Martin Kusch explores the origins of psychologism through the work of two major figures in the history of twentieth century philosophy, Gottlob Frege and Edmund Husserl. His sociological and historical reconstruction shows how the power struggle between the experimental psychologists and pure philosophers influenced the thought of these two philosophers, shaping their agendas and determining the success of their arguments for a sharp separation of logic from psychology. A move that was crucial in the creation of the distinct discipline of psychology and was responsible for the anti-naturalism found in both the analytic and the phenomenological traditions in philosophy. Students and lecturers in philosophy, psychology, linguistics, cognitive science and history will find this study invaluable for understanding a key moment in the intellectual history of the twentieth century.