Jules Henri Poincaré, the French mathematician and philosopher, was born into a distinguished family at Nancy. His cousin Raymond was both prime minister and president of the Third French Republic. At an early age Poincaré showed an interest in natural history and the classics, and at the age of fifteen he developed an interest in mathematics. However, he trained first as a mining engineer, studying mathematics on his own during this training. In 1879 he was appointed to teach courses in mathematical analysis in the Faculty of Science at Caen. In 1881 he moved to the University of Paris, where he was soon given charge of the courses in mathematics and experimental physics. He lectured on mechanics, mathematical physics, and astronomy. Poincaré wrote an enormous number of papers on mathematics and physics and several important books on the philosophy of...