Chapter 25
Difference and accident have this in common: that they are both predicated of several things as to what sort they are, and that the difference and the inseparable accident are always present in the things of which they are predicated. One of the distinguishing peculiarities of difference and accident is that the differences contain and are not contained, while the accidents are contained. For, on the one hand, both contain the species, as being predicated of several species; but the difference is not contained, because the same species does not admit of contradictory differences. On the other hand, the accident is contained, for the reason that the same species and the same individual will admit of several accidents which may oftentimes even be contradictory. Other distinguishing peculiarities are: that the difference does not admit of more or less, whereas the accidents on the contrary do, and that contradictory differences may not be combined, whereas contradictory accidents may.
See alternate