This question is one of the many interesting by-ways of St Thomas’s thought that one may stray through by reading his book, Questions about whatever you like (‘Quaestiones quodlibetales)’. It is IV, q. 3, art. 2. I’ve never seen it commented on, which is mainly why I bring it up.

I was inclined to think that the answer would be no: things which have gone out of existence cannot be restored, with the exception of any given human person, the continued existence of whose immortal soul after death means that he can, at the general resurrection, be brought back into being as precisely (‘numerically’) the same person.

But St Thomas doesn’t take this view. He distinguishes between ‘things’ whose identity is intrinsically dependent on their continuity and all other things. As examples of the first category he gives change and time. So  it’s part of the very notion of a given change that it is not interrupted but rather continuous; if it is interrupted and then later on the process of change is resumed, that makes 2 changes not 1. So if a ball which has been hit from one side of a tennis-court is caught by someone at the net, then that very same forward movement can never be restored; if the person who has caught the ball throws it onwards, that is a new movement, not the very same movement as before. And since time depends on change, therefore just as the very same change cannot be brought back into being, nor can the past time itself.

But with regard to other things – including what we normally mean by ‘things’, namely substances – their identity doesn’t depend on continuity in time. It depends, as far as I can see, on their form. This duck today is the same as that duck yesterday because they have numerically the same form.

So, once this duck has gone out of existence, can it be brought back into being? Not by any natural process, says St Thomas. The reason he gives is that ‘a natural agent cannot cause something to exist without a process of change’. As far as I can tell, the thought here seems to be that, since the very same change cannot be restored, nor can the very same thing that first came into being as a product of that change. The very same act of fertilisation by which duck A was brought into being cannot be restored; so any duck that is now brought into existence will not be duck A, but another duck.

However, he goes on, ‘God can restore things of this kind without a process of change, since it is within His power to produce effects without the intermediary causes, and so He can restore numerically the same things, even when they have ceased to exist {Sed Deus potest reparare huiusmodi et sine motu, quia in eius potestate est quod producat effectus sine causis mediis; et ideo potest eadem numero reparare, etiamsi in nihilum elapsa fuerint.}

So given that nature was able to bring duck A into being in the past by a series of changes, and since whatever nature can do, the Author of nature can do better, God can bring duck A into existence again even when it has once ceased to exist.

That at least is how I understand the question. But I may be wrong.

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