John Philoponus (/fᵻˈlɒpənəs/; Ἰωάννης ὁ Φιλόπονος; c. 490 – c. 570), also known as John the Grammarian or John of Alexandria, was an Alexandrian philologist, Aristotelian commentator and Christian theologian, author of a considerable number of philosophical treatises and theological works.

A rigorous, sometimes polemical writer and an original thinker who was controversial in his own time, John Philoponus broke from the Aristotelian–Neoplatonic tradition, questioning methodology and eventually leading to empiricism in the natural sciences. He was one of the first to propose a "theory of impetus" reminiscent of the modern concept of inertia over Aristotelian dynamics.

Later in life Philoponus turned to Christian apologetics, arguing against the eternity of the world, a theory which formed the basis of pagan attack of the Christian doctrine of Creation. He also wrote on Christology, and was posthumously condemned as a heretic by the Imperial Church in 680–81 because of what was perceived of as a tritheistic interpretation of the Trinity.

His by-name ὁ Φιλόπονος translates as "lover of toil", i.e. "diligent", in reference to a Monophysite sect in Alexandria, the philoponoi who were active in debating pagan (i.e. Neoplatonic) philosophers.

His posthumous condemnation limited the spread of his writing, but copies of his work did circulate in Greek or Latin versions in medieval Europe, influencing Bonaventure and Buridan. His work was also received in Arabic scholarly tradition, where he is known as Yaḥyā al-Naḥwī (i.e. "John the Grammarian"). His critique of Aristotle in the Physics commentary was a major influence on Giovanni Pico della Mirandola and Galileo Galilei, who cited Philoponus substantially in his works.

Philoponus' On Aristotle Categories 1-5 discusses the nature of universals, preserving the views of Philoponus' teacher Ammonius, as well as presenting a Neoplatonist interpretation of Aristotle's Categories. Philoponus treats universals as concepts in the human mind produced by abstracting a form or nature from the material individual in which it has its being. The work is important for its own philosophical discussion and for the insight it sheds on its sources. For considerable portions, On Aristotle Categories 1-5 resembles the wording of an earlier commentary which declares itself to be an anonymous record taken from the seminars of Ammonius. Unlike much of Philoponus' later writing, this commentary does not disagree with either Aristotle or Ammonius, and suggests the possibility that Philoponus either had access to this earlier record or wrote it himself. This edition explores these questions of provenance, alongside the context, meaning and implications of Philoponus' work. The English translation is accompanied by an introduction, comprehensive commentary notes, bibliography, glossary of translated terms and a subject index. The latest volume in the Ancient Commentators on Aristotle series, the edition makes this philosophical work accessible to a modern readership. Philoponus was a Christian writing in Greek in 6th century CE Alexandria, where some students of philosophy were bilingual in Syriac as well as Greek. In this Greek treatise translated from the surviving Syriac version, Philoponus discusses the logic of parts and wholes, and he illustrates the spread of the pagan and Christian philosophy of 6th century CE Greeks to other cultures, in this case to Syria. Philoponus, an expert on Aristotle's philosophy, had turned to theology and was applying his knowledge of Aristotle to disputes over the human and divine nature of Christ. Were there two natures and were they parts of a whole, as the Emperor Justinian proposed, or was there only one nature, as Philoponus claimed with the rebel minority, both human and divine? If there were two natures, were they parts like the ingredients in a chemical mixture? Philoponus attacks the idea. Such ingredients are not parts, because they each inter-penetrate the whole mixture. Moreover, he abandons his ingenious earlier attempts to support Aristotle's view of mixture by identifying ways in which such ingredients might be thought of as potentially preserved in a chemical mixture. Instead, Philoponus says that the ingredients are destroyed, unlike the human and divine in Christ. This English translation of Philoponus' treatise is the latest volume in the Ancient Commentators on Aristotle series and makes this philosophical work accessible to a modern readership. The translation in each volume is accompanied by an introduction, comprehensive commentary notes, bibliography, glossary of translated terms and a subject index.