1
Purpose and Plan
It must have been around 1950. I was accompanying Einstein on a walk from
The Institute for Advanced Study to his home, when he suddenly stopped, turned
to me, and asked me if I really believed that the moon exists only if I look at it.
The nature of our conversation was not particularly metaphysical. Rather, we
were discussing the quantum theory, in particular what is doable and knowable
in the sense of physical observation. The twentieth century physicist does not, of
course, claim to have the definitive answer to this question. He does know, how-
ever, that the answer given by his nineteenth century ancestors will no longer do.
They were almost exactly right, to be sure, as far as conditions of everyday life
are concerned, but their answer cannot be extrapolated to things moving nearly
as fast as light, or to things that are as small as atoms, or—in some respects—to
things that are as heavy as stars. We now know better than before that what man
can do under the best of circumstances depends on a careful specification of what
those circumstances are. That, in very broad terms, is the lesson of the theory of
relativity, which Einstein created, and of quantum mechanics, which he eventually
accepted as (in his words) the most successful theory of our period but which, he
believed, was none the less only provisional in character.
We walked on and continued talking about the moon and the meaning of the
expression to exist as it refers to inanimate objects. When we reached 112 Mercer
Street, I wished him a pleasant lunch, then returned to the Institute. As had been
the case on many earlier occasions, I had enjoyed the walk and felt better because
of the discussion even though it had ended inconclusively. I was used to that by
then, and as I walked back I wondered once again about the question, Why does
this man, who contributed so incomparably much to the creation of modern phys-
ics, remain so attached to the nineteenth century view of causality?
To make that question more precise, it is necessary to understand Einstein's
credo in regard not just to quantum physics but to all of physics. That much I
believe I know, and will endeavor to explain in what follows. However, in order
to answer the question, one needs to know not only his beliefs but also how they
came to be adopted. My conversations with Einstein taught me ,'ittle about that.
The issue was not purposely shunned; it simply was never raised. Only many
years after Einstein's death did I see the beginnings of an answer when I realized
5