One day in summer 1999, the comedian Stewart Lee was riding through London in a taxi when the driver turned to him. “I think,” said the driver, “that all homosexuals should be killed.”

Somewhat taken aback by the abruptness of this announcement, Lee asked why. “Well,” said the driver. “Because homosexuality is immoral.”

Politely, Lee suggested that notions of morality are far from immutable. For example, he said, modern Western society derives many of its fundamental principles – in ethics, aesthetics, philosophy and more – from ancient Greece: a society in which love between two men was regarded as the purest love of all.

The driver, however, was unimpressed.

“Yes, well,” he huffed. “You can prove anything with facts, can’t you?”