“The new philosophy puts all in doubt,” wrote the poet John Donne. Early 17th-century poetry and drama abounded in expressions of confusion and dismay about the world, God, and man. The gently questioning essays of the 16th-century French philosopher Michel de Montaigne, musing on human folly and fanaticism, continued to be popular long after his time, for they were no less relevant to the generation that suffered from the Thirty Years’ War. Unsettling scientific views were gaining a hold. As the new astronomy of Copernicus and Galileo, with its heliocentric view, was accepted, the firm association between religious beliefs, moral principles, and the traditional scheme of nature was shaken. In this process, mathematics occupied the central position. It was, in the words of René Descartes, “the general science which should explain all that can be known about quantity and measure, considered independently of any application to a particular subject.” It enabled its practitioners to bridge gaps between speculation and reasonable certainty: Johannes Kepler thus proceeded from his study of conic sections to the laws of planetary motion. When, however, Fontenelle wrote of Descartes, “Sometimes one man gives the tone to a whole century,” it was not merely of his mathematics that he was thinking. It was the system and philosophy that Descartes derived from the application of mathematical reasoning to the mysteries of the world—all that is meant by Cartesianism—which was so influential. The method expounded in his Discourse on Method (1637) was one of doubt: all was uncertain until established by reasoning from self-evident propositions, on principles analogous to those of geometry. It was serviceable in all areas of study. There was a mechanistic model for all living things.

A different track had been pursued by Francis Bacon, the great English lawyer and savant, whose influence eventually proved as great as that of Descartes. He called for a new science, to be based on organized and collaborative experiment with a systematic recording of results. General laws could be established only when research had produced enough data and then by inductive reasoning, which, as described in his Novum Organum (1620), derives from “particulars, rising by a gradual and unbroken ascent, so that it arrives at the most general axioms last of all.” These must be tried and proved by further experiments. Bacon’s method could lead to the accumulation of knowledge. It also was self-correcting. Indeed, it was in some ways modern in its practical emphasis. Significantly, whereas the devout humanist Thomas More had placed his Utopia in a remote setting, Bacon put New Atlantis (1627) in the future. “Knowledge is power,” he said, perhaps unoriginally but with the conviction that went with a vision of mankind gaining mastery over nature. Thus were established the two poles of scientific endeavour, the rational and the empirical, between which enlightened man was to map the ground for a better world.

Bacon’s inductive method is flawed through his insufficient emphasis on hypothesis. Descartes was on strong ground when he maintained that philosophy must proceed from what is definable to what is complex and uncertain. He wrote in French rather than the customary Latin so as to exploit its value as a vehicle for clear and logical expression and to reach a wider audience. Cartesian rationalism, as applied to theology, for example by Nicholas Malebranche, who set out to refute the pantheism of Benedict de Spinoza, was a powerful solvent of traditional belief: God was made subservient to reason. While Descartes maintained his hold on French opinion, across the Channel Isaac Newton, a prodigious mathematician and a resourceful and disciplined experimenter, was mounting a crucial challenge. His Philosophiae Naturalis Principia Mathematica (1687; Mathematical Principles of Natural Philosophy) ranks with the Discourse on Method in authority and influence as a peak in the 17th-century quest for truth. Newton did not break completely with Descartes and remained faithful to the latter’s fundamental idea of the universe as a machine. But Newton’s machine operated according to a series of laws, the essence of which was that the principle of gravitation was everywhere present and efficient. The onus was on the Cartesians to show not only that their mechanics gave a truer explanation but also that their methods were sounder. Christiaan Huygens was both a loyal disciple of Descartes and a formidable mathematician and inventor in his own right, who had worked out the first tenable theory of centrifugal force. His dilemma is instructive. He acknowledged that Newton’s assumption of forces acting between members of the solar system was justified by the correct conclusions he drew from it, but he would not go on to accept that attraction was affecting every pair of particles, however minute. When Newton identified gravitation as a property inherent in corporeal matter, Huygens thought that absurd and looked for an agent acting constantly according to certain laws. Some believed that Newton was returning to “occult” qualities. Eccentricities apart, his views were not easy to grasp; those who actually read the Principia found it painfully difficult. Cartesianism was more accessible and appealing.

Gradually, however, Newton’s work won understanding. One medium, ironically, was an outstanding textbook of Cartesian physics, Jacques Rohault’s Traité de physique (1671), with detailed notes setting out Newton’s case. In 1732 Pierre-Louis de Mauperthuis put the Cartesians on the defensive by his defense of Newton’s right to employ a principle the cause of which was yet unknown. In 1734, in his Philosophical Letters, Voltaire introduced Newton as the “destroyer of the system of Descartes.” His authority clinched the issue. Newton’s physics was justified by its successful application in different fields. The return of Halley’s comet was accurately predicted. Charles Coulomb’s torsion balance proved that Newton’s law of inverse squares was valid for electromagnetic attraction. Cartesianism reduced nature to a set of habits within a world of rules; the new attitude took note of accidents and circumstances. Observation and experiment revealed nature as untidy, unpredictable—a tangle of conflicting forces. In classical theory, reason was presumed to be common to all human beings and its laws immutable. In Enlightenment Europe, however, there was a growing impatience with systems. The most creative of scientists, such as Boyle, Harvey, and Leeuwenhoek, found sufficient momentum for discovery on science’s front line. The controversy was creative because both rational and empirical methods were essential to progress. Like the literary battle between the “ancients” and the “moderns” or the theological battle between Jesuits and Jansenists, the scientific debate was a school of advocacy.

If Newton was supremely important among those who contributed to the climate of the Enlightenment, it is because his new system offered certainties in a world of doubts. The belief spread that Newton had explained forever how the universe worked. This cautious, devout empiricist lent the imprint of genius to the great idea of the Enlightenment: that man, guided by the light of reason, could explain all natural phenomena and could embark on the study of his own place in a world that was no longer mysterious. Yet he might otherwise have been aware more of disintegration than of progress or of theories demolished than of truths established. This was true even within the expanding field of the physical sciences. To gauge the mood of the world of intellect and fashion, of French salons or of such institutions as the Royal Society, it is essential to understand what constituted the crisis in the European mind of the late 17th century.

At the heart of the crisis was the critical examination of Christian faith, its foundations in the Bible, and the authority embodied in the church. In 1647 Pierre Gassendi had revived the atomistic philosophy of Lucretius, as outlined in On the Nature of Things. He insisted on the Divine Providence behind Epicurus’ atoms and voids. Critical examination could not fail to be unsettling because the Christian view was not confined to questions of personal belief and morals, or even history, but comprehended the entire nature of God’s world. The impact of scientific research must be weighed in the wider context of an intellectual revolution. Different kinds of learning were not then as sharply distinguished, because of their appropriate disciplines and terminology, as they are in an age of specialization. At that time philomaths could still be polymaths. Newton’s contemporary, Gottfried Wilhelm Leibniz—whose principal contribution to philosophy was that substance exists only in the form of monads, each of which obeys the laws of its own self-determined development while remaining in complete accord with all the rest—influenced his age by concluding that since God contrived the universal harmony this world must be the best of all possible worlds. He also proposed legal reforms, invented a calculating machine, devised a method of the calculus independent of Newton’s, improved the drainage of mines, and laboured for the reunification of the Roman Catholic and Lutheran churches.