Full Idea
Everyone's certain knowledge assures him that he is something that actually exists. ...Therefore there is some real Being, and since non-entity cannot produce any real being, it is an evident demonstration that from Eternity there has been something.
Gist of Idea
We exist, so there is Being, which requires eternal being
Source
John Locke (Essay Conc Human Understanding (2nd Ed) [1694], 4.10.03)
Book Reference
Locke,John: 'Essay Concerning Human Understanding', ed/tr. Nidditch,P.H. [OUP 1979], p.620
A Reaction
This is a cosmological proof, deriving God as a necessary precondition from the observation that something exists. It is similar to, but not as good as, Aquinas's Third Way (Idea 1431).
Related Idea
Idea 21271 Way 3: contingent beings eventually vanish, so continuity needs a necessary being [Aquinas]