There is no quantum world. ... only an abstract quantum de-
scription. It is wrong to think that the task of physics is to find
out how nature is. Physics concerns what we can say about
nature.

Niels Bohr

Physics is an attempt conceptually to grasp reality as it is thought
independently of its being conceived. In this sense one speaks
of physical reality.

Albert Einstein

We have been using Bohr's remarks as a summary statement of anti-
realism. There are two parts to his remarks, one about nature itself and
the other about an aspect of our relation with nature, knowledge. On the
first part, there is no quantum world. On the second, we cannot know
how nature really is; we can only know nature as it appears to us.

Einstein's comment is a claim of realism. It too has two parts, one about
nature itself and the other about our knowledge of nature. On the first,
there is an independent reality, an existing state of nature in all realms of
sizes, quantum and classical. Furthermore, we can know about it. We can
transcend the appearances and come to know how nature really is.

Our concern is not with Bohr or Einstein; it is with realism and anti-
realism. The issue is not what these people really thought. I have just been
using these men and their provocative ideas to get your attention on the
question of nature and knowledge. The real issue is whether there is any-
thing in modern physics that favors the sort of anti-realism claimed by
Bohr or the realism proposed by Einstein. Is there anything in the exper-
iments or the principles of quantum mechanics or relativity that indicates
that there is (or is not) a real world out there, independent of our obser-
vations or thoughts, or anything that indicates that we can (or cannot)
know how nature really is?

Of course the physics could be neutral on these questions, supporting
neither one claim nor the other. These could be purely philosophical ques-
tions. They might even be unanswerable questions, immune to resolution
by evidence or logic. Then realism or anti-realism would be a matter of
speculation and personal taste (and not worth reading a book about). It
is also possible that the physics presents mixed evidence about realism,
supporting realism in some cases but anti-realism in others. All of this is
possible, but none of it is the case. The issue of realism is not a matter of

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