Full Idea
The two first principles of reasoning are: the principle of contradiction, and the principle of the need for giving a reason.
Gist of Idea
The two basics of reasoning are contradiction and sufficient reason
Source
Gottfried Leibniz (A Specimen of Discoveries [1686], p.75)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.75
A Reaction
Could animals have any reasoning ability (say, in solving a physical problem)? Leibniz's criteria both require language. Note the overlapping of the principle of sufficient reason (there IS a reason) with the contractual idea of GIVING reasons.