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, however, 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 physics, 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 little 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