Full Idea
A proposition will count as being justified a priori as long as no appeal to experience is needed for the proposition to be justified - once it is understood.
Gist of Idea
A priori justification requires understanding but no experience
Source
Laurence Bonjour (In Defence of Pure Reason [1998], §1.2)
Book Reference
Bonjour,Laurence: 'In Defense of Pure Reason' [CUP 1998], p.10
A Reaction
Could you 'understand' that a square cannot be circular without appeal to experience? I'm losing faith in the pure a priori.