René Descartes is generally considered the father of modern philosophy. He was the first major figure in the philosophical movement known as rationalism, a method of understanding the world based on the use of reason as the means to attain knowledge. Along with empiricism, which stresses the use of sense perception rather than pure reason, rationalism was one of the main intellectual currents of the Enlightenment, a cultural movement spanning the seventeenth and eighteenth centuries that revolutionized the Western world. In tandem with men like John Locke, John Hobbes, and Voltaire, Descartes spurred society to re-examine its traditions and institutions, leading to massive social upheaval. Both the American and French Revolutions were based on Enlightenment theories, and the ways we approach science, math, philosophy, and the idea of the self were radically transformed during the period.

Descartes was born in 1596 in La Haye, a small village near Tours, France. The son of an aristocratic family, Descartes was enrolled at age six in the Jesuit College at La Flèche in Anjou. Because Descartes had always been somewhat sickly, his teachers allowed him to stay in bed until noon every day. Descartes attributed his most important ideas to this habit, and said he did his best thinking when he spent the morning in bed. Despite the religious underpinnings of the school, it was open to the free study of humanities and science. Descartes immersed himself in a wide range of subjects, excelling especially in mathematics.

At La Flèche, Descartes’ professors favored the Aristotelian method of study, which held that nature was inherently stable and ordered and that one could rely on information derived from sense perceptions to deduce truths. Descartes would later question this fundamental tenet of his education. The college also taught mathematics separately from the study of physical world, which was founded on philosophy, rather than what we now consider scientific method. Descartes had doubts about this divide, and one of the major results of his later work was the use of mathematics in the study of physics.

After leaving La Flèche, Descartes enrolled in the University of Poitiers, and he obtained a law degree in 1616. Despite his ill health, he then enlisted in the military. His military service, along with his family’s modest wealth, gave Descartes the opportunity to travel. He happily settled in one foreign locale after another for most of his life. While in Holland in 1618, Descartes composed a brief treatise on music, titled Compendium Musicae, not published until after his death. The next year, Descartes traveled in Germany, where, in a stove-heated room on November 10, 1619, he had a vision of a new system of mathematics and science. He would later tell the story of this revelation in Discourse on the Method.

In 1628, Descartes began to compose Rules for the Direction of the Mind, a short treatise outlining a new method of thought. By using a set of rational principles, Descartes had been able to eliminate many of his own doubts about fundamental ideas. Although the book was originally intended to be composed of three sections of twelve rules, Descartes only completed the first twelve. These first twelve deal with simple propositions. The incomplete second set covers a method for dealing with “perfectly understood problems”—that is, problems that can be expressed through simple mathematical equations. The third section was intended to deal with “imperfectly understood problems,” problems too complex to be reduced to an equation. Descartes hoped to show that even these problems could be expressed through mathematics.

In 1633, the Inquisition issued a formal condemnation of the work of the Italian scientist Galileo. He argued, contrary to the traditional notion that Earth was the center of the universe, that Earth revolves around the sun. Galileo was condemned to death for heresy, but his sentence was later reduced to house arrest. At the time, Descartes was working on The World, a study he thought would revolutionize the study of physics. After Galileo’s house arrest, Descartes voluntarily suppressed The World, fearing the wrath of the Catholic Church.

It wasn’t until 1636, when Descartes was forty, that he published his first work, Discourse on the Method, a discussion of how he had made use of the rules he’d begun to lay out in Rules for the Direction of the Mind. Discourse on the Method relates the series of revelations Descartes had in 1619 while in the stove-heated room in Germany. After confessing how he came to doubt all his knowledge, Descartes shows how he used his rules to solve profound problems. He resolves the problem of personal existence in one of the most famous philosophical statements of all time, Cogito ergo sum, or “I think, therefore I exist.” He also offers rational proofs that the human mind is separate from the body, that the mind outlives the body, and that God exists. The Discourse was meant to serve as an introduction to three essays Descartes had been laboring over—Optics, Meteorology, and Geometry—which contain science now regarded obsolete. The Discourse, however, remains one of the world’s most influential works of philosophy.

The work that cemented Descartes’ fame was Meditations on First Philosophy (1641). Here Descartes addresses the concerns and attempted refutations various readers sent to him after reading the Discourse. The theories in Meditations would change the way people thought about their minds and bodies and the relationship between the two, but Meditations also contains arguments that later became known as the “Cartesian Circle” because of the apparent circularity of their logic. Meditations was followed by Principles of Philosophy (1644), which attempts to reduce the universe to its mathematical foundation. By the time Principles was widely read in Europe, Descartes was the toast of continental intellectual circles and was awarded a pension by the king of France.

But as his fame grew, so did the demands on his time. In 1649 Descartes moved from Holland to Stockholm, Sweden, at the request of Sweden’s nineteen-year-old Queen Christina, and agreed to work as her philosophy tutor. Ignoring Descartes’ poor health and his preference for staying in bed until noon, Queen Christina scheduled her lessons with him for 5:00 a.m. Lack of sleep and inhospitable living conditions took their toll on Descartes, and, in 1650, he died of pneumonia at age fifty-four. Despite his attempts to stay on the church’s good side, Descartes’ books were placed on the Index of Prohibited Books after his death, so for years no Catholic was allowed to read them.

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