When we think of any of the various “paradoxes” that have been put forward in quantum mechanics, it is clear that measurement is a key issue. In the last chapter, measurement lay at the heart of the discussion over the Einstein–Podolsky–Rosen paradox and over the existence of hidden variables. Indeed, almost from the beginning, the issue of measurement has been part of the discussion of quantum mechanics. In addition, this has been complicated by the argument over reality and when and where it arises in the quantum system. All of this points to a conundrum—what is a measurement? If we are to say that a measurement is defined by the experimental apparatus, as I would suspect Bohr would have us believe, then how are we supposed to express the possible outcomes? How does the experiment interact with the quantum system? What is meant by collapse of the wave function? These are not separate questions, but are strongly interrelated with each other. Moreover, they are also connected with what we believe the wave function to be. It is hard to not ask yourself, “What does this all mean?” Perhaps before proceeding, we should remind ourselves about the character of physical law and science, as expressed by Richard Feynman [1]:

What is necessary “for the very existence of science,” and what the characteristics of nature are, are not to be determined by pompous preconditions, they are determined always by the material with which we work, by nature herself. We look, and we see what we find, and we cannot say ahead of time successfully what it is going to look like. The most reasonable possibilities often turn out not to be the situation. If science is to progress, what we need is the ability to experiment, honesty in reporting results—the result must be reported without somebody saying what they would like the results to have been—and finally … the intelligence to interpret the results. An important point about this intelligence is that it should not be sure ahead of time what it must be. It can be prejudiced, and say “That is very unlikely; I don’t like that.” Prejudice is different from absolute certainty. I do not mean absolute prejudice—just bias. As long as you are only biased it does not make any difference, because if your bias is wrong a perpetual accumulation of experiments will perpetually annoy you until they cannot be disregarded any longer. They can only be disregarded if you are absolutely sure ahead of time of some precondition that science has to have. In fact it is necessary for the very existence of science that minds exist which do not allow that nature must satisfy some preconceived conditions.