always possible for it not to exist, that possibility would have a limit. Once that limit was passed, its existence would become necessary and its nonexistence impossible, even though the circumstances remained the same. This is impossible. It therefore remains that, if its nonexistence is possible, it must always be possible for it not to exist. Anything possible, when assumed to exist, may give rise to a false conclusion. An impossibility, however, never arises when it is assumed not to exist. Yet assuming this nonexistence does give rise to an impossibility.

The explanation is as follows: we suppose that one of the two alternatives open to a possible thing—namely, perpetual existence—has been realized, while it nevertheless remains capable of the form’s perpetual nonexistence. The realization of that possibility cannot be impossible, for if its realization were impossible, it would not be possible. Yet, on the assumption of its perpetual existence, its perpetual nonexistence is impossible. Otherwise, the thing would be both nonexistent and existent throughout an infinite time, which is impossible.

Admittedly, its nonexistence after its existence can be assumed. But that nonexistence is not perpetual; rather, it is a newly arising nonexistence.

Since this is impossible under the supposition—not merely false without being impossible, but impossible—it is impossible to judge that something capable of perpetual existence can fail to exist. Its existence is therefore necessary, which is what was to be demonstrated.”