Full Idea
To speak against reason is to speak against truth, for reason is a chain of truths.
Gist of Idea
Opposing reason is opposing truth, since reason is a chain of truths
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 2.21)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.199
A Reaction
Truth has a talismanic quality here (which it didn't always have). This is a lovely slogan for defenders of the Enlightenment. It forces modern critics of the Enlightenment (Adorno etc) to launch an attack on truth, which is a doomed line.