Full Idea
An idea is true if what it represents is possible; false if the representation contains a contradiction.
Gist of Idea
True ideas represent what is possible; false ideas represent contradictions
Source
Gottfried Leibniz (Reflections on Knowledge, Truth and Ideas [1684], p.287)
Book Reference
Leibniz,Gottfried: 'Leibniz Selections', ed/tr. Wiener,Philip P. [Scribners 1951], p.287
A Reaction
Odd in the analytic tradition to talk of a single idea or concept (rather than a proposition or utterance) as being 'true'. But there is clearly a notion of valid or legitimate or useful concepts here. Hilbert said true just meant non-contradictory.
Related Ideas
Idea 19423 By an 'idea' I mean not an actual thought, but the resources we can draw on to think [Leibniz]
Idea 5644 In Hegel's logic it is concepts (rather than judgements or propositions) which are true or false [Hegel, by Scruton]
Idea 15716 If axioms and their implications have no contradictions, they pass my criterion of truth and existence [Hilbert]
Idea 19439 God produces possibilities, and thus ideas [Leibniz]
Idea 21410 That a concept is not self-contradictory does not make what it represents possible [Kant]