“I am not a crook.” Richard Nixon resigned as president of the US 40 years ago this week, and of all the things he said in his political career this quote is the one that lives on. He did also popularise the phrase “the silent majority”, although it is seldom attributed to him.

To state the obvious: we remember the first quote because it seems to us in hindsight to have been so audacious a lie. It takes its place with “I did not have sexual relations with that woman, Miss Lewinsky” (another US president, Bill Clinton) and the promise to “cut out the cancer of bent and twisted journalism in our country with the simple sword of truth and the trusty shield of British fair play”, by a man jailed after admitting perjury and perverting the course of justice (Nixon biographer Jonathan Aitken).

But in the moment it was uttered, it is reasonable to assume it would have been pretty effective as a piece of rhetoric. “I am not a crook” is a special use of ethos (the speaker’s connection with the audience). Logically speaking, it is redundant: if you are accused of X, to say X is untrue is to deny, rather than to disprove, the charge. But by adding the barefaced lie to the mix you raise the stakes. It puts doubt in your audience’s mind. Indignation sounds – even though it is not – like evidence of innocence.