by virtue of the very nature of contingency. For a contingent thing is, by its very nature, that which exists only through something else, not through itself; and this nature does not distinguish between individual instances and their aggregate.
As for an event having a beginning, or something past having an end, this is known of each individual event and each individual thing that has passed. As for whether the genus is likewise, its nature belongs to each individual instance, and there is no aggregate of events and past occurrences subsisting in external reality such that one could ask: Does the judgment applying to its individual instances apply to that aggregate or not? Its individual instances exist successively. If successive events are supposed, knowing this does not entail that they cannot continue perpetually.
But if events are supposed to occur together at a single instant, or to be bounded, it is said: “This aggregate has a beginning.” If things that have elapsed, or things whose endpoints are bounded, are supposed to be gathered together, it is said: “They have an end.”
As for things that cannot coexist, whether of the former or the latter kind, they have no collective existence in external reality. Their individual instances are brought together only in the mind, not in external reality.
This is clarified by considering that which exists only in succession, one instance after another. If it is said, “Each of its individual instances follows another instance,” this does not establish that the whole follows something else. For the judgment was not that the genus follows something outside that genus; rather, we judged only that the individual instances of the genus follow one another.
Likewise, if we say, “Each of its individual instances was preceded by nonexistence,” we have not judged that the genus was preceded by nonexistence—as we judged above that the genus of things that have come into being depends on an agent, and that the genus of contingent things depends on what is not contingent, or on an agent or a necessary existent, and so forth.
This matter is discussed at length in its proper place.