Bohr's Conception of the Quantum Mechanical State of a System and Its Role in the Framework of Complementarity

Abstract

What Niels Bohr called the `epistemological lesson' of `complementarity' was the result of reasoning analogically from the classical conception of a mechanical state to a new quantum mechanical conception of an `object' in a mechanical state. Bohr proposed to redefine the `objectivity' essential for scientific description in terms of the epistemological demand for unambiguously communicable descriptions of observational results, a move which has profound consequences for how we can understand the concept of the quantum mechanical state and the nature of the `system' which is `in' this state. Here it is argued that the old notion of the `object' which is in a classical mechanical state is drawn from a substance/property ontology derived from Aristotle's analysis of categorical propositions. In moving to describing a system in a quantum mechanical state, the system that is `in' such a state can no longer be so regarded as a substance possessing properties. Bohr argues that the concept refers to an interaction which has a feature of wholeness or `individuality' that implies that the distinction between `object system' and `observing system' is relative to the context of the description. This conclusion, in turn, implies the need for a combination of complementary modes of description; however, because of his reticence in making ontological claims, he failed to develop this dimension of his new framework of complementarity.