 In the last part of the 5th century, Diogenes of Apollonia (active after 440 BCE) revived and revised the Milesian system of cosmology, claiming that “all the things that are are alterations from the same thing and are the same thing” (64B2/LM28D3); he identified this single basic substance with air, like Anaximenes more than a century before (Graham 2006, Laks 2008, 2008a). Diogenes takes care to give arguments for the reality and properties of his basic principle. In DK64B2/LM28D3 he says that only things that are alike can affect one another. If there were a plurality of basic substances, each differing in what Diogenes calls their “own proper nature,” there could be no interaction between them. Yet the evidence of the senses is clear: things mix and separate and interact with one another. Thus, all things must be forms of some one single thing. Like Anaxagoras, Diogenes claims that the cosmic system is ordered by intelligence, and he argues that that “which possesses intelligence (noēsis) is what human beings call air” (DK64B5/LM28D10). Humans and animals live by breathing air, and are governed by it —in them air is both soul and intelligence, or mind (DK64B4/LM28D2). Moreover, Diogenes argues, air governs and rules all things and is god (DK64B5/LM28D10). Thus, like Anaxagoras, Diogenes has a theory grounded in intelligence, although Diogenes is more fully committed to teleological explanations, insofar as he states explicitly that intelligence (noēsis) orders things in a good way (DK64B3/LM28D56). In presenting his arguments, Diogenes fulfills his own requirement for a philosophical claim. In DK64B1/LM28D2 he says, “In my opinion, anyone beginning a logos (account) ought to present a starting principle (archē) that is indisputable and a style that is simple and stately.” He notes that his theory that air is soul and intelligence “will have been made clearly evident in this book” (DK64B4/LMD9). 