  Atomism in the form in which it first emerged in Ancient Greece was a metaphysical thesis, purporting to establish claims about the ultimate nature of material reality by philosophical argument. Versions of atomism developed by mechanical philosophers in the seventeenth century shared that characteristic. By contrast, the knowledge of atoms that is now taken for granted in modern science is not established by a priori philosophical argument but by appeal to quite specific experimental results interpreted and guided by a quite specific theory, quantum mechanics. If metaphysics involves an attempt to give an account of the basic nature of material reality then it is an issue about which science rather than philosophy has most to say. A study of the path from philosophical atomism to contemporary scientific atomism helps to shed light on the nature of philosophy and science and the relationship between the two.