1637: Descartes publishes his Discourse on the Method for Guiding One’s Reason and Searching for Truth in the Sciences, the source of the famous quote, “I think, therefore I am.” He outlines his rules for understanding the natural world through reason and skepticism, forming the foundation of the scientific method still in use today.

Born in France in 1596, Rene Descartes was raised with the standard Jesuit education of the day — Latin, Greek, mathematics, classical philosophy — and even studied to be a lawyer like his father. But at age 20, he quit law in order to travel. During a brief stint in the army, Descartes decided to devote his life to tearing down everything he had been taught was true and building it back up on a firm foundation, a project he said came to him in a dream.

“I gave up completely the study of the humanities and [resolved] not to search for any other science apart from what could be found in myself or the great book of the world,” he wrote in Part 1 of the Discourse on Method.

In the Discourse, Descartes described four rules he established to make sure he always came to true conclusions.

1. Doubt everything. “The first was never to accept anything as true if I did not know clearly that it was so … and to include nothing in my judgments apart from whatever appeared so clearly and distinctly to my mind that I had no opportunity to cast doubt on it.”
2. Break every problem down into smaller parts.
3. Solve the simplest problems first, and build from there.
4. Be thorough. “The final rule was: In all cases, to make such comprehensive enumerations and such general reviews that I was certain not to omit anything.”

By following these simple guidelines, he said, “There cannot be anything so remote that it cannot eventually be reached nor anything so hidden that it cannot be uncovered.”

The first precept of doubting everything, including the evidence of his senses and the thoughts in his head, led Descartes to the one fundamental truth on which he could base everything else: He existed. The senses sometimes lie, Descartes reasoned, and thoughts that occur in dreams feel just as real as thoughts that come while awake. But the fact that Descartes wondered about the truth of his thoughts and sensations meant that something must be doing the wondering.

“While I thus wished to think that everything was false, it was necessarily the case that I, who was thinking this, was something,” he wrote. “From the very fact that I was thinking of doubting the truth of other things, it followed very evidently and very certainly that I existed.” He expressed this thought concisely and memorably as cogito ergo sum: I think, therefore I am.

Descartes also expressed his preference for mathematics as the basis and language of his new method, “because of the certainty and self-evidence of the way it reasons.” In one of the essays that was published with the Discourse, titled Geometry, Descartes introduced the Cartesian coordinate system, which influenced the development of calculus and is still widely used.

Despite its fame and influence, the Discourse on Method was just a sketch of a larger argument. For years before the Discourse’s publication, Descartes had been working on a comprehensive treatise on a new physics called The World, or a Treatise on Light.

But just before it was ready to go to print in 1633, Descartes got wind of Galileo’s condemnation by the Inquisition for his ideas about the Earth revolving around the sun. Realizing that his own book could raise some of the same theological problems, Descartes canceled the publication and hid the manuscript. The first part wasn’t published until 1662, 12 years after the author’s death. And Descartes was right to worry: In 1663, the pope placed The World and the rest of Descartes’ works on the Index of Prohibited Books.

Still, the Discourse on Method is widely regarded as one of the most influential works in the history of science, and as marking the beginning of the Scientific Revolution. “For it is not enough to have a good mind; it is more important to use it well.”

Source: Discourse on the Method for Guiding One’s Reason and Searching for Truth in the Sciences, Various

Image: Detail from Dispute of Queen Cristina Vasa (left) and René Descartes (right) by Nils Forsberg (1842-1934) after Pierre-Louis Dumesnil the Younger (1698-1781)