Every ten years, Alan Chalmers draws on his experience as a teacher and researcher to improve and update the text that strives to answer the philosophical question in it's title: What is This Thing Called 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 that question. The most significant feature of this fourth edition is the extensive postscript, in which Chalmers uses the results of his recent research on the history of atomism to illustrate and enliven key themes in the philosophy of science. This new edition ensures that the book holds its place as the leading introduction to the philosophy of science for the foreseeable future.