"What I said about the rejection of freedom when I was sixteen assumed physical determinism more dogmatically than I can do now, though I still think it the most probable hypothesis as regards macroscopic phenomena. I do not know of anybody else who has dealt with the topic with which you propose to deal. I could hardly say that the notion of freedom is a key notion in my ethical thinking. I think freedom important, but with increasing density of population it has to undergo increasing limitations."