Early Enlightenment French philosopher and mathematician René Descartes invented a new genre of philosophy, we might say, one that would dominate the century to come. Before Locke, Leibniz, or Kant, Descartes stood out as a "theist rationalist.” Rather than trusting in revelation, he leaned solely on logic and reason, creating a set of “rules for the direction of the mind,” the title of one of his books. He believed we might think our way—solely unaided by unreliable external sources—to belief in God and “all the knowledge that we may need for the conduct of life.”
Descartes’ proofs of God may not sound so convincing to modern ears, slipping as they do into the language of faith when convenient. But in other respects, he seems distinctly contemporary, or at least like a contemporary of Ludwig Wittgenstein. He believed that philosophy suffered from improper definitions and lacked clarity of thought. And like the early 20th-century logical positivists, he put tremendous store in logic and mathematics as analytic tools for acquiring knowledge about the world. These, along with the scientific method Descartes championed, were indeed the sole means of acquiring such knowledge.
Descartes, then, has become known for introducing the radical “method of doubt,” which supposedly strips away all prejudice and preconception, every article of belief, to get at the most fundamentally ascertainable core of knowledge. Upon doing this in his 1637 Discourse on Method, the French philosopher famously found that the only thing he could say for certain was that he must exist because he could see himself doubting his existence—cogito ergo sum, “I think therefore I am.” The process involved casting aside all authority and tradition, which made Descartes a hero to French Revolutionists. His freethinking also made him very much the enemy of many in the Catholic church.
Describing in Discourse on Method how he had abandoned all reliance on other texts and resolved to derive the answers to his questions from experience and reason, he seemed to dismiss the authority not only of church hierarchy and dogma but of scripture itself. Rather than fixing God at the center of the universe, Descartes used the “Archimedean point” of his own certain existence to anchor “an epistemologically unsteady world.” Nonetheless, he was committed to keeping faith intact, even as he seemingly demolished the foundations of its existence, including—for Catholics—the cherished idea that priests could turn bread into flesh.
It might have been an attempt at self-preservation or appeasement, but it seems more to reflect sincere belief: in the Meditations on First Philosophy, Descartes sought to prove the existence of God in much the same way as he had proved his own existence, through circular reasoning and arguments that split mind and matter into two distinct camps. Descartes created a dualist view of the world that became a major problem in his philosophy. At the time, many of his critics were less concerned with this ontological puzzle than they were with the possibility of his heretical thought interfering in world affairs.
Descartes’ radical doubt threatened not only church doctrine but also church politics. One scholar claims to have found evidence that a Catholic priest—fearing the French freethinker would jeopardize the conversion of Sweden’s Queen Christina to Catholicism—murdered Descartes with an arsenic-laced communion wafer. If so, it would have been a cruelly ironic death, perhaps by design, for the man who dared to write in the Meditations that transubstantiation—one of the Church’s central supernatural teachings—should be “rejected by theologians as irrational, incomprehensible and hazardous for the faith,” and to hope for a time when “my theory will be accepted in its place as certain and indubitable.”