Porphyry is best known for his contributions to philosophy. Apart from writing 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".