 Natorp’s most influential writings are in the interpretation of Plato, the philosophy of science, and in the philosophy of psychology. Starting with his habilitation (“Descartes’s Theory of Knowledge: A Study in the Pre-history of Criticism”) in 1881, Natorp extended the reach of the Marburg school as historians of philosophy beyond Kant to other philosophers who in various ways anticipated the Kantian themes important to the Marburgers. Natorp’s most significant work in this vein was his book Plato’s Theory of Ideas (1903), which gave a serious scholarly argument for the dating of Plato’s dialogues, in addition to a novel “critical” reading of Platonic ideas. In the philosophy of science, his systematic work The Logical Foundations of the Exact Sciences (1910) gives a systematic philosophy of logic, arithmetic, geometry, and physics that engages with contemporary science and mathematics in a more comprehensive and detailed way than any of Cohen’s works. Last, in a series of works in the philosophy of psychology (culminating in General Psychology According to the Critical Method (1912b)), Natorp defended a novel conception of psychology as the science of the subjective.