That normal science proceeds on the basis of perceived similarity to exemplars is an important and distinctive feature of Kuhn’s new picture of scientific development. The standard view explained the cumulative addition of new knowledge in terms of the application of the scientific method. Allegedly, the scientific method encapsulates the rules of scientific rationality. It may be that those rules could not account for the creative side of science—the generation of new hypotheses. The latter was thus designated ‘the context of discovery’, leaving the rules of rationality to decide in the ‘context of justification’ whether a new hypothesis should, in the light of the evidence, be added to the stock of accepted theories. 