Full Idea
The objection which is commonly brought against empiricism is that it is impossible on empiricist principles to account for our knowledge of necessary truths.
Gist of Idea
Empiricism, it is said, cannot account for our knowledge of necessary truths
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.4)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.96
A Reaction
This criticism goes back at least to Leibniz. Ayer's distinctive contribution to empiricism (with help) is to emphasise that we can only know necessities if they are tautologies. Hume always challenged our knowledge of natural necessities.