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He then used a hypothesis of local realism to conclude that the other particle had these properties already determined. The principle he proposed is that if it is possible to determine what the answer to a position or momentum measurement would be, without in any way disturbing the particle, then the particle actually has values of position or momentum.
More: http://en.wikipedia.org/wiki/Albert_Einstein#Einstein.E2.80.93Podolsky.E2.80.93Rosen_paradox