Full Idea
Leibniz distinguished two fundamental principles of rationality - the principle of non-contradiction and the principle of sufficient reason.
Clarification
'Non-contradiction' is the refusal to accept contradictory ideas; 'sufficient reason' is the assumption that there is a reason for everything
Gist of Idea
For Leibniz rationality is based on non-contradiction and the principle of sufficient reason
Source
report of Gottfried Leibniz (works [1690]) by José A. Benardete - Metaphysics: the logical approach Ch.18
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.136