I was very rarely angry as a teacher. But one thing that got through my defences was overhearing my pupils comparing intelligences. They love to discuss who is cleverer than who, as much at university as at school. With the proposals for a revival of grammar schools, it seems the game has become a national habit.

A pernicious conception of “intelligence” is involved when people compare cleverness in this way. It defines intelligence as a univocal concept, as though it referred to one single quality, every instance of which could be measured against the same bar.

One boy I taught – let us call him Matthew – performed so abysmally that, after two years, my principles were buckling. I was starting to view him as lacking in mental quality. After his final exams, when my judgement of his abilities would have been fixed forever, I saw him in the end-of-year play. He was outstanding. I was ashamed, and after the show I embraced him with penitence.