Discourse on the Method is Descartes’ attempt to explain his method of reasoning through even the most difficult of problems. He illustrates the development of this method through brief autobiographical sketches interspersed with philosophical arguments.

Part 1 contains “various considerations concerning the sciences.” First, all people possess “good sense,” the ability to distinguish truth from fiction. Therefore, it is not a lack of ability that obstructs people but their failure to follow the correct path of thought. The use of a method can elevate an average mind above the rest, and Descartes considered himself a typical thinker improved by the use of his method. Descartes benefited from a superior education, but he believed that book learning also clouded his mind. After leaving school, he set off traveling to learn from “the great book of the world” with an unclouded mind. He comes to the conclusion that all people have a “natural light” that can be obscured by education and that it is as important to study oneself as it is to study the world.

In part 2, Descartes describes his revelation in the “stove-heated room.” Contemplating various subjects, he hits on the idea that the works of individuals are superior to those conceived by committee because an individual’s work follows one plan, with all elements working toward the same end. He considers that the science he learned as a boy is likely flawed because it consists of the ideas of many different men from various eras. Keeping in mind what he has learned of logic, geometry, and algebra, he sets down the following rules: (1) to never believe anything unless he can prove it himself; (2) to reduce every problem to its simplest parts; (3) to always be orderly in his thoughts and proceed from the simplest part to the most difficult; and (4) to always, when solving a problem, create a long chain of reasoning and leave nothing out. He immediately finds this method effective in solving problems that he had found too difficult before. Still fearing that his own misconceptions might be getting in the way of pure reason, he decides to systematically eliminate all his wrong opinions and use his new method exclusively.

In part 3, Descartes puts forth a provisional moral code to live by while rethinking his views: (1) to obey the rules and customs of his country and his religion and never take an extreme opinion; (2) to be decisive and stick with his decisions, even if some doubts linger; (3) to try to change himself, not the world; and (4) to examine all the professions in the world and try to figure out what the best one is. Not surprising, Descartes determines that reasoning and searching for the truth is, if not the highest calling, at least extremely useful. For many years after his revelation, Descartes traveled widely and gained a reputation for wisdom, then retired to examine his thoughts in solitude.

In part 4, Descartes offers proofs of the existence of the soul and of God. Contemplating the nature of dreams and the unreliability of the senses, he becomes aware of his own process of thinking and realizes it is proof of his existence: I think, therefore I exist (Cogito ergo sum). He also concludes that the soul is separate from the body based on the unreliability of the senses as compared with pure reason. His own doubts lead him to believe that he is imperfect, yet his ability to conceive of perfection indicates that something perfect must exist outside of him—namely, God. He reasons that all good things in the world must stem from God, as must all clear and distinct thoughts.

Part 5 moves from discussion of a theory of light to theories about human anatomy. Descartes considers the fact that animals have many of the same organs as humans yet lack powers of speech or reason. He takes this difference to be evidence of humankind’s “rational soul.” He considers the mysterious connection of the soul to the body and concludes that the soul must have a life outside the body. Therefore it must not die when the body dies. Because he cannot conceive of a way that the soul could perish or be killed, he is forced to conclude that the soul is immortal.