Overview
The Isagoge, or Introduction, is a short handbook by Porphyry of Tyre written in Greek and addressed to a certain Chrysaorius. Its stated purpose is to set out, in the form of an elementary introduction, the five terms — or “predicables” — that a student needs before reading Aristotle’s Categories: genus, species, difference, property, and accident. The treatment is compact and doctrinal in tone, moving from definitions and divisions of each of the five terms to a systematic comparison of how each one relates to and differs from the others. It is not a dialogue, and it contains no extended discussion of any particular natural or metaphysical substance; it is a technical textbook of logic.
The Preface and the Question of Universals
Porphyry opens by apologizing for the difficulty of the task and by stating what he will not do. He mentions the “deep and lofty” questions that arise over genus and species — whether they subsist in reality or exist only in bare thought, whether, if they subsist, they are corporeal or incorporeal, and whether they are separated from sensible things or present in them — and he declares that he will decline to answer them, because such inquiries exceed the scope of an elementary introduction. He promises instead to explain how the Peripatetics, that is, the Aristotelians, treated the five terms from the point of view of logic. This deliberate refusal to settle the ontological status of universals is the passage that later tradition fastened upon as the locus of the so-called problem of universals; within the work itself it functions as a scruple about method and audience.
Genus
The discussion of genus begins from the observation that “genus” is used in several senses. Porphyry distinguishes, among others, the sense in which a group of people descended from one ancestor is called a genus (as the Heraclidae are named from Heracles), and the sense in which the source of generation is called a genus. He then isolates the sense relevant to logic: that which is predicated of many things differing in species, in answer to the question “What is it?” — as animal is predicated of man and of ox.
From this he develops the familiar apparatus of generic and specific levels. There is a “most generic” genus, which is itself above every other genus and has none above it; there is a “most specific” species, which is below every other and has no species below it; and between them lie the “subordinate” genera and species, each of which is a species relative to what is above it and a genus relative to what is below it. He illustrates this with a descending chain — substance, body, animated body, animal, rational animal, man — and notes that individuals fall below the lowest species. He also sets out criteria by which one may tell what is a genus and defends the points against possible objections.
Difference
Difference (Greek diaphora) is treated in its various senses. There is the common or generic difference, which distinguishes one thing from another in a loose sense; there is the proper difference, by which one thing exceeds another in a certain respect; and there is the most proper difference, which is used in dividing a genus into its species, as rational and irrational divide animal. This last is the difference that is predicated of the species in answer to the question “What sort of thing is it?” rather than “What is it?” Porphyry explains how differences are used to divide genera, how some differences are constitutive of species and some merely mark off one species from another, and how precisely the proper differentia is to be distinguished from others.
Species
Species (Greek eidos) is presented primarily as that which is predicated of many things differing in number, in answer to “What is it?” — as man is predicated of individual men. Porphyry also records the other senses of the word: the form of a thing, and that which is subordinate to a genus. He explains the relation of species to the genus above it and to the individuals below it, and how species at different levels of a hierarchy can be at once species and genera.
Property
The treatment of property (Greek idion) carefully distinguishes several senses. A property may be that which belongs to some one species alone but not to the whole of it; it may be that which belongs to a whole genus; it may be that which belongs to a species alone and at all times, such as the capacity to laugh for man; and it may be the “most proper” sense, in which the property is coextensive with the species and convertible with it. The last sense is the one relevant to the logical apparatus, and Porphyry shows how it differs from the other senses and from difference and accident.
Accident
Accident (Greek symbebēkos) is defined as that which may be present or absent from a thing without the thing being destroyed. Porphyry divides it into the separable and the inseparable. Inseparable accidents can be so either by nature or by convention; he notes that this distinction gives rise to some difficulty, but the broad point is that an accident is what is predicated of a subject in answer to “How is it?” rather than “What is it?” He distinguishes accidents that are proper to individuals and accidents that attach to species.
Common Features and Distinctions Among the Five Terms
The second main part of the work systematically compares the five terms. Porphyry first identifies what the five have in common: all are predicated of many things (though in different ways — genus of species and individuals, species of individuals, difference of species and individuals, property and accident of species and individuals). He then runs through each pair, showing in turn how genus differs from species, species from difference, and so on through genus and property, genus and accident, species and difference, species and property, species and accident, difference and property, difference and accident, and property and accident. The comparisons turn on such considerations as whether a term is predicated essentially or accidentally, whether it is predicated in the “What is it?” or “What sort of thing is it?” mode, whether it belongs to a whole or only to a part, and whether it is convertible with its subject.
The work closes with these pairwise distinctions rather than with a general conclusion; the last section treats the common and divergent features of property and accident. Throughout, the exposition is plain and summary in style, arranged for memorization and for use as a first text in logic. The Isagoge offers no substantial argument for or against the existence of universals, no theory of knowledge, and no metaphysical system of its own; its contents are confined to the definitions, divisions, and mutual relations of the five predicables, together with Porphyry’s brief and notable explanation of why he leaves the deeper questions aside.
The Five Terms as Instruments of Definition and Division
The Isagoge treats the five terms not as a metaphysical system but as a working vocabulary for definition and division. Genus and difference together supply the formula of a species; species names the whole so defined; property marks what is coextensive with a species but not part of its essence; accident marks what can be present or absent without destroying the subject. The text repeatedly tests terms by asking whether they are predicated essentially or accidentally, whether they answer “What is it?” or “What sort of thing is it?”, and whether they are convertible with their subject. These tests make the work a manual for sorting predicates rather than a treatise on being.
The Graded Order of Predication
The discussion of genera and species establishes a graded order: individuals are subjects of predication, species are predicated of individuals, genera are predicated of species, and a term can be a species at one level and a genus at another. Difference divides a genus and completes a species. The work stresses that predication is not mere naming: the same thing may be described through genus, species, difference, property, and accident under different logical modes. This is why the pairwise comparisons matter—they keep the beginner from confusing coextension with identity, or accidental accompaniment with essence.
The Deliberate Limits of the Inquiry
A distinctive feature of the Isagoge is its refusal to settle the metaphysical status of genera and species. Porphyry raises the questions whether they subsist or exist only in thought, whether they are corporeal or incorporeal, and whether they are separable from sensible things, but he treats these as beyond the scope of an elementary introduction. The result is a text that is logically precise and metaphysically reticent: it gives a compact set of distinctions while leaving the problem of universals open. Its significance lies in that combination—it supplies a teachable scheme of the five predicables without itself prescribing a solution to the deeper questions it identifies.
