion were decreed by God and were sustained by his activity. Earth, air, fire, and water were simply four among many natural kinds, all distinguished simply by the characteristic sizes, shapes, positions, and motions of their parts.
Although Descartes nominally subscribed to the biblical story of creation, in his natural philosophy he presented the hypothesis that the universe began as a chaotic soup of particles in motion and that everything else was subsequently formed as a result of patterns that developed within this moving matter. Thus, he conceived that many suns formed, around which planets coalesced. On these planets, mountains and seas formed, as did metals, magnets, and atmospheric phenomena such as clouds and rain. The planets themselves are carried around the sun in their orbits by a fluid medium that rotates like a whirlpool or vortex. Objects fall to earth not because of any intrinsic “form” that directs them to the center of the universe, and also not because of a force of attraction or other downward-tending force. Rather, they are driven down by the whirling particles of the surrounding ether. Descartes insisted that all cases of apparent action at a distance, including magnetism, must be explained through the contact of particle on particle. He explained magnetism as the result of corkscrew-shaped particles that spew forth from the poles of the earth and flow from north to south or vice versa, causing magnetized needles to align with their flow. To explain magnetic polarity, Descartes posited that the particles exiting from the south pole are threaded in one direction and those from the north are threaded oppositely (like the oppositely threaded spindles on bicycle pedals).
In writings that were published only posthumously (but were read by friends and followers during his lifetime, e.g., 5:112), he developed an extensive physiological description of animal bodies, in which he explained the functions of life in a purely mechanical manner, without appeal to a soul or vital principle.
In mechanizing the concept of living thing, Descartes did not deny the distinction between living and nonliving, but he did redraw the line between ensouled and unensouled beings. In his view, among earthly beings only humans have souls. He thus equated soul with mind: souls account for intellection and volition, including conscious sensory experiences, conscious experience of images, and consciously experienced memories. Descartes regarded nonhuman animals as machines, devoid of mind and consciousness, and hence lacking in sentience. Consequently, Descartes was required to explain all of the powers that Aristotelians had ascribed to the vegetative and sensitive soul by means of purely material and mechanistic processes. These mechanistic explanations extended, then, not merely to nutrition, growth, and reproduction, but also to the functions of the external and internal senses, including the ability of nonhuman animals to respond via their sense organs in a situationally appropriate manner: to approach things that are beneficial to their body (including food) and to avoid danger (as the sheep avoids the wolf).
In the Treatise on Man and Passions, Descartes described purely mechanical processes in the sense organs, brain, and muscles, that were to account for the functions of the sensitive soul. These processes involved “animal spirits,” or subtle matter, as distilled out of the blood at the base of the brain and distributed down the nerves to cause muscle motions in accordance with brain structures and current sensory stimulation. The brain structures that mediate behavior may be innate or acquired. Descartes ascribed some things that animals do to instinct; other aspects of their behavior he explained through a kind of mechanistic associative memory. He held that human physiology is similar to nonhuman animal physiology, as regards both vegetative and (some) sensitive functions—those sensitive functions that do not involve consciousness or intelligence:
Now a very large number of the motions occurring inside us do not depend in any way on the mind. These include heartbeat, digestion, nutrition, respiration when we are asleep, and also such waking actions as walking, singing, and the like, when these occur without the mind attending to them. When people take a fall, and stick out their hands so as to protect their head, it is not reason that instructs them to do this; it is simply that the sight of the impending fall reaches the brain and sends the animal spirits into the nerves in the manner necessary to produce this movement even without any mental volition, just as it would be produced in a machine. (7:229–30)
Many of the behaviors of human beings are actually carried out without intervention from the mind.
The fact that Descartes offered mechanistic explanations for many features of nature does not mean that his explanations were successful. Indeed, his followers and detractors debated the success of his various proposals for nearly a century after his death. His accounts of magnetism and gravity were challenged. Leibniz challenged the coherence of Descartes' laws of motion and impact. Newton offered his own laws of motion and an inverse square law of gravitational attraction. His account of orbital planetary motions replaced Descartes' vortexes. Others struggled to make Descartes' physiology work. There were also deeper challenges. Some wondered whether Descartes could actually explain how his infinitely divisible matter could coalesce into solid bodies. Why shouldn't collections of particles act like whiffs of smoke, that separate upon contact with large particles? Indeed, how do particles themselves cohere?
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