Full Idea
If I lie and say "It is raining", my utterance is intelligible to me as a lie precisely because I understand that the utterance commits me to the truth of a proposition I do not believe to be true.
Gist of Idea
We can't understand something as a lie if beliefs aren't commitment to truth
Source
John Searle (Rationality in Action [2001], Ch.6.II)
Book Reference
Searle,John R.: 'Rationality in Action' [MIT 2001], p.184