 Bohr’s firmness about the use of classical concepts for the descriptions of measurement can be seen as his response to the measurement problem. This problem arises from the fact that quantum mechanics itself cannot account for why experiments on objects in a state of superposition always produce a definite outcome. Hence if one does not argue for spontaneous collapse of the wave function, hidden variables, or many worlds, one needs to supplement quantum mechanics with a classical description of measuring instruments in terms of clocks and rods. Henrik Zinkernagel (2015, 2016) may seem to get close to Bohr’s view when he argues that Bohr not so much solved the measuring problem as he dissolved it. According to his interpretation, Bohr believed in a quantum world, but only relative to a particular classical description and a certain classical world. The distinction between classical and quantum (both ontic and epistemic) is contextual. He thinks that the measurement problem is ultimately a consequence of ontological quantum fundamentalism (that everything is quantum). Because if everything is quantum – and correctly described by quantum formalism (what else would it mean to call everything quantum?) – then a measurement ends up in a superposition whether we describe the apparatus classical or not. One could say with Zinkernagel that Bohr believed all objects can be treated as quantum objects, but they cannot all be treated as quantum objects at the same time. Borrowing a conception from the two Russian physicists, Landau and Lifshitz, Zinkernagel claims that only some parts of the measuring device are entangled with the object in question, but those parts which are not entangled exists as a classical object. Depending on the context, objects cannot be treated as quantum objects in those situations in which they acts as measuring apparatuses. In these situations the classical treatment of the measuring device provides us with a frame of reference of space and time with respect to which an atomic object has a position, and, mutatis mutandis, with respect to which it has energy and momentum. Such a frame of reference is necessary for our ability to define and measure a particular property. In Bohr’s own words: “in each case [of measurement] some ultimate measuring instruments, like the scales and clocks which determine the frame of space-time coordination on which, in the last resort, even the definitions of momentum and energy quantities rest, must always be described entirely on classical lines, and consequently kept outside the system subject to quantum mechanical treatment” (CC, p. 104). What characterizes a frame of reference is that it establishes the conditions for the ascription of a well-defined position or a well-defined momentum, and treated classically measuring instruments act exactly as frames of reference. The implication is that Bohr did not exclude the application of quantum theory to any system. Every system can in principle be treated quantum mechanically, but since we always need a frame of reference to describe experimental outcomes, not all systems can be treated quantum mechanically at once.