'The idea which I shall present here came to me more or less out of the blue. I was on a train some five years ago, on my way to spend a day at Headingley, and I was reading a book about the death camp Sobibor... The particular, not very appropriate, conjunction involved for me in this train journey...had the effect of fixing my thoughts on one of the more dreadful features of human coexistence, when in the shape of a simple five-word phrase the...