Einstein, Podolsky, and Rosen presented their argument in 1935. They did not present it as a paradox, but rather as a demonstration that quantum mechanics does not, even in principle, describe all there is. The quantum-mechanical state, even if completely specified, does not encode all true (physically significant) information. This conclusion is, in some form, common to almost all interpretations of the theory. Thus, von Neumann would say that the state does not tell us which possible acausal transition will actually happen in a measurement, and the modal interpretation says that the state does not give full information about the values observables have or come to have. Indeed, how could indeterminism be reflected otherwise? But the EPR paper purports to establish a more specific conclusion about the values of incompatible observables. As we shall see, the fascination with the argument concerns a cluster of problems, centring on the non-classical correlations which we encountered already in the chapter on the empirical basis of quantum mechanics (Chapter 4) and have come across at many junctures since.
The structure of the EPR argument is both simple and clear. It is already set out in the abstract at the beginning, which I quote here in full:
In a complete theory there is an element corresponding to each element of reality. A sufficient condition for the reality of a physical quantity is the possibility of predicting it with certainty, without disturbing the system. In quantum mechanics in the case of two physical quantities described by non-commuting operators, the knowledge of one precludes