Niels Bohr on the wave function and the classical/quantum divide

• Bohr׳s symbolic understanding of the wave function includes some ontological connotations.
• The collapse of the wave function is a formal notion, not a physical process.
• Classical descriptions are necessary for Bohr׳s dissolution of the measurement problem.
• Whether an object is quantum or classical depends on the context.

It is well known that Niels Bohr insisted on the necessity of classical concepts in the account of quantum phenomena. But there is little consensus concerning his reasons, and what he exactly meant by this. In this paper, I re-examine Bohr׳s interpretation of quantum mechanics, and argue that the necessity of the classical can be seen as part of his response to the measurement problem. More generally, I attempt to clarify Bohr׳s view on the classical/quantum divide, arguing that the relation between the two theories is that of mutual dependence. An important element in this clarification consists in distinguishing Bohr׳s idea of the wave function as symbolic from both a purely epistemic and an ontological interpretation. Together with new evidence concerning Bohr׳s conception of the wave function collapse, this sets his interpretation apart from both standard versions of the Copenhagen interpretation, and from some of the reconstructions of his view found in the literature. I conclude with a few remarks on how Bohr’s ideas make much sense also when modern developments in quantum gravity and early universe cosmology are taken into account.