Full Idea
If certain possibles never exist, then existing things are not always necessary; otherwise it would be impossible for other things to exist instead of them, and so all things that never exist would be impossible.
Gist of Idea
If non-existents are possible, their existence would replace what now exists, which cannot therefore be necessary
Source
Gottfried Leibniz (On Freedom [1689], p.106)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.106
A Reaction
A neat argument, though it is not self-evident that when possibles came into existence they would have to replace what is already there. Can't something be possible, but only in another world, because this one is already booked?