Quantum theory grew up, from Planck to Heisenberg and Schroedinger, in response to a welter of new experimental phenomena: measurements of the heat radiation spectrum, the photoelectric effect, specific heats of solids, radioactive decay, the hydrogen spectrum, and confusingly much more. Yet this theory, emerging from the mire and blood of empirical research, radically affected the scientific world-picture. If it did describe a world 'behind the phenomena', that world was so esoteric as to be literally unimaginable. The very language it used was broken: an analogical extension of the classical language that it discredits, and redeemed at best by the mathematics that it tries to gloss.
Interpretation of quantum theory became genuinely feasible only after von Neumann's theoretical unification in 1932. Von Neumann himself, in that work, attempted to codify what he took to be the common understanding. Astonishingly, the attempt led him to assert that in measurement something happens which violates Schroedinger's equation, the theory's cornerstone. As he saw very clearly, interpretation enters a circle when its main principle is Born's Rule for measurement outcome probabilities, while at the same time measurements are processes in the domain of the theory itself. Behold the enchanted forest: every road leads into it, and none leads out—or does the hero's sword cleave the wood by magic?
An empiricist bias will be evident throughout this book, but my own interpretation of quantum mechanics does not begin until Chapter 9 . The first three chapters provide philosophical background; though they overlap my Laws and Symmetry, I have tried to make them interesting in their own right. The next four chapters mainly outline the achievements of foundational research, though with an eye to the philosophical issues to come. The negative part is to show that the phenomena themselves, and not theoretical motives, can suffice to eliminate Common Cause models of the observable world. The positive part is the conclusion that there are adequate descriptions of