If everyone always lies to you, the consequence is not that you believe the lies, but that no one believes anything at all anymore — and rightly so, because lies, by their very nature, have to be changed, to be “re-lied,” so to speak. So a lying government which pursues different goals at different times has constantly to rewrite its own history. That means that the people are deprived not only of their capacity to act, but also of their capacity to think and to judge. And with such a people you can then do what you please.

Hannah Arendt (1906–1975) German-American philosopher, political theorist

Interview (1973-10) with Roger Errera, Office de Radiodiffusion-Télévision Française (ORTF)
    (Source)

Parts of this interview were turned into an episode of the French TV series "Un certain regard," directed by Jean-Claude Lubtchansky, first broadcast 1974-07-06. The interview was edited and retranslated in 2013 [tr. Andrew Brown], and collected in Arendt, Thinking without a Bannister: Essays in Understanding, 1953-1975 (2018) [ed. Jerome Kohn]. That is the text quoted.

This section was published in The New York Review of Books (1978-10-26), but in a somewhat altered form:

If everybody always lies to you, the consequence is not that you believe the lies, but rather that nobody believes anything any longer. This is because lies, by their very nature, have to be changed, and a lying government has constantly to rewrite its own history. On the receiving end you get not only one lie -- a lie which you could go on for the rest of your days -- but you get a great number of lies, depending on how the political wind blows. And a people that no longer can believe anything cannot make up its mind. It is deprived not only of its capacity to act but also of its capacity to think and to judge. And with such a people you can then do what you please.