Euclid's Elements and the axiomatic method
also in The history of mathematics
Euclid gathered the geometry and number theory of his age into thirteen books built from a handful of definitions, postulates, and step-by-step proofs. The Elements set the model for how mathematics is done, starting from stated assumptions and deducing everything else, and it served as a textbook for over two thousand years. Little is known of Euclid himself, and much of the content he organized rather than discovered.