Full Idea
A dictate of reason is the name we give to certain propositions which we hold to be true without investigation, and of which we think ourselves so firmly convinced we should be incapable of seriously testing them even if we wanted to.
Gist of Idea
A priori propositions are those we could never be seriously motivated to challenge
Source
Arthur Schopenhauer (Parerga and Paralipomena [1851], I:12)
Book Reference
Schopenhauer,Arthur: 'Essays and Aphorisms [from Pand P]', ed/tr. Hollingdale,R.J. [Penguin 1970], p.119
A Reaction
This is closer to the cautious way modern thinkers are inclined to express the idea. Even Quine would be reasonably happy with this.