Full Idea
If it is possible to derive a proof purely from general laws, which themselves neither need nor admit of proof, then the truth is a priori.
Gist of Idea
A truth is a priori if it can be proved entirely from general unproven laws
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §03), quoted by Tyler Burge - Frege on Apriority (with ps) 1
Book Reference
Burge,Tyler: 'Truth, Thought, Reason (on Frege)' [OUP 2001], p.359
A Reaction
Burge brings out the contrast with Kant, for whom a priori truths are derived from particular facts, not general ones.