The history of science is as uncertain and as relative a piece of knowledge as the history of any other pursuit or institution. The usual rules are, first, 'back to the sources' and, second, to interpret the sources by subtracting obvious or latent personal and ideological bias. The philosophy of science, even more than the philosophy of literature or sociology, determines what history of science the historian of science will find. The tenuous relationship between the sources and the story of art, literature, society, politics and economics upon which historians of these subjects can rely, at least to start with, is infinitely more tenuous when one is dealing with the source material of the history of science. When philosophers of science had to admit that this simple disregard of the history of science was untenable, they fell back upon a different line of reasoning.