What exactly is Niels Bohr's interpretation of quantum physics? And what general philosophical position underlies it? In the following pages I try to present clear and thorough answers to these questions. Bohr's interpretation is undoubtedly of major importance, but there is no universal agreement about what that interpretation is, owing largely to the notorious obscurity of his writings and the fact that his own views are often confused with those of others within the Copenhagen tradition which he founded. My reading is based solely on Bohr's own writings, and it is supported with liberal citations; I defend it against rival readings, but I try to keep polemical discussion to a minimum. When it illuminates Bohr's views, I have made use of material in the Niels Bohr Archive, much of which has now been published in Niels Bohr: Collected Works (six vols., eds. Léon Rosenfeld & Erik Rüdinger, North-Holland, Amsterdam, 1972–85). All translations are my own, unless the contrary is stated.

Bohr's interpretation of quantum physics, I believe, contrary to a widely held view, was not the outcome of a ready-made philosophy; rather, it developed gradually from his day-to-day grappling with the problems of the physics. It is best understood, therefore, when seen against the historical background of these problems. My account of this background in the first few chapters is selective, highlighting only those ideas and problems which played a prominent part in the formation of Bohr's views.