Full Idea
Leibniz actually characterises reason as the faculty for apprehending priori, necessary truths.
Gist of Idea
Reason is the faculty for grasping apriori necessary truths
Source
report of Gottfried Leibniz (works [1690]) by Tyler Burge - Frege on Apriority (with ps) 2
Book Reference
Burge,Tyler: 'Truth, Thought, Reason (on Frege)' [OUP 2001], p.371
A Reaction
No wonder it is called the Age of Reason when the claims are this grandiose.