Pythagoras (c.580-500 BC). Illustration of the Ancient Greek mathematician and philosopher, Pythagoras. Pythagoras is most famous for the mathematical proof, concerning triangles, that is named after him (Pythagorean theorem). The movement that he founded was one of the first to discover the mathematical concept of irrational numbers. They realised that the square root of two cannot be represented as a fraction of two whole numbers. Much of Pythagoras' life is shrouded in myth and legend. He established an eccentric religious community in southern Italy based on the mystical belief that 'all is number'.