Discourse on the Method of Rightly Conducting One’s Reason and of Seeking Truth
The Discourse on Method is best known as the source of the famous quotation “cogito ergo sum”, “I think, therefore I am.” …. It is a method which gives a solid platform from which all modern natural sciences could evolve. With this work, the idea of skepticism was revived from the ancients such as Sextus Empiricus and modified to account for a truth that Descartes found to be incontrovertible. Descartes started his line of reasoning by doubting everything, so as to assess the world from a fresh perspective, clear of any preconceived notions.
(Summary by Wikipedia)