Discourse on the Method

Doubt is where Rene Descartes begins in this philosophical and autobiographical treatise: he resolves to question everything in order to look at the world afresh, free of preconceived notions. From that starting point he proves to his own satisfaction that the self exists, the source of the famous line I think, therefore I am, which appears in Part IV, then deduces the existence and nature of God and gives a fresh account of human and animal nature and of the physical world. Descartes wrote it in French rather than Latin to reach a wider readership. It was meant as an introduction to three works, among them La Geometrie, which holds his early ideas for Cartesian coordinates.

Yuval Noah Harari has said in an interview that it changed his life.