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

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. 1 The Conceptual Framework for the Descrip-