Anaximander, in the sixth century BCE, had a theory of evolution, and maintained that men are descended from fishes. This was philosophy because it was a speculation unsupported by detailed evidence, but Darwin's theory of evolution was science, because it was based on the succession of forms of life as found in fossils, and upon the distribution of animals and plants in many parts of the world. A man might say, with enough truth to justify a joke: 'Science is what we know and philosophy is what we don't know.' But it should be added that philosophical speculation as to what we do not yet know has shown itself a valuable preliminary to exact scientific knowledge. The guesses of the Pythagoreans in astronomy, of Anaximander and Empedocles in biological evolution, and of Democritus as to the atomic constitution of matter, provided the men of science in later times with hypotheses which, but for the philosophers, might never have entered their heads. We may say that, on its theoretical side, philosophy consists, at least in part, in the framing of large general hypotheses which science is not yet in a position to test; but when it becomes possible to test the hypotheses they become, if verified, a part of science, and cease to count as philosophy.