More than any other writer of the twentieth century, Henri Poincar brought the elegant, but often complicated, ideas about science and mathematics to the general reader. A genius who throughout his life solved complex mathematical calculations in his head, and a writer gifted with an inimitable style, Poincar rose to the challenge of interpreting the philosophy of science to scientists and nonscientists alike. His lucid and welcoming prose made him the Carl Sagan of his time. This volume collects his three most important books: Science and Hypothesis (1903); The Value of Science (1905); and Science and Method (1908).