René Descartes published 'La Géométrie' as an appendix to his philosophical work 'Discourse on the Method,' revolutionizing mathematics by establishing analytic geometry. Descartes introduced a coordinate system (now known as the Cartesian coordinate system) that allowed geometric shapes to be described using algebraic equations. This breakthrough unified algebra and geometry, enabling the representation of geometric objects as equations and vice versa. His innovation made it possible to solve geometric problems algebraically and created the foundation for calculus, analytic geometry, and eventually modern mathematical physics.