One of the most important philosophers of the thirteenth century, Siger of Brabant was a pioneer of a radical return to the philosophy of Aristotle. He said that his main philosophical method was to determine as precisely as possible the authentic teachings of the ancient philosophers. In this process the philosopher given the most attention was Aristotle, and it is Siger's constant concern, especially in his earlier writings, to determine exactly what Aristotle said. This enterprise entailed a radical separation between philosophy and religion.

Siger accepted the truths of the Catholic faith but also realized that philosophy can be carried out independently of faith. Thus, for example, in his Quaestiones in Tertium De Anima (Questions on the Third Book of [Aristotle's] On the Soul, 1269-1270) and De Aeternitate Mundi (On the Eternity of the World, 1271-1272) Siger holds that philosophy demonstrates that the world is eternal. This conclusion...