EINSTEIN'S VISION 467
feeling sure that in nature is actualized the ideal of mathematical simplicity' [E4].
As early as 1927, Heisenberg stressed, in a letter to Einstein, that the latter's
concept of simplicity and the simplicity inherent in quantum mechanics cannot be
realized at the same time. 'If I have understood correctly your point of view, then
you would gladly sacrifice the simplicity [of quantum mechanics] to the principle
of [classical] causality. Perhaps we could comfort ourselves [with the idea that]
the dear Lord could go beyond [quantum mechanics] and maintain causality. I do
not really find it beautiful, however, to demand more than a physical description
of the connection between experiments' [HI].
As Einstein's life drew to a close, doubts about his vision arose in his mind.
'The theory of relativity and the quantum theory... seem little adapted to
fusion into one unified theory,' he remarked in 1940 [E34]. He wrote to Born,
probably in 1949, 'Our respective hobby-horses have irretrievably run off in dif-
ferent directions.... Even I cannot adhere to [mine] with absolute confidence'
[E35]. In the early 1950s, he once said to me that he was not sure whether dif-
ferential geometry was to be the framework for further progress, but if it was then
he believed he was on the right track.* To his dear friend Besso he wrote in 1954,
'I consider it quite possible that physics cannot be based on the field concept, i.e.,
on continuous structures. In that case, nothing remains of my entire castle in the
air, gravitation theory included, [and of] the rest of modern physics' [E37]. I doubt
whether any physicist can be found who would not agree that this judgment is
unreasonably harsh. In one of the last of the many introductions Einstein wrote
for books by others, he said:
My efforts to complete the general theory of relativity... are in part due to the
conjecture that a sensible general relativistic [classical] field theory might per-
haps provide the key to a more complete quantum theory. This is a modest
hope, but certainly not a conviction. [E38]
But, as Helen Dukas told me, Einstein once said at the dinner table (she did not
recall the year) that he thought physicists would understand him a hundred years
later. Nor can I escape the impression that he was thinking about himself when
he wrote the following lines about Spinoza:
Although he lived three hundred years before our time, the spiritual situation
with which Spinoza had to cope peculiarly resembles our own. The reason for
this is that he was utterly convinced of the causal dependence of all phenomena,
at a time when the success accompanying the efforts to achieve a knowledge of
the causal relationship of natural phenomena was still quite modest. [E39].*V. Bargmann informs me that Einstein made similar remarks to him in the late 1930s. A related
comment is found in a letter to Infeld: 'I tend more and more to the opinion that one cannot come
further with a continuum theory" [E36].