In this book, Ulrich Steinvorth offers a fresh analysis of rationality as the core part of Western thinking. Western rationality includes a critique of tradition and collectivism and a defence of human rights and individualism, but is impregnated in all its elements by a conception of the self that was formed by Locke and utilitarianism. This conception is compatible with classical physics, but is no help in understanding the facts of human psychology and history. Steinvorth argues that Descartes' conception of the self offers an alternative. When freed from the dualism in which Descartes conceived it, it achieves what the Lockean conception does not. In particular, it allows understanding the human craving for extraordinariness and the achievements of the West in science and art as well as its political disasters in the twentieth century. Moreover, it enables us to understand why individualism - a hallmark of modernity - became an ideal that implies universal rights; how individualism could peak in the ideal of equal liberty; and why it is now in decline. Most importantly, the Cartesian concept of the self is shown to offer a way of protecting modernity against the dangers that it now encounters.