In writing an article that argued both for and against the European Union, Boris Johnson was following a solidly classical precedent — that the finest exponents of the art of persuasion were those able to argue equally convincingly on both sides of any question.

An anonymous document entitled Dissoi Logoi (‘Two-sided arguments’, c. 4th Century BC) provided a long list of examples: ‘Death is bad for those who die, but good for the undertakers and the grave-diggers. Farming, when it makes a handsome success of producing crops, is good for the farmers, but bad for the merchants… It is shameful for a husband to adorn himself with white lead and wear gold ornaments, but proper for a wife. It is proper to do good to friends, but shameful to do it to enemies. And it is shameful to slaughter friends or fellow citizens, but proper to slaughter the foe.’

But is this all just verbal clever-dickery? The Greek philosopher Carneades, for example, once outraged strait-laced Romans by arguing on one day in favour of, and the next day against, justice. But a later writer explained his performance by saying that he was simply demonstrating to the Romans that they had not thought clearly enough about the subject.

This was Socrates’ preferred approach. When, for example, he was told that the Delphic oracle had said ‘no one is wiser than Socrates’, he did not believe it. So he decided to test the thesis and, when he had done so, concluded that he was wiser than anyone in this single respect: that he knew he knew nothing, while everyone else thought they knew something but clearly did not. In other words, self-examination was everything; ‘the unexamined life is not worth living for a human’, as he famously put it.

So one can be comforted that Boris has examined his thoughts on both sides of the argument. Just like Shami, in fact, who knows that it is proper for her to send her own child to a public school, but…