 Peter John Olivi was one of the most original and interesting philosophers and theologians of the thirteenth century. Although not as clear and systematic as Thomas Aquinas, and not as brilliantly analytical as John Duns Scotus, Olivi’s ideas are equally original and provocative, and their philosophical value is nowadays recognized among the specialists in medieval philosophy. He is probably best known for his psychological theories, especially his voluntarist conception of the freedom of the will, but his influence extends also to other areas of philosophy, from metaphysics to practical philosophy.