Full Idea
Nothing happens without a sufficient reason why it should be thus rather than otherwise.
Gist of Idea
There is always a reason why things are thus rather than otherwise
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], 3.2)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.211