Full Idea
Nothing ever takes place without its being possible for one who knew everything to give some reason why it should have happened rather than not.
Gist of Idea
For every event it is possible for an omniscient being to give a reason for its occurrence
Source
Gottfried Leibniz (Letter on Freedom [1689], p.112)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.112
A Reaction
Presumably there will be GOOD reason why genocide occurs. Note that there is a reason for every 'event'. Is there a reason for every truth? Presumably not, or there would have to be reasons for self-evident truths.