On the influence of Ernst Mach and the speculative origins of theoretical physics.

'As far as Mach is concerned, I must distinguish between Mach's general influence and his influence on me': Einstein underlines the importance both of Mach's specialist scientific work, such as the discovery of shock waves ('based on a really brilliant optical method') and also his influence on the foundations of physics in softening the dogmatism of the 18th and 19th centuries and advancing the primary importance of the empirical: 'he showed clearly that the most important problems in physics are not of a mathematical-deductive sort but those which concern the basic principles'. His weakness however was in his neglect of the constructive element in scientific thought: 'He thought in a certain sense that theories spring from discovery and not from invention'.

Einstein also acknowledges the importance of Mach's influence on his own development – partly thanks to Besso himself, who first brought Mach's works on mechanics and the theory of heat to his friend's attention. As for Mach's influence on Einstein's work, he finds it harder to define, and considers that David Hume may have had a greater direct influence, though he notes 'I am not in a position to analyse what is anchored in my unconscious thought'. Mach vigorously opposed the theory of special relativity, which seemed to him 'impermissibly speculative. He did not know that this speculative character is attached also to Newton's mechanics, or indeed to any imaginable theory. There is only a difference of degree between theories in so far as the path of thought from the basic principles to empirically verifiable consequences is more or less long and complicated'.

The letter moves on to a number of other observations and assertions by Besso, noting the importance of Minkowski in introducing four-dimensional tensor theory to relativity, 'without which the mathematical formulation of the general theory of relativity would have been unfeasible'. On the subject of Brownian motion, Einstein notes that he derived it from mechanics, without realising that something similar had already been observed. He mentions also the work of Niels Bohr on the orbits of electrons, though he does not see any link with his own work on quanta, noting in a parenthesis that 'I do not consider the current statistical quantum theory the right path, in spite of its great practical success. I am in the same attitude as the Jews with respect to the Messiah'. He goes with a brief discussion of morality, noting that with his belief in unlimited causality he naturally feels a certain scepticism about 'sin', and concludes with a statement of his belief that Besso could have achieved something of significance in science if he had been sufficiently focused: 'A butterfly is not a mole; but no butterfly should regret this'.