His interest in telling the truth or in lying presupposes that there is a difference between getting things wrong and getting them right, and that it is at least occasionally possible to tell the difference. Someone who ceases to believe in the possibility of identifying certain statements as true and others as false can have only two alternatives. The first is to desist both from efforts to tell the truth and from efforts to decieve. This would mean refraining from making any assertion whatever about the facts. The second alternative is to continue making assertions that purport to describe the way things are but that cannot be anything except bullshit.
Absolute skepticism is the thing to try.
Probably physics is a fiction. Don't let anything in but sets. Nobody has yet suggested that the way out of the Liar paradox is to give up talking, but I expect it's only a matter of time.