One of the keys to the achievement of a new worldview in the seventeenth century was the development of better ways of obtaining knowledge about the world. Two important thinkers, Francis Bacon (1561–1626) and René Descartes (day-KAHRT) (1596–1650), were influential in describing and advocating for improved scientific methods based, respectively, on experimentation and mathematical reasoning.

English politician and writer Francis Bacon was the greatest early propagandist for the new experimental method. Rejecting the Aristotelian and medieval method of using speculative reasoning to build general theories, Bacon argued that new knowledge had to be pursued through empirical research. The researcher who wants to learn more about leaves or rocks, for example, should not speculate about the subject but should rather collect a multitude of specimens and then compare and analyze them to derive general principles. Bacon formalized the empirical method, which had already been used by Brahe and Galileo, into the general theory of inductive reasoning known as empiricism. Bacon’s work, and his prestige as lord chancellor under James I, led to the widespread adoption of what was called “experimental philosophy” in England after his death. In 1660 followers of Bacon created the Royal Society (still in existence), which met weekly to conduct experiments and discuss the latest findings of scholars across Europe.

On the continent, more speculative methods retained support. The French philosopher René Descartes was a multitalented genius who made his first great discovery in mathematics. As a twenty-three-year-old soldier serving in the Thirty Years’ War, he experienced a life-changing intellectual vision one night in 1619. Descartes saw that there was a perfect correspondence between geometry and algebra and that geometrical spatial figures could be expressed as algebraic equations and vice versa. A major step forward in the history of mathematics, Descartes’s discovery of analytic geometry provided scientists with an important new tool.

Descartes used mathematics to elaborate a highly influential vision of the workings of the cosmos. Accepting Galileo’s claim that all elements of the universe are composed of the same matter, Descartes began to investigate the basic nature of matter. Drawing on ancient Greek atomist philosophies, Descartes developed the idea that matter was made up of identical “corpuscules” that collided together in an endless series of motions. All occurrences in nature could be analyzed as matter in motion and, according to Descartes, the total “quantity of motion” in the universe was constant. Descartes’s mechanistic view of the universe depended on the idea that a vacuum was impossible, which meant that every action had an equal reaction, continuing in an eternal chain reaction.

Although Descartes’s hypothesis about the vacuum was proved wrong, his notion of a mechanistic universe intelligible through the physics of motion proved inspirational. Decades later, Newton rejected Descartes’s idea of a full universe and several of his other ideas, but retained the notion of a mechanistic universe as a key element of his own system.

Descartes’s greatest achievement was to develop his initial vision into a whole philosophy of knowledge and science. The Aristotelian cosmos was appealing in part because it corresponded with the evidence of the human senses. When the senses were proven to be wrong, Descartes decided it was necessary to doubt them and everything that could reasonably be doubted, and then, as in geometry, to use deductive reasoning from self-evident truths, which he called “first principles,” to ascertain scientific laws. Descartes’s reasoning ultimately reduced all substances to “matter” and “mind” — that is, to the physical and the spiritual. The devout Descartes believed that God had endowed man with reason for a purpose and that rational speculation could provide a path to the truths of creation. His view of the world as consisting of two fundamental entities is known as Cartesian dualism. Descartes’s thought was highly influential in France and the Netherlands, but less so in England, where experimental philosophy won the day.

Both Bacon’s inductive experimentalism and Descartes’s deductive mathematical reasoning had their faults. Bacon’s inability to appreciate the importance of mathematics and his obsession with practical results clearly showed the limitations of antitheoretical empiricism. Likewise, some of Descartes’s positions demonstrated the inadequacy of rigid, dogmatic rationalism. For example, he believed that it was possible to deduce the whole science of medicine from first principles. Although insufficient on their own, Bacon’s and Descartes’s extreme approaches are combined in the modern scientific method, which began to crystallize in the late seventeenth century.