Einstein's lasting conviction that quantum mechanics was not a theory of principle did not impede him from recognizing that this theory was highly successful. As early as 1927, he publicly expressed his judgment that wave mechanics is 'in amazing agreement with the facts of experience' [E1]. In 1936 he wrote, 'It seems clear . . . that the Born statistical interpretation of the quantum theory is the only possible one' [E2], and in 1949 declared, 'The statistical quantum theory [is] the most successful theory of our period' [E3]. Then why was he never convinced by it?
I believe Einstein indirectly answered this question in his 1933 Spencer lecture--perhaps the clearest and most revealing expression of his way of thinking in later life. The key is to be found in his remarks on Newton and classical mechanics. In this lecture [E4], Einstein noted that ' Newton felt by no means comfortable about the concept of absolute space, . . . of absolute rest . . . [and] about the introduction of action at a distance.' Then he went on to refer to the success of Newton's theory in these words: 'The enormous practical success of his theory may well have prevented him and the physicists of the eighteenth and nineteenth centuries from recognizing the fictitious character of the principles of his system.' It is important to note that by fictitious Einstein meant free inventions of the human mind. Whereupon he compared Newton's mechanics with his own work on general relativity: 'The fictitious character of the principles is made quite obvious by the fact that it is possible to exhibit two essentially different bases [Newtonian mechanics and general relativistic mechanics] each of which in its consequences leads to a large measure of agreement with experience.' (Remember that these words were spoken long before it was realized how markedly the predictions of Newtonian mechanics differ from those of general relativity when strong grativational fields come into play.)
In the Spencer lecture, Einstein mentioned the success not only of classical mechanics but also of the statistical interpretation of quantum theory. 'This conception is logically unexceptionable and has led to important successes.' But, he added, 'I still believe in the possibility of giving a model of reality which shall represent events themselves and not merely the probability of their occurence.'