Paul Natorp's monumental work, Plato's Theory of Ideas. An Introduction to Idealism (1903, 2nd ed. 1921), is a fascinating and highly original work which merits renewed study not only as a significant document of German Neokantianism, but as an important contribution to our understanding of Plato.Natorp's interpretation, far from being an uncritical projection of Kant back onto Plato, is a remarkable attempt to dissociate the theory of ideas from its Aristotelian reception, still dominant today, which sees the ideas as transcendent substances, and to argue that Plato's theory is basically a theory of explanation.In particular, Natorp argues that Plato's ideas are simply explanations, or laws, rather than substances; and what the ideas explain is in the first instance the possibility and nature of thought and knowledge, and only as a consequence the nature of reality. Natorp emphasizes the broadly Kantian origins of his approach, but he takes issue with Kant on a number of crucial points, and this not least is what allows him to defend a transcendental interpretation of Plato's theory of ideas.This volume contains a complete translation of Plato's Theory of Ideas; an introduction which summarizes Natorp's interpretation of Plato, its relation to Aristotle and Kant, and its continued importance; along with a postscript which positions Natorp within German, and in particular Marburg, Neokantianism.