- The genus of the subject of the proposition: "Socrates is an animal."
- The species of the subject: "Socrates is a man"
- The subject's specific difference: "Socrates is rational."
- An essential trait or property (proprium) of the subject: "Socrates is risible (i.e., able to laugh)."
- An unessential trait or accident of the subject: "Socrates is white."
When one thing is predicated of another [in a proposition consisting of a subject and a predicate], all that which is predicable of the predicate will be predicable also of the subject. Thus, 'man' is predicated of the individual man [e.g., "this individual is a man"]; but 'animal' is predicated of 'man' [e.g., "man is an animal"]. Therefore, it [i.e., animal] will be predicable of the individual man also [e.g., "this individual in an animal"]: for the individual man is both 'man' and 'animal'.
From Porphyry's Isagoge to Aristotle's Categories, 1-2:
[Preface to the Work.] Since it is necessary, Chrysaorius, to know about [the five predicables, namely] genus, specific difference, species, exclusive property, and accident, both for understanding Aristotle’s theory of the categories and for assigning definitions, and generally for questions of classification or demonstration (the study of all of which is most useful), I shall give you a concise exposition, and I shall attempt, briefly and in the manner of an introduction, to approach what the ancients said, leaving alone the deeper questions, but making a few reasonable conjectures about simple ones. [...]
[The Five Predicables.] [O]f predicates [within propositions], some are (A) predicable of one thing alone, as individuals, for instance, "Socrates," and "this man," and "this thing;" but others are (B) predicable of many, as are (1) genera, (2) species, (3) differences, (4) properties, and (5) accidents, predicated in common, but not peculiarly to any one thing. Now (1) genus is such as "animal," (2) species as "man," (3) difference as "rational," (4) property as " risible," (5) accident as "white," "black," "to sit."
[The First Two Predicables: Genera & Species] [G]enus is that which is predicated of many things differing in species, in answer to [questions concerning] what a thing is, e. g. "animal." [...] That is called species also, which is under the genus stated, according to which we are accustomed to call "man" a species of "animal," "animal" being genus, but "white" a species of "colour," and "triangle" of "figure."
[Subaltern Genera & Species.] They, therefore, define species thus: (1) "species is what is arranged under genus, and of which genus is predicated in reply to [questions concerning] what a thing is." [They] also [define it] thus, (2) "species is what is predicated of many things differing in number in reply to [questions concerning] what a thing is." This [second] definition [of species], however, belongs to the most specific [species] (species specialissima), and which is species only, but no longer genus also, but the other [definition] will pertain to such as are not the most specific [species] (species specialissima).