This major study of Kant provides a detailed examination of the development and function of the doctrine of transcendental illusion in his theoretical philosophy. The author shows that a theory of 'illusion' plays a central role in Kant's arguments about metaphysical speculation and scientific theory. Indeed, she argues that we cannot understand Kant unless we take seriously his claim that the mind inevitably acts in accordance with ideas and principles that are 'illusory'. Taking this claim seriously, we can make much better sense of Kant's arguments and reach a deeper understanding of the role he allots human reason in science.