This chapter looks at Kuhn’s position on substantive philosophical issues such as realism, relativism and relativism. We begin by looking at Kuhn’s notorious “different worlds” thesis in Structure, which lead to charges of idealism, before moving on to examine Kuhn’s later-career identification with Kant and how justified this was. We then look at Kuhn’s reasons for rejecting the correspondence theory of truth, before concluding with Kuhn’s reasons for rejecting the traditional concept of progress in science and replacing it with an ‘evolutionary’ conception of progress.