Simon Capelin, of Cambridge University Press, suggested that I send him my papers on quantum philosophy and let him make them into a book. I have done so. The papers, from the years 1964–1986, are presented here in the order, as far as I now can tell, in which they were written. But of course that is not the order, if any, in which they should be read.

Papers 18 and 20, ‘Speakable and unspeakable in quantum mechanics’ and ‘Six possible worlds of quantum mechanics’, are nontechnical introductions to the subject. They are meant to be intelligible to nonphysicists. So also is most of paper 16, ‘Bertlmann's socks and the nature of reality’, which is concerned with the problem of apparent action at a distance.

For those who know something of quantum formalism, paper 3, ‘The moral aspect of quantum mechanics’, introduces the infamous ‘measurement problem’. I thank Michael Nauenberg, who was co-author of that paper, for permission to include it here. At about the same level, paper 17, ‘On the impossible pilot wave’, begins the discussion of ‘hidden variables’, and of related ‘impossibility’ proofs.

More elaborate discussions of the ‘measurement problem’ are given in paper 6, ‘On wavepacket reduction in the Coleman–Hepp model’, and in 15, ‘Quantum mechanics for cosmologists’. These show my conviction that, despite numerous solutions of the problem ‘for all practical purposes’, a problem of principle remains. It is that of locating precisely the boundary between what must be described by wavy quantum states on the one hand, and in Bohr's ‘classical terms’ on the other.