The ancient sources situate the maturity of Anaximander of Miletus a little before the middle of the sixth century BC. Like Thales, of whom he is said to have been the disciple, he is credited by the biographical tradition with political activity, connected to the colonial expansion of Miletus. Again like Thales, various inventions are attributed to him, notably the gnômôn, the construction of a “sphere” (i.e. a tridimensional model of the universe), and a geographical map. His doctrine, unlike Thales’, has outlines we can grasp. Only a single sentence of his has been transmitted in its original wording. But the fairly numerous testimonia indicate that Anaximander recounted the generation of the world and of its constitutive parts all the way to living beings, explained its present function, and envisaged its disappearance. Thus he stands at the origin of a new kind of investigation bearing upon the totality of the world. One tradition calls Anaximander the first Greek to have written a treatise on nature. Theophrastus called the style of the phrase he transmits “poetic”; nevertheless, this must have been a text in prose. The ‘unlimited,’ from which everything that exists derives and to which everything returns, and ‘separation’ are the two concepts that