How Did René Descartes Contribute to the Scientific Revolution?
Descartes' Discourse on Method, Meditations, and Principles of Philosophy: the cogito, the mechanical philosophy, the vortex theory, and contributions to optics, geometry, and physiology.
How Did René Descartes Contribute to the Scientific Revolution?
René Descartes was, one of the most influential natural philosophers of the seventeenth century. The Discourse on Method (1637), the Meditations on First Philosophy (1641), and the Principles of Philosophy (1644) were, in their own time, the most widely read philosophical works in Europe, and the Cartesian natural philosophy — the view that the physical world could be explained by the motion of corpuscles of matter, governed by laws of impact and conservation of motion — was the dominant framework for the physical sciences in the second half of the seventeenth century. Descartes was also a brilliant mathematician, the inventor of analytic geometry, and a contributor to optics, physiology, and meteorology. This page looks at Descartes’ contributions to the Scientific Revolution: the method of the Discourse, the metaphysics of the Meditations, the mechanical philosophy of the Principles, and the work in mathematics, optics, and physiology. The biographical and contextual notes are on the Bacon and Descartes page. The contrast with Bacon’s inductive method is treated in What Was Francis Bacon’s Scientific Method?. The broader context of the Scientific Revolution is in the page on the key figures and the philosophy of science section.
The Method of the Discourse
The Discourse on Method was published in 1637, in French, in Leiden, with three appendices on optics, meteorology, and geometry. The book was an intellectual autobiography: it described the method that Descartes had developed, and it illustrated the method by applying it to the three appendices. The method was, in brief, a procedure for arriving at certain knowledge, by beginning from self-evident first principles and deducing conclusions from them. The method was set out in four rules, which Descartes summarized as follows. First, “never to accept anything as true that I did not know to be evidently so: that is to say, carefully to avoid precipitation and prejudice, and to include in my judgments nothing more than what presented itself so clearly and so distinctly to my mind that I had no occasion to doubt it.” Second, “to divide each of the difficulties that I examined into as many parts as possible, and as was requisite in order to resolve them better.” Third, “to conduct my thoughts in an orderly manner, beginning with the simplest and most easy objects in order to ascend gradually to the knowledge of the most complex.” Fourth, “to make enumerations so complete, and reviews so general, that I would be certain of having omitted nothing.” The method was the opposite of Bacon’s inductive method. Bacon began with the data and tried to reason up to general principles. Descartes began with general principles and tried to reason down to the data. The two methods were, complementary, and the dispute between the two camps was resolved in favor of a combination of the two. The combination was the new model of natural philosophy, and it was the model that the Scientific Revolution produced. The contrast is treated in detail in What Was Francis Bacon’s Scientific Method?.
The Cogito and the Meditations
The Meditations on First Philosophy was published in Paris in 1641, in Latin. The book was the metaphysical foundation of the Cartesian natural philosophy. The book opens with the famous “First Meditation,” in which Descartes resolves to doubt everything that can be doubted, including the existence of the external world, the existence of his own body, and the existence of God. The doubt is methodological, not skeptical: Descartes is not arguing that nothing can be known; he is arguing that the only path to certain knowledge is to begin from what cannot be doubted. The result of the doubt is the famous cogito, ergo sum — “I think, therefore I am” — which appears in the “Second Meditation.” The cogito is, Descartes argues, the one thing that cannot be doubted: even if I am deceived about everything else, the very act of being deceived implies that I exist. The cogito is the foundation of the Cartesian philosophy, and it was the foundation of the modern tradition of subjectivist philosophy. From the cogito, Descartes proceeded, in the later Meditations, to argue for the existence of God, the existence of the external world, and the distinction between mind and body. The argument for the existence of God is, in brief, that the idea of a perfect being could not have originated in an imperfect being, and must therefore have originated in a perfect being itself. The argument for the existence of the external world is, in brief, that God is not a deceiver, and would not allow us to have strong, clear, distinct perceptions of the external world if those perceptions were systematically false. The distinction between mind and body is, in brief, that the mind is a thinking substance, and the body is an extended substance, and the two are, in their natures, distinct. The arguments were, in their own time, controversial, and they have been controversial ever since. But they were the founding documents of the modern tradition of rationalist philosophy, and they were the foundation of the modern conception of the self.
The Mechanical Philosophy
The Principles of Philosophy was published in Amsterdam in 1644, in Latin. The book was the Cartesian natural philosophy: it set out, in a systematic form, the laws of motion, the theory of matter, the theory of the solar system, and the theory of the Earth. The book was, in the second half of the seventeenth century, the most widely used textbook in natural philosophy on the Continent. The central claim of the Cartesian natural philosophy is that the physical world can be explained by the motion of corpuscles of matter, governed by laws of impact and conservation of motion. Matter, for Descartes, was extension: anything that had spatial extent was matter, and the essence of matter was extension. The properties of bodies — their colors, sounds, smells, tastes, and even their “secondary qualities” like warmth and coldness — were, in Descartes’ view, effects of the motion of the corpuscles on the senses of the observer, not properties of the bodies themselves. The only true properties of bodies were the mechanical ones: extension, shape, motion, and the arrangements of their parts. The laws of motion were the heart of the Cartesian physics. The first law was the principle of inertia: “every body, considered in itself, is always at rest, or in uniform motion in a straight line.” The second law was the conservation of motion: “every body, in motion, tends to continue its motion in a straight line.” The third law was the rule of impact: “if a body in motion collides with another body, if its force of motion is less than the force of resistance of the other body, it is deflected; if it is greater, it carries the other body with it.” The laws were the first systematic attempt to articulate the principles of mechanics. The principle of inertia was, in particular, a major step beyond the Aristotelian physics, which held that the natural state of a body was rest, and that motion required a continuous mover. The principle of inertia was the foundation of Newton’s first law of motion. The conservation of motion was the ancestor of the modern principle of conservation of momentum. The rule of impact was the ancestor of the modern theory of collisions. The vortex theory of the solar system was one of the most distinctive features of the Cartesian natural philosophy. The theory held that the planets were carried around the Sun by huge whirlpools, or vortices, of subtle matter. The vortices, in turn, were carried by larger vortices around the Earth, and so on, in an infinite hierarchy. The theory was the Cartesian alternative to the gravitational theory of Newton. The theory was widely accepted in the seventeenth century, especially in France and the Low Countries, but it was eventually refuted by Newton, in Book II of the Principia, on the grounds that it could not account for the observed motions of the planets and comets. The full reception of the vortex theory is treated in the Newton page and in What Is in Newton’s Principia Mathematica?.
