Porphyry’s Introduction offers a careful explanation of how the five predicables—Genus, Difference, Species, Property, and Accident—relate to one another. These comparisons help us understand how each term functions in logic, and how they are similar or different when used to describe the natures of things.
Genus and Difference
Both genus and difference can be said of species and of individuals. They are not destroyed when their subjects cease to exist, because they describe general natures that remain in thought.
Yet, genus and difference are not equal. A genus is said of more things than a difference, for it is broader. The genus contains the difference, not the other way around. Genus is also prior to difference, both in order and in understanding. Each answers a different question: the genus tells us what kind of thing something is in general, while the difference tells us how it is distinct from others of the same genus. Every species has only one genus, but it may have several differences. In nature, genus stands to matter as difference stands to form.
Genus and Species
Genus and species share certain traits. Both are said of many things, and both exist before the individual things that belong to them. Each is a kind of whole that contains its subjects.
However, the genus comprehends the species, while the species does not comprehend the genus. The genus is prior to the species, and is said of the species univocally. Though the genus exceeds the species in extension, the species exceeds the genus by its own proper differences. The genus is said of things that differ in species, not of things entirely unlike one another.
Species and Difference
Species and difference are alike because both are said of all their subjects equally and always. They describe what belongs to the essence of a thing.
Still, they are not the same. They answer different questions: species describes what the thing is, while difference describes how it is. Difference can belong to many species, but a species belongs only to its individual members. Difference is prior to species, for species are formed when differences are added to genera. Differences combine to make species, but species do not combine to make other species.
Genus and Property
Both genus and property are said of a species, and both are said univocally. The genus applies equally to all its species, just as a property applies equally to all the individuals of its species.
Yet, they are distinct. The genus comes before the property in order of understanding. The genus is said of many species, but the property belongs to only one. A property is said reciprocally of its subject, meaning that if something has that property, it is that species, and if it is that species, it has that property. A property belongs to its species always and alone, while the genus does not belong alone. If a property is destroyed, the genus remains; but if a genus is destroyed, its properties disappear with it, because they depend on the species that arise from the genus.
Difference and Property
Difference and property share that both are said of all their subjects equally and always. They describe what is always true of the things to which they belong.
But they are not the same. A difference can belong to many species, but a property belongs only to one. A property is said reciprocally of its subject, but a difference is not. The property marks something constant in the nature of the species, while difference serves to divide genera into species.
Species and Property
Species and property are said reciprocally of one another, since every member of the species has the property, and every thing having the property belongs to that species. Both are found equally in all their subjects, allowing no degrees.
Still, species and property are different. A species may serve as the genus for other things, but a property cannot be the property of something else. Species is prior to property. The species is always actually present in its subjects, but a property may be present only potentially. Their definitions also differ, since they explain different aspects of a thing’s nature.
Genus and Accident
Genus and accident are both said of many things, but they are unlike in nature. The genus comes before the species, but accidents come after them. Genus is present equally in all the members of a species, but accidents vary. Accidents belong to individuals, while genera exist before individuals in understanding. Genus and accident also answer different questions about a thing: the genus tells what it is, the accident tells what happens to it or what it has.
Difference and Accident
Difference and accident are both said of many things, and inseparable accidents are said of all their subjects always. Yet they differ in kind. Differences define what a thing is and are not comprehended by species, while accidents can both belong to and be contained by other things in some way. Accidents are said in degrees—some more, some less—but differences are not. Contradictory differences cannot exist in one subject, but contrary accidents may.
Species and Accident
Species and accident are both said of many things. Still, they answer different questions. A thing can belong to only one species, but can have many accidents. Species come before accidents, and are said equally of all their members. Accidents, however, vary in degree. Some may be stronger or weaker in different subjects.
Property and Accident
Property and inseparable accident are both found in all their subjects. Each depends on its subject for existence. Yet, they differ. A property belongs to only one species, while an accident may belong to many. A property is said reciprocally of its subject, but an accident is not. The property is present equally in all cases, while accidents admit degrees and variation.
In these comparisons, Porphyry teaches how the five predicables are related and distinct. Each helps the mind to speak rightly about the essence and qualities of things. The genus tells what kind a thing is; the difference tells how it is distinguished; the species names the full essence; the property describes what belongs always and alone; and the accident shows what may come and go.
Mr. William C. Michael, O.P.
Headmaster
Classical Liberal Arts Academy
