On the eve of the Council of Constantinople in 553, John Philoponus, the Alexandrian philosopher and prolific commentator on Aristotle, entered the controversy over the Chalcedonian definition of faith. By clarifying the terms of the debate, he intended to lay the groundwork for a defence of miaphysitism as the appropriate way of understanding the Incarnation. This monograph elucidates the argument of Philoponus' Arbiter by locating it within the Christological discussions of the fifth and sixth centuries and by highlighting its indebtedness to the Neoplatonic commentators on Aristotle. The Christian reception of an Aristotelian philosophy in the sixth century facilitated the emergence of a 'scholastic' theology, of which Philoponus is an important representative. The reader will also find here a treatment of a number of philological and historical issues concerning Philoponus' Christological writings, an English translation of the Arbiter, and a critical edition of newly discovered Greek fragments of this work.