Analytic Geometry
The third appendix to the Discourse on Method was the Geometry, in which Descartes developed the method of analytic geometry. The method is the foundation of modern mathematics: it allows geometric problems to be translated into algebraic problems, and vice versa, by means of coordinate systems. The basic idea is simple. A point in the plane can be described by two numbers, its x and y coordinates. A curve in the plane can be described by an equation relating x and y. A circle of radius 1, for instance, is described by the equation x² + y² = 1. A line is described by an equation of the form y = mx + b, where m is the slope and b is the y-intercept. The method allows the geometer to use the powerful tools of algebra to solve geometric problems, and it allows the algebraist to use the visual intuition of geometry to solve algebraic problems. Descartes did not invent coordinate systems — they had been used, in a limited way, by the Greeks, by Oresme in the fourteenth century, and by Fermat, who developed analytic geometry independently. Descartes did, however, develop the method in a systematic way, and he showed how it could be applied to a wide range of problems. The method was, one of the most important mathematical innovations of the seventeenth century, and it was the foundation of the differential and integral calculus. Descartes also made important contributions to the theory of equations. He developed the law of signs for the number of positive and negative roots of a polynomial, the method of factoring polynomials, and the method of finding the roots of a polynomial by successive approximations. The contributions to algebra were, as important as the contributions to geometry, and they were the foundation of the modern theory of equations.
Optics
The first appendix to the Discourse on Method was the Dioptrics, a treatise on optics. The treatise was the first systematic treatment of geometric optics, and it included an account of the law of refraction, the theory of the rainbow, the construction of lenses, and the design of telescopes and microscopes. The law of refraction — that the angle of incidence is proportional to the angle of refraction, for a given pair of media — had been known since antiquity, in the form given by Ptolemy. Descartes, in the Dioptrics, derived a more accurate law, the law of sines, which says that the sine of the angle of incidence is proportional to the sine of the angle of refraction, with a constant of proportionality that depends on the two media. The law of sines is the basis of the modern theory of refraction, and it is the law that governs the design of lenses, telescopes, and microscopes. Descartes was not, in fact, the first to discover the law of sines. The law had been discovered, in a slightly different form, by the Dutch mathematician Willebrord Snell, and Descartes may have seen Snell’s unpublished manuscripts. The priority dispute was bitter, and it was one of the first of the many priority disputes that plagued the seventeenth century. The law of sines is now known in Continental Europe as Descartes’ law, and in the English-speaking world as Snell’s law, or sometimes as the Snell-Descartes law. Descartes’ account of the rainbow was the first correct account. He showed that the rainbow is produced by the refraction and reflection of sunlight in raindrops, and he derived the angular radius of the primary rainbow (about 42 degrees) and the secondary rainbow (about 51 degrees). The account was the model for the new mathematical natural philosophy, in which a phenomenon is explained by a combination of physical hypothesis and mathematical analysis.
Physiology and the Mind-Body Problem
Descartes also made important contributions to physiology. The Treatise on Man (1662, published posthumously) was the first systematic treatise on physiological psychology, and it included an account of the nervous system, the brain, the senses, and the motor responses. The treatise was the foundation of the modern science of reflex action, and it was the foundation of the modern conception of the body as a machine. The mind-body problem was, for Descartes, the central problem of philosophy. The mind, he argued, is a thinking substance, and the body is an extended substance, and the two are, in their natures, distinct. The mind and the body interact, in some way, at the pineal gland, which Descartes believed to be the seat of the soul. The interaction was the most mysterious part of the Cartesian philosophy, and it was the source of some of the most persistent problems in modern philosophy.
The Long-term Influence
Descartes’ contributions to the Scientific Revolution were, enormous. The Cartesian method was, for two centuries, the dominant method in Continental natural philosophy, and it was the foundation of the modern rationalist tradition. The mechanical philosophy was, for two centuries, the dominant framework for the physical sciences, and it was the foundation of the modern conception of nature as a system of matter in motion. The analytic geometry was, for two centuries, the foundation of modern mathematics, and it was the foundation of the differential and integral calculus. The contributions to optics, physiology, and meteorology were, in their own time, the leading edge of the new science. The Cartesian natural philosophy was replaced by the Newtonian. Newton’s gravitational theory, with its action at a distance, was, less mechanistic than the Cartesian vortex theory, and the Newtonians accused the Cartesians of reintroducing occult qualities. But the Cartesian program — the reduction of nature to matter in motion, governed by mathematical laws — survived the Newtonian revolution, and it is the program of the modern physical sciences. See the Newton page and in What Is in Newton’s Principia Mathematica?.