Full Idea
In order to proceed from mathematics to physics the principle of sufficient reason is necessary, that nothing happens without there being a reason why it should be thus rather than otherwise.
Gist of Idea
The principle of sufficient reason is needed if we are to proceed from maths to physics
Source
Gottfried Leibniz (Letters to Samuel Clarke [1716], §2)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.207