Kuhn and Feyerabend formulated the problem, Dilworth provides the solution. In the fourth edition of this highly original book, Craig Dilworth answers the questions raised by the incommensurability thesis. Logical empiricism cannot account for theory conflict. Popperianism cannot account for how one theory is a progression beyond another. Dilworth's Perspectivist conception of science covers both bases with a concept of scientific progress based on both rationalism and empiricism.