Now well into its fourth decade, What is this thing called science? has become something of a classic the world over (it's available in nineteen languages). Each decade Alan Chalmers has drawn on his experience as a teacher and researcher to improve and update the text. The most significant feature of this new, fourth, edition is the addition of an extensive postscript, in which Chalmers uses the results of his recent research into the history of atomism to illustrate and enliven key themes in the philosophy of science. Identifying the qualitative difference between knowledge of atoms as it figures in contemporary science and metaphysical speculations about atoms common in philosophy since the time of Democritus proves to be a highly revealing and instructive way to pinpoint key features of the answer to the question What is this thing called science?' This new edition ensures that the book holds its place as the leading introduction to the philosophy of science for the foreseeable future.