The differences, by virtue of which things are different from one another, should be considered in two ways.
Within the genera themselves
Within the genera themselves, e.g. “Wherein does justice differ from courage, and wisdom from temperance?”; for all these belong to the same genus.
In connection with a different genus
In connection with a different genus, provided these different genera are not too far apart, e.g. “Wherein does perception differ from knowledge?”; for when the genera are very far apart, the differences are entirely obvious.
Bibliography: Aristotle Topics (107b.39 to 108a.6)
Translation: George Kotsalis