One of the central tasks of the philosophy of science is to give an account of scientific progress. We have more reason than our ancestors to wonder whether economic and political history show forth the increasing moral perfection of mankind; skeptics and pessimists conclude that the "Myth of Progress' has been exploded once and for all. But myths do not die easily, and the history of ideas seems to provide examples of the actual growth of knowledge and not merely of changes in intellectual fashion. In the history of science, paradigmatically, we can trace out both the growing acceptance of theories of increasing scope, generality and explanatory power and the declining influence of theories which fail to square with the facts of nature. Surely this is intellectual progress from ignorance to knowledge! The philosophical question becomes: How is scientific progress possible?