Descartes opens by asserting that everyone is equally well endowed with reason. Following scholastic philosophy, he claims that we are essentially rational animals, and while we may differ with respect to our accidental, or non-essential, properties, we must all share the same form, or essential properties. Since we are all equally human, we must all be equally rational. People have different opinions and arrive at the truth with varying degrees of success not because some people are better equipped with reason than others but because different people apply their reason in different ways.
Descartes proposes to share a method that he discovered in his youth that he believes has helped him increase his knowledge to the greatest possible extent given his own limitations. While he does not feel his reason is any better than anyone else's he does feel he's discovered a very effective method of applying it. Some people may not find this method useful, but he proposes to put it forth not as a guideline that all must follow, but merely as a description of the path he has followed in the hope that some might similarly profit from it.
Because his method is so tied up with the manner in which he's lived, his account is of necessity autobiographical. He begins with his youth, growing up in one of the finest schools in Europe. The education he gained there, he was told, would provide him with certain knowledge of everything useful in life. But at the end of his education he only found himself riddled with doubts, feeling he'd learnt nothing but an awareness of his own ignorance. He had been a strong student at one of the finest schools in one of the most enlightened ages in history, so he doubts that his disappointment was a result of not learning this certain knowledge that he'd been promised. Rather, he suggests, there was no such knowledge to be learned.
He did not discard the works he had learned in school, but he resolved not to study any further. He lauds the virtues of studying ancient texts, fables, history, oratory, poetry, mathematics, morality, theology, philosophy, and the other sciences, but also explains why they do not prove ultimately satisfying. Too many texts from or about other times can distance one from one's own time, while oratory and poetry seem to rely on innate skill rather than careful study. He deeply admired mathematics, but did not perceive its higher uses since it was mostly applied in engineering. Morality was usually poorly reasoned and studying theology was not likely to unlock the secrets of heaven. Philosophy has been disputed over for millennia without any real agreements, and Descartes doubts that he could settle what the greatest minds of past generations have failed to achieve. Lastly, the sciences are built upon the premises of philosophy and so are as uncertain as their foundations.
Instead, Descartes decided to abandon his books and see what he could learn in traveling the world. He learnt that people have all sorts of different customs, and what might seem strange in his native France is well accepted in great nations abroad. This helped him to mistrust anything that he had learned simply through custom and example and to trust his reason above all. One day, Descartes resolved to pursue studies within himself rather than in the world, to look inward and see what he could dig out by means of his reason. In this study, he feels he has had far greater success than anything he has learned from books or traveling.
Descartes was educated at the Jesuit college of La Fleche—considered one of the finest schools of the age—from the age of ten to eighteen or nineteen. He then took a doctorate in law at the University of Poitiers. The travels he describes after his years of study took place mostly in Holland and Germany, serving in the armies of Maurice of Nassau and then Maximilian of Bavaria. He left France in 1618, at the age of twenty-two, and returned in 1622.
Descartes describes a growing trend in the youth of his generation, something that Thomas Kuhn calls a "paradigm shift." There was a growing dissatisfaction with, and skepticism toward, the scholastic philosophy that had been inherited from Aristotle. This is not so much to say that people stopped being interested in the things that previous generations had been interested in. Rather, people grew dissatisfied with a paradigm of knowledge, of an understanding of what knowledge was, of how it could be learned, and of what value it could have.
Aristotelian logic, and hence Aristotelian science, works according to a method of syllogism and demonstration. One starts with a premise that one knows with certainty by means of intuition, and then one deduces consequences from it by means of a syllogism. A syllogism is a kind of logical argument with three steps and three terms. For example, "All y's are z; x is a y; therefore, x is a z." If we are certain about the first two statements, then we can deduce the third statement with equal certainty.
According to Aristotle, and to the two thousand-year old tradition that used his ideas, scientific knowledge is certain knowledge deduced from certain premises. This is the kind of knowledge that Descartes was promised as a part of his education and that he came to find unsatisfying. We might be inclined to sympathize with Descartes on this one. Among the irrefutable demonstrations of Aristotelian philosophy are the assertions that the earth is the center of the universe, that women are naturally inferior to men, and that the world is made up of the four elements of earth, air, fire, and water.
Aristotelian scholasticism was not overthrown by a set of scientific discoveries. Rather, these discoveries were a result of a revolution in the way we think about science. Galileo and Descartes were two of the early exponents of a new scientific method that relies on hypothesis and experiment rather than on demonstration and syllogism. This method does not pretend to provide certainty, but only proposes theories and models that fit the facts and provide plausible explanations of natural phenomena. It took a long while before people came to accept that a sound theory, and not certainty, was the highest possible aspiration of science. For instance, one of the main arguments the Inquisition made against Galileo was that his claim that the earth went around the sun was not demonstrated knowledge. They were perfectly happy to accept that it was a plausible theoretical model, but they were trapped in an ancient worldview according to which theoretical models and demonstrations of certainty were two very different things. Galileo was accused of claiming that his model was a demonstration of certainty rather than a theoretical model.
The Discourse on the Method is thus a rather tricky book, because it is part of a revolution in its early stages. Descartes not only must pay lip service to Aristotelian philosophy, but he also has not entirely freed himself from that mindset. For instance, we find him arguing early on that we are all equally rational because reason is a form, and not an accident, of human nature. The distinction between form and accident is quintessentially Aristotelian. The idea is that we have essential properties—like reason—without which we would not be what we are. A human being without reason is not a human being. We also have accidental properties—like legs—without which we could still be human beings. As humans we can only differ with respect to our accidental properties, but not with respect to our form (our essential properties). Thus, we must all have reason, and have it to an equal extent.
This assertion also identifies Descartes as a rationalist philosopher. The early modern period in philosophy, of which Descartes is the founding father, was split roughly into two camps: the British empiricists and the Continental rationalists. Empiricists, such as John Locke, asserted that the mind is a blank slate at birth, and all knowledge comes from experience. Descartes, on the other hand, maintains that there is a certain something—our native intellect or reason—that we are born with and all share.