'What are light quanta?'. A reflective letter with memories of Jewish culture, his late sister Maja and the happy days he shared with Besso in the Swiss federal patent office.

Einstein disclaims Besso's description of him as 'mighty' ('gewaltiger') with the Yiddish word 'nebbich ... that expressive word of our fathers'. After all, 'a whole 50 years of deliberate brooding have not brought me closer to the answer to the question "what are light quanta?". Nowadays any fool thinks he knows the answer, but he deceives himself. In relation to the natural generalisation of of the gravitation equations, I am now sure of it, but I cannot work out whether any kind of physical truth is hiding in it. It is all constructed on the non-symmetrical tensor g_ik (≠ g_ki)'.

Einstein teases Besso for having converted to the Christian faith ('You will certainly not go to hell, even if you have had yourself baptised'), though he is impressed that he is learning Hebrew: 'As a goy, you are not obliged to learn the language of our fathers, whilst I as a "Jewish saint" must feel ashamed at the fact that I know next to nothing of it. But I prefer to feel ashamed than to learn it'.

The letter also refers to Einstein's late sister, Maja (who had died in June 1951 after living the last 12 years of her life in Princeton with Einstein), and her husband Paul Winteler: 'He writes sometimes and in general shows an attachment to Maja of which I would never have believed him capable. I also miss her very much. During the years of her illness we read a good part of the best books of all times together. But her absolute favourite was Bertrand Russell – mine too, for that matter'. From time to time, Einstein makes public pronouncements against the growing militarism in the US, which has brought his name into a certain amount of discredit: 'this gives me the comforting impression of not having too much neglected my duty (as sometimes in the old days in the blessed patent office). I remember that time with great pleasure'.