Are we living in a quantum world? Bohr and quantum fundamentalism

Abstract: The spectacular successes of quantum physics have made it a commonplace to assert that we live in a quantum world. This idea seems to imply a kind of "quantum fundamentalism" according to which everything in the universe (if not the universe as a whole) is fundamentally of a quantum nature and ultimately describable in quantum-mechanical terms. Bohr's conception of quantum mechanics has traditionally been seen as opposed to such a view, not least because of his insistence on the necessity of the concepts of classical physics in the account of quantum phenomena. Recently, however, a consensus seems to be emerging among careful commentators on Bohr to the effect that he, after all, did subscribe to some version of quantum fundamentalism. Against this consensus, and by re-examining the historical record, I will defend a variant of the traditional reading of Bohr in which (1) the answer to what an object is (quantum or classical) depends on the experimental context; and (2) in principle, any physical system can be treated quantum-mechanically but not all systems can be treated that way at the same time.