I say:

This is a sound observation. When one of two things replaces and displaces the other, if one must be a positive reality and the other an absence, then motion and rest must likewise be one positive reality and one absence—which contradicts what the argument seeks to establish. If, however, both may be positive realities or both may be absences, the proof is invalid. That proof is his statement:

“For one to replace the other implies that one of them is a positive reality: either what is displaced is a positive reality or, if it is not, what displaces it is a positive reality, because removing an absence establishes a presence.”

On this supposition, however, an absence can be displaced by another absence, and a positive reality by another positive reality.

If it is said:

“Rather, both must be positive realities, or one of them must be a positive reality. They cannot both be absences, because an absence cannot be displaced by another absence as a positive reality can be displaced by another positive reality, or an absence by a positive reality, or vice versa.”

The reply is:

Two absences may indeed be mutually incompatible, just as they may necessarily accompany one another. Just as the absence of a condition entails the absence of what depends upon it, so, where one of a set of things must exist, the absences of its members are incompatible with their all being absent. If a genus exists only through the existence of a species possessing a differentia, then, while the genus exists, all its species and differentiae cannot be absent. Thus, the absence of some is incompatible with the absence of them all.

This is like what is said concerning a division into alternatives, that is, a disjunctive conditional proposition. It may exclude both joint truth and joint falsity, as when someone says:

“A number is either even or odd.”

It may exclude only joint truth, as when someone says:

“A body is either black or white.”

Or it may exclude only joint falsity. Thus, whatever excludes only joint falsity