the Aids to the Study of the Intelligibles (Ἀφορμαὶ πρὸς τὰ νοητά; Sententiae Ad Intelligibilia Ducentes), a basic summary of Neoplatonism, he is especially appreciated for his Introduction to Categories (Introductio in Praedicamenta or Isagoge et in Aristotelis Categorias commentarium), a very short work often considered to be a commentary on Aristotle's Categories, hence the title. According to , however, the correct title is simply Introduction (Εἰσαγωγή Isagoge), and the book is an introduction not to the Categories in particular, but to logic in general, comprising as it does the theories of predication, definition, and proof. The Introduction describes how qualities attributed to things may be classified, famously breaking down the philosophical concept of substance into the five components genus, species, difference, property, accident. As Porphyry's most influential contribution to philosophy, the Introduction to Categories incorporated Aristotle's logic into Neoplatonism, in particular the doctrine of the categories of being interpreted in terms of entities (in later philosophy, "universal"). Boethius' Isagoge, a Latin translation of Porphyry's "Introduction", became a standard medieval textbook in European schools and universities, which set the stage for medieval philosophical-theological developments of logic and the problem of universals. In medieval textbooks, the all-important Arbor porphyriana ("Porphyrian Tree") illustrates his logical classification of substance. To this day, taxonomy benefits from concepts in Porphyry's Tree, in classifying living organisms (see cladistics). The Introduction was translated into Arabic by Abd-Allāh Ibn al-Muqaffaʿ from a Syriac version. With the Arabicized name Isāghūjī (إيساغوجي) it long remained the standard introductory logic text in the Muslim world and influenced the study of theology, philosophy, grammar, and jurisprudence. Besides the adaptations and epitomes of this work, many independent works on logic by Muslim philosophers have been entitled Isāghūjī. Porphyry's discussion of accident sparked a long-running debate on the application of accident and essence. Philosophy from Oracles (De Philosophia ex Oraculis Haurienda) Porphyry is also known as an opponent of Christianity and defender of Paganism; his precise contribution to the philosophical approach to traditional religion may be discovered in the fragments of Philosophy from Oracles (Περὶ τῆς ἐκ λογίων φιλοσοφίας; De Philosophia ex Oraculis Haurienda), which was originally three books in length. There is debate as to whether it was written in his youth (as Eunapius reports) or closer in time to the persecutions of Christians under Diocletian and Galerius. Whether or not Porphyry was the pagan philosopher opponent in Lactantius' Divine Institutes, written at