Origen has been always studied as a theologian and too much credit has been given to Eusebius' implausible hagiography of him. This book explores who Origen really was, by pondering into his philosophical background, which determines his theological exposition implicitly, yet decisively. For this background to come to light, it took a ground-breaking exposition of Anaxagoras' philosophy and its legacy to Classical and Late Antiquity (Plato, Aristotle, Stoics, Origen, Neoplatonism), assessing critically Aristotle's distorted representation of Anaxagoras. Origen, formerly a Greek philosopher of note, whom Proclus styled an anti-Platonist, is placed in the history of philosophy for the first time. By drawing on his Anaxagorean background, and being the first to revive the Anaxagorean Theory of Logoi, he paved the way to Nicaea. He was an anti-Platonist because he was an Anaxagorean philosopher with far-reaching influence, also on Neoplatonists such as Porphyry. His theology made an impact not only on the Cappadocians, but also on later Christian authors. His theory of the soul, now expounded in the light of his philosophical background, turns out more orthodox than that of some Christian stars of the Byzantine imperial orthodoxy.