If, then, the matter is as we have described, we return to our argument and say:

The proof that acts must depend upon an agent is that we find some preceding others in existence and some coming after others. They exist at a time when they could instead have been nonexistent, and they are nonexistent at a time when they could instead have existed.

If this is their description, we say:

Whatever precedes another must do so either by virtue of itself, because of something inhering in it, neither by virtue of itself nor because of a cause, or because someone made it precede the other. The same alternatives apply to whatever comes later.

Upon examination, we find it impossible that whatever comes earlier should do so by virtue of itself. For a thing may precede another of its own kind, while something whose essence is like its own comes after it—as with substances, instances of blackness, and other like things. If whatever comes earlier did so by virtue of itself, it would have no greater claim to precedence than its counterpart; nor would whatever comes later have any greater claim to coming later than to coming earlier. For it has been soundly established that two things are alike when either can take the place of the other and serve in its stead, and the same determinations follow from the essence of one as from that of its counterpart.

Our knowledge that some like things precede others and some come after others—with whatever comes earlier having a greater claim to coming earlier than to coming later, and whatever comes later having a greater claim to coming later than to coming earlier—demonstrates that it cannot be the case that