Paul Rée (1849-1901), an interdisciplinary traveller and admirer of the French moralistes, but also of Schopenhauer and Darwin, was for a time one of the closest friends of Lou Andreas-Salomé and Friedrich Nietzsche. With his publications, Rée wanted to convert moral philosophy into moral science.This edition presents his books Psychologische Beobachtungen (Psychological Observations) (1875), Der Ursprung der moralischen Empfindungen ( The Origin of Moral Sensations) (1877), Die Entstehung des Gewissens (The Genesis of Conscience) (1885) and Die Illusion der Willensfreiheit (The Illusion of Free Will) (1885). Together with a sketch of Rée's biography and the history of his works, which makes hitherto unknown documents accessible (such as the official record of Rée's fatal fall), Treiber reveals numerous sources for Rée's texts. In addition, he reproduces available contemporary reactions to the publications.