so that things which differ have a single necessary concomitant. This is not objectionable.

Or that in which they differ is a necessary concomitant of what they share, so that what necessarily follows from one and the same thing would be different and mutually opposed. This is objectionable. Or what they share is an accidental attribute attaching to that in which they differ. This is not objectionable.

Or that in which they differ is an accidental attribute attaching to what they share. This too is not objectionable.”

The commentators on his words said:

“Any two things must differ in their identity and individuation, for if the individuation of this thing belonged to that one, this would be that very thing, not something other than it. Things may agree in some essential constituent, as with the highest genera, which do not agree in any essential constituent, although they may agree in some accidental attributes.

Thus, when they agree in an essential constituent, that in which they differ must be distinct from what they share. The identity of each is therefore composed of that which it shares with the other and that which distinguishes it from the other. Accordingly, either what they share is a necessary concomitant of that in which they differ, or the reverse; or what they share is an accidental attribute of that in which they differ, or the reverse.

The first is not objectionable, as with the differentiae of species subsumed under a genus: the nature of that genus is a necessary concomitant of the natures of those differentiae; and as with existence