Full Idea
That which can not-be at some time is not. So if everything can not-be, then once there was nothing in existence. If so, it would have been impossible for anything to have begun to exist. So there must be some being having of itself its own necessity.
Gist of Idea
Way 3: contingent beings eventually vanish, so continuity needs a necessary being
Source
Thomas Aquinas (Summa Theologicae [1265], Ia,Q02,Art3,Reply)
Book Reference
'The Existence of God', ed/tr. Hick,John [Macmillan 1964], p.84
A Reaction
[compressed] Why can't things take it in turns to not-be, so that something is always on duty? Maybe it is a feature of things that they bring other things into existence (e.g. virtual particles)?