This page provides a detailed explanation of the five predicables as taught in Porphyry’s Introduction. This work is also referred to by its Greek name, Isagoge.
Contents
What are the Five Predicables?
In his Introduction, Porphyry proposes five fundamental ways in which something may be predicated of a subject. These are called the five predicables: Genus, Species, Difference, Peculiarity, and Accident.
These distinctions are necessary for understanding how things are defined and how they are related to one another in logical reasoning. When we ask what a thing is, what kind of thing it is, or what belongs to it, we are making use of these predicables, whether we are aware of it or not.
Genus and species are used to express the essence of a thing. Genus signifies what is common to many things differing in species, while Species signifies the complete nature of a thing as predicated of individuals. Difference determines the kind of thing something is, distinguishing one species from another within the same genus. It is through difference that a genus is divided and a species is constituted. Peculiarity (or property) belongs to one species alone and to all its members, always. It flows from the nature of the species, though it does not enter into its definition. Accident is that which may be present or absent without destroying the subject. It does not belong to the essence of a thing, but exists in it and may vary.
These five predicables provide the foundation for definition, division, and demonstration in logic. In the chapter that follows, Porphyry compares them with one another, so that their similarities and differences may be clearly understood.
Definitions of the Five Predicables
In chapters 2-5 of the Introduction, Porphyry provides a definition for each of the five predicables:
- Genus: That which is predicated of many things differing in species, in answer to the question “What is this?” (Chapter 2)
- Species: That which is predicated of many things differing in number, in answer to the question “What is this?”; and that which is placed under a genus. (Chapter 2)
- Difference: That which is predicated of many things differing in species, in answer to the question “What kind of a thing is this?”; and that which separates things under the same genus. (Chapter 3)
- Peculiarity: That which belongs to one species alone, to every individual of that species, and always (though not always in act). (Chapter 4)
- Accident: That which may be present or absent without the corruption of its subject. (Chapter 5)
Comparisons of the Five Predicables
In chapter 6 of the Instroduction, Porphyry adds to the definitions of the five predicables to show “what things are common to them, and what are peculiar”. These comparisons are summarized below:
Genus vs. Difference
- Common:
- Genus & Difference both predicated of Species and individuals.
- Genus & Difference are not destroyed by the destruction of their subjects.
- Different:
- Genus is predicated of more things than Difference.
- Genus contains Difference and not vice-versa.
- Genera prior (in definition) to their Differences.
- Genus and Difference answer different questions.
- Only one proximate Genus for each species, but more than one Difference possible for each species.
- Genus is to matter as Difference is to form.
Genus vs. Species
- Common:
- Genus and Species both predicated of many things.
- Genus and Species are prior to their subjects.
- Genus and Species are both kinds of wholes.
- Different:
- Genus comprehends Species, but not vice-versa.
- Genera prior to Species.
- Genera predicated univocally of Species, but not vice-versa.
- Genera exceed Species by comprehending them, but Species exceed Genera by their proper differences
- Genera are predicated of things differing in species.
Genus vs. Peculiarity
- Common:
- Both Genus & Peculiarity are predicated of Species.
- Genus is equally predicated of all its Species, and Peculiarity of all the individuals of its Species.
- Predication of Genus and Peculiarity is Univocal.
- Different:
- Genus prior to Peculiarity.
- Genus predicated of many species, but Peculiarity of only one.
- Peculiarity predicated reciprocally, but not Genus.
- Peculiarity inherent in a species whole, alone and always, but Genus not alone.
- Destroying properties does not destroy Genera, but destroying Genera do, indirectly, destroy Peculiarities, as they belong in Species.
Genus vs. Accident
- Common:
- Both predicated of many things.
- Different:
- Genus prior to Species, but Accident posterior;
- Accidents not present in all subjects equally, as Genus.
- Accidents primarily inhere in individuals, but Genera prior to individuals.
- Genera and Accidents answer to different questions about their subjects.
Species vs. Difference
- Common:
- Species & Difference predicable of all their subjects equally.
- Species and Difference predicable of all their subjects always.
- Different:
- Species and Difference answer different questions about their subjects.
- Difference predicated of many Species, but Species only of its individuals.
- Difference prior (in definition) to Species;
- Differences together with Genus constitute Species, but Species do not join to form other Species.
Peculiarity vs. Difference
- Common:
- Peculiarity and Difference predicable of all their subjects equally.
- Peculiarity and Difference predicable of all their subjects always.
- Different:
- Difference predicated of many species, but Peculiarity of only one.
- Peculiarities predicated reciprocally of their subjects, but not differences.
Accident vs. Difference
- Common:
- Accident and Difference are both predicable of many things.
- Difference and inseparable Accident predicated with all of its subject always.
- Different:
- Differences comprehend, but not comprehended by Species, but Accidents do, in a manner, comprehend and are, in a manner, comprehended.
- Accidents predicable of their subjects in degrees, but not Differences. 3. Contrary Differences cannot be mingled in a subject, but contrary Accidents may be.
Peculiarity vs. Species
- Common:
- Both Species & Peculiarity predicated reciprocally of each other.
- Both present in all of their subjects equally, not allowing degree.
- Different:
- Species may be the Genus of other things, but Peculiarity cannot be the peculiarity of other things.
- Species prior to Peculiarity.
- Species always present “in actu” with its subject, but peculiarity sometimes only “in potestate”;
- Different definitions.
Accident vs. Species
- Common:
- Species & Accident both predicated of many things.
- Different:
- Answer to different questions about their subjects.
- A subject may partake of only one Species, but many Accidents.
- Species prior to Accidents.
- Species predicated equally of all subjects, but Accidents present in degrees.
Accident vs. Peculiarity
- Common:
- Both depend on their subjects for their existence;
- Peculiarity and Inseparable Accident are always present to all of their subjects.
- Different:
- Peculiarity may be predicated of only one species, but Accident may be predicated of many species;
- Peculiarity predicated reciprocally of its subject, but not Accident.
- Peculiarity predicated equally of its subjects, but Accident present in degrees.
Summary
The five predicables—genus, species, difference, peculiarity, and accident—provide a complete account of the ways in which something may be predicated of a subject. Each serves a distinct role in logical reasoning: some express what a thing is, others determine what kind of thing it is, and others describe what belongs to it without constituting its essence. By comparing these predicables with one another, Porphyry enables us to see their proper order and use. Genus and species express essence at different levels; difference determines and divides; peculiarity follows from the nature of a species; and accident remains outside the essence, capable of variation. Through these distinctions, confusion is removed and definitions are made precise. The study of the predicables is therefore not merely technical, but foundational. Without them, we cannot define clearly, divide correctly, or reason soundly about the nature of things. With them, we gain the principles necessary for all further study in logic and philosophy.
Mr. William C. Michael, O.P.
Headmaster
Classical Liberal Arts Academy
