Full Idea
No proposition, other than a tautology, can possibly be anything more than a probable hypothesis.
Gist of Idea
Only tautologies can be certain; other propositions can only be probable
Source
A.J. Ayer (Language,Truth and Logic [1936], Ch.1)
Book Reference
Ayer,A.J.: 'Language, Truth and Logic' [Penguin 1974], p.51
A Reaction
A nice clear empiricist rejection of all attempts to assert necessary truths about nature. This also seems to be a rejection of empiricist foundationalism. A problem case seems to be introspective observations, which seem irrefutable and obvious.