In The Structure of Scientific Revolutions, Thomas S. Kuhn presented a model of scientific development on which science is characterized by periods of unified research intermittently disrupted by revolutionary change of paradigm. Ever since Kuhn first proposed this model of scientific theory change, relativism, in one form or another, has been associated with his work. There has, for example, been widespread discussion of Kuhn's suggestion that scientific rationality varies relative to the changing rules and standards employed by different paradigms. There has also been much discussion of his account of conceptual change in science by philosophers who saw in it an extreme conceptual relativism of radically incommensurable conceptual schemes. Yet in recent years Kuhn has retreated from many of the claims which were responsible for these earlier reactions to his position. In his later work, Kuhn presents instead an ontological form of relativism, which involves an anti-realist denial of objective natural kinds.