Isagoge

A brief introduction to Aristotle's Categories, written by the Neoplatonist Porphyry, pupil and editor of Plotinus, for a Roman student named Chrysaorius. It explains five terms needed for logic and definition: genus, species, difference, property and accident, later known as the five predicables. Porphyry defines each, gives examples, and compares them in pairs, showing what they share and how they differ. He illustrates the order of genera and species with a ladder running from substance through body, living body, animal and rational animal down to man and individual men such as Socrates and Plato, later drawn as the Tree of Porphyry. At the start he sets aside, as too deep for an introduction, the questions of whether genera and species exist in reality or only in thought, whether they are bodily or not, and whether they exist apart from things. Through Boethius' Latin translation those questions became the medieval problem of universals.

It is the short text that launched the medieval debate over universals and served for over a thousand years as the first lesson in logic.