 In 1935 and 1936, Schrödinger published a two-part article in the Proceedings of the Cambridge Philosophical Society in which he discussed and extended an argument by Einstein, Podolsky, and Rosen. The Einstein-Podolsky-Rosen (EPR) argument was, in many ways, the culmination of Einstein’s critique of the orthodox Copenhagen interpretation of quantum mechanics and was designed to show that the theory is incomplete. (See the entries on the  Einstein-Podolsky-Rosen argument in quantum theory  and the  Copenhagen interpretation of quantum mechanics.)  In classical mechanics the state of a system is essentially a list of the system’s properties — more precisely, it is the specification of a set of parameters from which the list of properties can be reconstructed: the positions and momenta of all the particles comprising the system (or similar parameters in the case of fields). The dynamics of the theory specifies how properties change in terms of a law of evolution for the state. In a letter to Max Born, Wolfgang Pauli characterized this mode of description of physical systems as a ‘detached observer’ idealization (see The Born-Einstein Letters, Born, 1992; p. 218). On the Copenhagen interpretation, such a description is not possible for quantum systems. Instead, the quantum state of a system should be understood as a catalogue of what an observer has done to the system and what has been observed, and the import of the state then lies in the probabilities that can be inferred (in terms of the theory) for the outcomes of possible future observations on the system. Einstein rejected this view and proposed a series of arguments to show that the quantum state is simply an incomplete characterization of a quantum system. The missing parameters are sometimes referred to as ‘hidden parameters’ or ‘hidden variables.’