Full Idea
It is not the case that everything which happens is necessary; rather, everything which happens is certain after God made choice of this possible universe, whose notion contains this series of things.
Gist of Idea
Everything which happens is not necessary, but is certain after God chooses this universe
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1686.05)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.54
A Reaction
I think this distinction is best captured as 'metaphysical necessity' (Leibniz's 'necessity'), and 'natural necessity' (his 'certainty'). 'Certainty' seems a bad word, as it is either certain de dicto or de re. Is God certain, or is the thing certain?