Full Idea
To say 'I am not thinking' is self-stultifying since if it is said intelligently it must be false: but it is not self-contradictory. The proof that it is not self-contradictory is that it might have been false.
Gist of Idea
To say 'I am not thinking' must be false, but it might have been true, so it isn't self-contradictory
Source
A.J. Ayer (The Problem of Knowledge [1956], 2.iii)
Book Reference
Ayer,A.J.: 'The Problem of Knowledge' [Penguin 1966], p.45
A Reaction
If it doesn't imply a contradiction, then it is not a necessary truth, which is what it is normally taken to be. Is 'This is a sentence' necessarily true? It might not have been one, if the rules of English syntax changed recently.