The only lecture, I think, that I didn’t understand a word of was given by Wolfgang Pauli – as you know, the discoverer of electron spin and the exclusion principle, and he was talking... and there was a thing called the Kronecker delta, which came in, which is an easy enough idea if you understand it, but I’d never heard of it, so the rest was lost on me. But it turns out it’s a function which has the value of either one or zero, but it’s called the Kronecker delta function for a particular reason. But that was electrodynamics - I really didn’t understand any of that.

But he was a fluent lecturer. It was a physics named lecture, as I remember, and I can remember he had a good use of English. And there was one where he had said, or the story was told, that someone had asked him a question – that wasn’t in the lecture that I was at – and Pauli, who was Herr Professor in Switzerland, I think he was at the time, in Zurich; he said, ‘That question is impertinent, young man, but in the present context, it is also pertinent’. And he answered it!