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Pythagoras of Samos (c 569 BC - c 475 BC) - an ancient Greek mathematician. None of his writings survive, and we know relatively little about the mathematics which he did. He is said to have been the leader of a religious/scientific group and to have worked on arithmetic and geometry. The theorem which we know as Pythagoras' theorem was known long before Pythagoras' time, although Pythagoras himself might have been the first to prove it. Pythagoras is said to have been one of the first to believe that the world is fundamentally mathematical, that phenomena are ruled by mathematical processes. He was interested in the abstract nature of mathematical concepts such as the natural numbers, or the idea of a proof. The Pythagoreans are said to have discovered the irrational numbers, the five regular (Platonic) solids, and to have shown that the sum of the angles in a triangle is equal to two right angles.