Cartesianism

In some areas, Descartes's work was accepted as pathbreaking: above all his geometrical optics and his algebra (Isaac Newton learned his advanced mathematics in the first instance from Descartes's Geometry). But other areas attracted a more partisan response. We can distinguish three different kinds of concern in the Cartesian corpus in this respect: epistemology and metaphysics, cosmology, and physiology.

Although the Meditations is for modern readers the canonical text by Descartes, this focus is really a nineteenth-century development, and the most widely read and discussed of his works in the eighteenth century were the Discourse on Method, Principles of Philosophy, and the posthumously published Treatise on Man. Moreover, Descartes himself had played down the epistemological and metaphysical concerns that characterize the Meditations. Two themes dominate the Meditations—a skeptically driven epistemology and the mind/body problem—and the fortunes of these differ radically among later philosophers. Neither Baruch Spinoza (1632–1677) nor Gottfried Wilhelm von Leibniz (1646–1716) could see any value or legitimacy in beginning epistemology by answering radical skeptical problems (and Blaise Pascal [1623–1662] referred to Descartes's philosophy as "useless and uncertain"), and they abandoned the idea of a skeptically driven epistemology. Nicolas Malebranche (1638–1715) was a little closer to Descartes, although still at variance with him on key doctrines, and he was not so dismissive of skeptical beginnings for epistemology, but they work in a rather different way in his philosophy than they do in Descartes. Malebranche's philosophy vied with that of John Locke (1632–1794) at the end of the seventeenth century, and his influence on George Berkeley (1685–1753) and David Hume (1711–1776) was at least as great as that of Locke, putting skeptical issues to the fore. On the mind/body question, no one was satisfied with Cartesian dualism. Spinoza, Malebranche, and Leibniz each saw the question as being the key to success, and each offered their own distinctive solution to the question: neutral monism, occasionalism, and preestablished harmony, respectively. By the eighteenth century, epistemologically oriented issues of mind had receded from the philosophical arena to some extent, and Cartesianism was less associated with dualism.

Two natural-philosophical topics on which Descartes's views had caused immense controversy were his account of the solar system in book 3 of the Principles of Philosophy and his account of the formation of the earth in book 4. Descartes's response to the Copernicanism problem was not to make the sun the center of the cosmos, as Copernicus had done, but to make the sun one of an indefinite (perhaps infinite) number of solar systems, each containing planets like ours that may harbor beings with souls. The idea that our sun was simply one star among many and could not be held to be the center of the cosmos gained credence in the course of the later seventeenth century (for those who held that the universe was infinite there was no issue, of course, since something infinite cannot have a center since every point is equidistant from an infinitely remote boundary). Even more radical was his view of the formation of the earth. The Bible had presented a creation story in which the fabric of the earth, plants, and animals, had a function in a unique, highly designed system. Descartes, by contrast, has a general theory of planet formation: planets derive from stars that have formed a coating of hard matter around their periphery and are squeezed out by surrounding solar systems, in effect becoming massive pieces of refuse that find a home in other solar systems in orbits that depend on their size and speed. No exception is made for the Earth in Descartes's account, which completely robs the formation of earth of any teleology. The account was the first nonmythological treatment of the formation of the earth and was of great influence in the eighteenth century, not least in Georges-Louis Buffon's (1707–1778) account.

More generally, Descartes's cosmology—in which planets were carried around their suns by means of the vortical motion of a swiftly circulating medium in which they were immersed—was taken very seriously, and was the dominant cosmological system before Newton's Principia. Indeed, in continental Europe, it was not abandoned in favor of Newtonianism for some time, primarily because it did not involve any appeal to action at a distance, which was universally perceived to be the most problematic general feature of Newton's system. Moreover, Descartes's system was far more comprehensive than Newton's, tying his vortex theory of planetary motion in with phenomena such as magnetism and static electricity.

Descartes's rejection of teleology was also manifest in his account of embryology, set out in the Treatise on Man and in more detail in The Description of the Human Body. There he insists on a radically mechanistic approach, denying that the fetus strives to realize an end or goal in the development process. The inert and initially undifferentiated matter making up the horse fetus, for example, develops into a horse not because it somehow contains "horseness" or because it is able to shape itself into a horse but because the distinctive mechanical and physio-chemical processes in the womb of a horse cause the matter to develop in a certain way. In this way, Descartes opened up the question of the physiology of fetal development. Embryology is in fact just one topic in Descartes's comprehensive attempt to mechanize physiology, and in the Treatise on Man a variety of physiological processes are construed in such a way that we need only postulate inert matter being acted upon by mechanical forces. From this derives his infamous doctrine of bêtes machines (animal machines), whereby the behavior of animals, who lack minds (in the sense of awareness of their "cognitive" states) in Descartes's account, can be accounted for fully in terms of the mechanically describable interaction between their constituent parts. In 1747 Julien Offray de La Mettrie (1709–1751) published his Man Machine, in which he purported to apply the Cartesian theory of animal machines to human beings (actually his materialism was different from Cartesian mechanism), claiming that this could be done successfully, so that it was unnecessary to postulate a human soul to account for human thoughts and behavior. This solidified the image of Descartes as a dangerous materialist, and it was only in the nineteenth century that the idea of Descartes as a dualist was generally revived.

By the twentieth century, interest in Descartes in Anglo-phone philosophy was largely confined to his skeptically driven epistemology and his dualist account of mind. The revival of interest in empiricist epistemology, helped by the rise of positivism, resulted in skepticism being taken much more seriously as a philosophical problem (for example, in A. J. Ayer). In the philosophy of mind, various alternatives to dualism—such as epiphenomenalism, behaviorism, and materialism—were devised, with the effect that Cartesian dualism was often set up as a straw man by which to contrast one's own theory (as in Gilbert Ryle's defense of behaviorism). By contrast, in French and German philosophy, interest centered rather on Descartes's idea of a self as independent of the world in which it finds itself, as a locus of subjectivity that is given prior to any interactions that it has with other subjects. The ethical and political aspects of this understanding have been explored either in their own right (by Jean-Paul Sartre, for example) or in combination with a Kantian account of ethical autonomy (such as by Jürgen Habermas).