Full Idea
To say that a proposition is true a priori is to say that it is a tautology.
Gist of Idea
To say that a proposition is true a priori is to say that it is a tautology
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.4)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.115
A Reaction
This is Ayer's splendidly clearcut anti-rationalism. However, one might concede that one cannot know a priori about remote possible worlds (though I'm not so sure), but still claim a priori extrapolations from our current experiences.