6 INTRODUCTORYthat, nearly a decade before the discovery of modern quantum mechanics, he had
been the first to understand that the nineteenth century ideal of causality was
about to become a grave issue in quantum physics. However, while I know more
now about the evolution of his thinking than I did when I walked with him, I
would not go so far as to say that I now understand why he chose to believe what
he did believe. When Einstein was fifty years old, he wrote in the introduction to
the biography by his son-in-law Rudolph Kayser, 'What has perhaps been over-
looked is the irrational, the inconsistent, the droll, even the insane, which nature,
inexhaustibly operative, implants in an individual, seemingly for her own amuse-
ment. But these things are singled out only in the crucible of one's own mind.'
Perhaps this statement is too optimistic about the reach of self-knowledge. Cer-
tainly it is a warning, and a fair one, to any biographer not to overdo answering
every question he may legitimately raise.
I should briefly explain how it happened that I went on that walk with Einstein
and why we came to talk about the moon. I was born in 1918 in Amsterdam. In
1941 I received my PhD with Leon Rosenfeld in Utrecht. Some time thereafter I
went into hiding in Amsterdam. Eventually I was caught and sent to the Gestapo
prison there. Those who were not executed were released shortly before VE Day.
Immediately after the war I applied for a postdoctoral fellowship at the Niels Bohr
Institute in Copenhagen and at The Institute for Advanced Study in Princeton
where I hoped to work with Pauli. I was accepted at both places and first went
to Copenhagen for one year. Soon thereafter, I worked with Bohr for a period of
several months. The following lines from my account of that experience are rele-
vant to the present subject: 'I must admit that in the early stages of the collabo-
ration I did not follow Bohr's line of thinking a good deal of the time and was in
fact often quite bewildered. I failed to see the relevance of such remarks as that
Schroedinger was completely shocked in 1927 when he was told of the probability
interpretation of quantum mechanics or a reference to some objection by Einstein
in 1928, which apparently had no bearing whatever on the subject at hand. But
it did not take very long before the fog started to lift. I began to grasp not only the
thread of Bohr's arguments but also their purpose. Just as in many sports a player
goes through warming-up exercises before entering the arena, so Bohr would
relive the struggles which it took before the content of quantum mechanics was
understood and accepted. I can say that in Bohr's mind this struggle started all
over every single day. This, I am convinced, was Bohr's inexhaustible source of
identity. Einstein appeared forever as his leading spiritual partner—even after the
latter's death he would argue with him as if Einstein were still alive' [PI].
In September 1946 I went to Princeton. The first thing I learned was that, in
the meantime, Pauli had gone to Zurich. Bohr also came to Princeton that same
month. Both of us attended the Princeton Bicentennial Meetings. I missed my first
opportunity to catch a glimpse of Einstein as he walked next to President Truman
in the academic parade. However, shortly thereafter, Bohr introduced me to Ein-
stein, who greeted a rather awed young man in a very friendly way. The conver-
sation on that occasion soon turned to the quantum theory. I listened as the two