Full Idea
The fundamental proof of necessary truths comes from the understanding alone, and other truths come from experience or from observations of the senses. Our mind is capable of knowing truths of both sorts, but it is the source of the former.
Gist of Idea
Proofs of necessity come from the understanding, where they have their source
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 1.01)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.80
A Reaction
Interesting because it not only spells out that necessary truths are known a priori, but also explicitly says that the understanding is the 'source' of the truths, or at least the source of their proofs. He also says possibilities derive from essences.