Full Idea
There are two kinds of truths: of reasoning and of facts. Truths of reasoning are necessary and their opposites impossible. Facts are contingent and their opposites possible. A necessary truth is known by analysis.
Gist of Idea
Truths of reason are known by analysis, and are necessary; facts are contingent, and their opposites possible
Source
Gottfried Leibniz (Monadology [1716], §33)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.184