Discourse on the Method

The Discourse on the Method (1637) is one of the rare philosophical texts written as autobiography. Descartes tells the story of his dissatisfaction with the learning of the schools, his travels through the world as a kind of living book, and his eventual resolution to doubt everything he could doubt in order to find something certain.

The method itself has four rules: accept only what is clear and distinct, divide problems into their simplest parts, work from simple to complex, and review the whole to avoid omissions. Part IV applies this method to metaphysics, yielding the cogito: even if everything he believed about the external world were false, the very act of doubting proves that he exists as a thinking thing. From this first certainty Descartes reconstructs knowledge, proving the existence of God and using divine veracity to underwrite clear and distinct perception as a criterion of truth.

Part V applies the method to physics and biology, including the beast-machine hypothesis: animals are automata, driven by mechanism with no inner experience. Part VI announces the ambition of the new natural philosophy, to make men "masters and possessors of nature," with medicine and practical mechanics as its chief fruits.

The Discourse launched the program that defines modern philosophy: the demand for certainty from a standpoint outside tradition and authority, and the reconstruction of knowledge from a single indubitable point. Locke, Hume, and Kant spent the century following Descartes trying to honor that demand while dismantling the particular structure he built.