Full Idea
If the proof of a truth can be derived exclusively from general laws, which themselves neither need nor admit of proof, then the truth is a priori.
Gist of Idea
An a priori truth is one derived from general laws which do not require proof
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §03)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.4
A Reaction
Presumably the unproved general laws from which the derivation comes are more securely a priori, as are the principles used to make the derivation. As Frege says, he is trying to spell out Kant's view; see Idea 9345.
Related Idea
Idea 9345 Propositions involving necessity are a priori, and pure a priori if they only derive from other necessities [Kant]