In Rules for the Direction of the Mind, Descartes wrote: “The first principles themselves are given by intuition alone, while, on the contrary, the remote conclusions are furnished only by deduction. . . . These two methods are the most certain routes to knowledge, and the mind should admit no others. All the rest should be rejected as suspect of error and dangerous.”
Descartes was embracing the Euclidean ideal: Start from self-evident axioms, proceed by infallible deductions. But in his own research, Descartes forgot the Euclidean ideal. Nowhere in the Geometry do we find the label Axiom, Theorem, or Proof.
In classical Greece, and again in the Renaissance and after, mathematicians distinguished two ways of proceeding—the “synthetic” and the “analytic.” The synthetic way was Euclid’s: from axioms through deductions to theorems. In the analytic mode, you start with a problem and “analyze” it to find a solution. Today we might call this a “heuristic” or “problem-solving” approach.
In formal presentation of academic mathematics, the synthetic was and still is the norm. Foundationist schools of the nineteenth and twentieth centuries identify mathematics with its synthetic mode—true axioms followed by correct deductions to yield guaranteed true conclusions.
In his Rules for the Direction of the Mind, Descartes insists on the synthetic method. But his own research, in the Geometry, uses only the analytic mode. He solves problems. He finds efficient methods for solving problems. Never does he bother with axioms.
Descartes’s conviction of the certainty of mathematics might lead readers to expect that at least Descartes’s own mathematics is error-free. But of course, as we will see, the Geometry, like every other math book, has mistakes. Certitude is only a goal.
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You won’t find in the Geometry the method we teach nowadays as Cartesian or “analytic” geometry. Our analytic geometry is based on rectangular coordinates (which we call “Cartesian”). To every point in the plane we associate a pair of real numbers, the “x” and “y” coordinates of the point. To an equation relating x and y corresponds a “graph”—the set of points whose x and y coordinates satisfy the equation. For an equation of first degree, the graph is a straight line. For an equation of second degree, it’s a circle or other conic section. Our idea is to solve geometric problems by reducing them to algebra. Nowhere in Descartes’s book do we see these familiar horizontal and vertical axes! Boyer says it was Newton who first used orthogonal coordinate axes in analytic geometry.
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The conceptual essence of analytic geometry, the “isomorphism” or exact translation between algebra and geometry, was understood more clearly by Fermat than by Descartes. Fermat’s analytic geometry predated Descartes’s, but it wasn’t published until 1679. The modern formulation comes from a long development. Fermat and Descartes were the first steps. Instead of systematically developing the technique of orthogonal coordinate axes, the Geometry studies a group of problems centering around a problem of Pappus of Alexandria (third century A.D.). To solve Pappus’s problem Descartes develops an algebraic-geometric procedure. First he derives an algebraic equation relating known and unknown lengths in the problem. But he doesn’t then look for an algebraic or numerical solution, as we would do. He is faithful to the Greek conception, that by a solution to a geometric problem is meant a construction with specified instruments. When possible Descartes uses the Euclidean straight edge and compass. When necessary, he brings in his own instrument, an apparatus of hinged rulers. Algebra is an intermediate device, in going from geometric problem to geometric solution. Its role is to reduce a complicated curve to a simpler one whose construction is known. He solves third and fourth-degree equations by reducing them to second degree—to conic sections. He solves certain fifth-and sixth-degree equations by reducing them to third degree. A modern reader knows that the general equation of fifth degree can’t be solved by extraction of roots. So he’s skeptical about Descartes’s claim that his hinged rulers can solve equations of degree six and higher. Descartes was mistaken on several points. In themselves, these are of little interest today. But they discredit his claim of absolute certainty. Descartes’s mathematics refutes his epistemology.
Emily Grosholz and Carl Boyer point out errors in the Geometry. “When he turns his attention to the locus of five lines, he considers only a few cases, not bothering to complete the task, because, as he says, his method furnishes a way to describe them. But Descartes could not have completed the task, which amounted to giving a catalogue of the cubics. . . . Newton, because he was able to move with confidence between graph and equation, first attempted a catalogue of the cubics; he distinguished seventy-two species of cubics, and even then omitted six” (Grosholz, referring to Whiteside).
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Descartes claimed his Method was infallible in science and mathematics. He was more cautious with religion. He didn’t derive Holy Scripture or divine revelation by self-evident axioms and infallible deductions. When he heard that Galileo’s Dialogue on the Two Chief Systems was condemned by the Holy Church, he suppressed his first book, Le Monde, even though he was living in Holland, safe from the Church. (Galileo was kept under house arrest at first. For three years he had to recite the seven penitential psalms every week.) Descartes wrote to Father Mersenne, “I would not want for anything in the world to be the author of a work where there was the slightest word of which the Church might disapprove.”
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Like Pascal, Newton, and Leibniz, Descartes may have valued his contributions to theology above his mathematics. His struggle against skeptics and heretics is the major half of his philosophy, more explicit than his battles with scholastics. “In Descartes’ reply to the objections of Father Bourdin, he announced that he was the first of all men to overthrow the doubts of the Sceptics . . . he discovered how the best minds of the day either spent their time advocating scepticism, or accepted only probable and possibly uncertain views, instead of seeking absolute truth. . . . It was in the light of this awakening to the sceptical menace, that when he was in Paris Descartes set in motion his philosophical revolution by discovering something so certain and so assured that all the most extravagant suppositions brought forward by the sceptics were incapable of shaking . . . in the tradition of the greatest medieval minds, (he) sought to secure man’s natural knowledge to the strongest possible foundation, the all-powerful eternal God” (Popkin, p. 72). The essence of the Meditations is a proof that the world exists by first proving Descartes exists, and then, by contemplating Descartes’s thoughts, proving that God exists and is not a deceiver. Once a non-deceiving God exists, everything else is easy.
