Abstract
Although microscopic systems are described by means of quantum mechanics, the outcomes obtained when performing measurements on such systems “appear” to us as being “classical” (Giulini et al. 2003). This situation is puzzling. How does the classical appearance of these measurement outcomes emerge (problem A)? And why, after all, couldn’t we describe these measurement outcomes by means of quantummechanics (problem B)? Problem A can be tackled within the domain of physics. As a matter of fact, it has been partly solved by means of decoherence theory (Zurek 1981, 1982, 1991; Blanchard et al. 2000; Joos et al. 2003). Problem B falls under the competency of epistemology, as it deals with the respective role of classical physics and quantum mechanics. Concerning this topic of the quantum- classical cut, the contribution of Bohr and especially his thesis of the necessary use of classical concepts have been very influential during the twentieth century. Many scholars in the past have examined Bohr’s writings concerning this thesis (Folse 1985; Honner 1987; Murdoch 1987; Faye 1991; Chevalley 1991; Scheibe 1993). Recently, this thesis has been discussed in relation to decoherence theory. More precisely, Camilleri and Schlosshauer (2015) asked if Bohr’s thesis is challenged by decoherence theory. Their inquiry leads them to conclude that this is not the case. In other words, Bohr’s viewis still defendable. This chapter aims at showing that Bohr’s view is not only defendable but can throw light on the problems associated with the quantum- classical cut. First, I will recall that Bohr’s way of dealing with this cut is not ontological but epistemological. I will then put forward four conceptual distinctions and argue that those are essential to understand and discuss Bohr’s thesis of the necessary use of classical concepts. Finally, on the basis of this clarification, the relation between Bohr’s thesis and decoherence theory will be discussed.