I saw it on the plane yesterday.  Very funny, very insightful, somewhat disjointed at the end.

Forgive me for doing the game theorist’s equivalent of spoiling a time-travel movie by pointing out the paradoxes, but the main premise of the story rests on some shaky epistemology.

Nobody ever lies, but what really drives the plot is that everybody is perfectly credulous.  Does it follow?  Partially:  if everybody has always been honest and you cannot conceive of the possiblity that anyone would lie, then yes you should assume that everything you hear is truthful.

But that’s not the same as the truth.  Take for example the scene in which Gervais’ character lies to the bank teller.  He tells her that he has $800 in his account.  She looks up his account on the computer which says that he has only $300.  What should she conclude?

She has to consider the possible scenarios that could have led to this contradiction and decide which is most likely.  She must believe that he is being truthful and the movie assumes that this leads her to the conclusion that the computer is wrong and he is right.  But surely people have often been mistaken even if they have always been truthful.  And so another scenario is that the computer is right and he is making an honest mistake.

Indeed it is probably more common that people make honest mistakes than computer records are wrong.  So that is the most likely explanation and its what the teller should have concluded.  And this kind of inferential dilemma underlies most of the movie.

In a world where everybody is honest but nobody is omniscient, someone who starts lying will sooner be viewed as delusional than a prophet.