 In the original EPR article, two particles are prepared from a source in a certain ‘pure’ quantum state of the composite system (a state that cannot be expressed as a mixture or probability distribution of other pure quantum states, and cannot be reduced to a pure quantum state of each particle separately). After the particles move apart, there are ‘matching’ correlations between both the positions of the two particles and their momenta: a measurement of either position or momentum on a particular particle will allow the prediction, with certainty, of the outcome of a position measurement or momentum measurement, respectively, on the other particle. These measurements are mutually exclusive: either a position measurement can be performed, or a momentum measurement, but not both simultaneously. The subsequent measurement of momentum, say, after establishing a position correlation, will no longer yield any correlation in the momenta of the two particles. It is as if the position measurement disturbs the correlation between the momentum values, and conversely. Apart from this peculiarity that either correlation can be observed, but not both for the same pair of quantum particles, the position and momentum correlations for the quantum particles are exactly like the classical correlations between two billiard balls after a collision. Classical correlations can be explained by a common cause, or correlated ‘elements of reality.’ The EPR argument is that quantum mechanics is incomplete because these common causes or elements of reality are not included in the quantum state description. 