Johannes Philoponus (c.490–575), Greek philosopher and theologian, who worked in Alexandria (philoponus, ‘workaholic’, just a nickname). A Christian from birth, he was a pupil of the Platonist Ammonius, and is the first Christian Aristotelian. As such, he challenged Aristotle on many points where he conflicted with Christian doctrine, e.g. the eternity of the world, the need for an infinite force, the definition of place, the impossibility of a vacuum, and the necessity for a fifth element to be the substance of the heavens. Johannes composed commentaries on Aristotle’s Categories, Prior and Posterior Analytics, Meteorologics, and On the Soul; and a treatise Against Proclus: On the Eternity of the World. There is dispute as to whether the commentaries exhibit a change of mind (away from orthodox Aristotelianism) on these questions. J.M.D.