PRELIMINARIES 363opposition of one against the most basic views held by the other continued una-
bated until the end of Einstein's life. At issue were the criteria by which one should
judge the completeness of the description of the physical world. Their discussions
have not affected the evolution of physical theory. Yet theirs will be remembered
as one of the great debates on scientific principle between two dominant contem-
porary figures.
The dialogue between Bohr and Einstein had one positive outcome: it forced
Bohr to express the tenets of complementarity in increasingly precise language.
This debate will be one of the themes of Chapter 25, which deals with Einstein's
objections to quantum mechanics.
A point made earlier bears repeating here: Einstein's own visions on physics
issues were often in opposition to the mainstream, but they were never negative.
So it was in the case of quantum mechanics. After 1930 he considered this theory
to be consistent and successful but incomplete. At the same time, he had his own
aspirations for a future theory of particles and fields. I shall try to make clear in
Chapter 26 what these were.
I do not believe that Einstein presented valid arguments for the incompleteness
of quantum theory, but neither do I think that the times are ripe to answer the
question of whether the quantum-mechanical description is indeed complete, since
to this day the physics of particles and fields is a subject beset with many unre-
solved fundamental problems. Among these, there is one that was most dear to
Einstein and with which he (and all of us to date) struggled in vain: the synthesis
of quantum physics with general relativity. In the survey given in Chapter 2, I
noted that we still have far to go in regard to this synthesis. The assessment of
Einstein's view of this problem, to be given in Chapter 26, must therefore neces-
sarily be tentative.
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