Descartes claims to have found a particularly effective method of guiding his reason that has helped him to make many significant discoveries in his scientific research. He undertakes to explain his method by means of autobiography: he tells the story of his intellectual development and of how he came upon this method.
He developed his method largely in reaction to the schooling in Aristotelian philosophy that he received at the hands of the Jesuits. He had been told that he would find knowledge and certainty in his schooling, but came out thoroughly dissatisfied. He had found no certainty, only ever-increasing doubts, so he left school, and traveled the world, learning about different people and different customs.
The real turning point comes on November 10, 1619, when he spends a day alone in a room with his thoughts. He decides to call into doubt all his former beliefs and opinions, holding on only to certain guiding principles and certain moral maxims that would help him live productively during this period of doubt. Applying these principles to algebra and geometry he has great success, discovering analytic geometry.
After nine years of travel, he settles in Holland and begins a systematic philosophical investigation. He finds he can doubt pretty much everything except the fact that he exists. The very act of doubting suggests to him that he must exist, or else he would not be able to doubt. He concludes: "I am thinking, therefore I exist." His knowledge of this claim is a "clear and distinct perception": it is not something that he learns through reasoning, but something that he simply knows because he is incapable of doubting it. He concludes further that he is essentially a thinking thing, and that his soul is distinct from his body. He also provides two arguments to prove the existence of God.
Descartes claims he has also developed a set of scientific principles that have allowed him to make a great many discoveries. He had initially planned to publish these in a work entitled The World, but suppressed the manuscript when he learnt of Galileo's condemnation by the Inquisition. Instead, he provides a brief summary of the sorts of things he discusses in that work. He claims that he would rather remain free from controversy during his lifetime so that he can devote his energy to further research rather than bitter disputes. The three essays—on optics, meteorology, and geometry—are meant to serve as examples of how his method can be applied. He also hopes that his publication of these essays will lead others to contribute their thoughts in those fields as well.