Of things that are, some are said of a subject, and are not in a subject, e.g. man is said of a subject, the individual man, but is not in a subject; others are not said of a subject, yet they are in a subject.
In a subject
By “in a subject” I mean what is incapable of existing separately from what it is in, and is not as a part of this thing; for example the individual grammar is in a subject, the soul, but is not said of a subject, and the individual white is in a subject, the body — for all colors are in a body — but is not said of a subject.
Of a subject and in a subject
Again, others are said of a subject and are in a subject, e.g. scientific knowledge is both in a subject, the soul, and it is said of a subject, the body; others are neither in a subject nor are said of a subject, e.g. the individual man or the individual horse — since nothing of this sort is either in a subject or said of a subject.
Generally, those that are individual and numerically one are not said of any subject, however nothing prevent some of them from being in a subject, since the individual grammar belongs to things being in a subject.
When one thing is predicated of another as of a subject, whatever is said of the predicate will be said of the subject also; thus man is predicated of the individual man and animal of man, so animal will be predicated of the individual man also, since the individual man is also an animal.
Now those that belong to different genera which are not subordinate to each other (subordinate genera) have also different differentiae as for the subject that are predicated of, such as animal and knowledge; for pedestrian, feathery, aquatic are differentiae of animal, yet none of these are differentiae of knowledge, since one sort of knowledge does not differ from another by being two-footed.
However, of those that are subordinate to others, there is nothing to prevent them from having the same differentiae, since the higher genera are predicated of the lower, so that all the differentiae of the predicate will be differentiae of the subject also.
Bibliography: Aristotle, Categories (1a.20 to 1b.24)
Translation: George Kotsalis