By rigorously formalizing the Einstein-Podolsky-Rosen (EPR) argument, and Bohrs reply, one can appreciate that both arguments were technically correct. Their opposed conclusions about the completeness of quantum mechanics hinged upon an explicit difference in their criteria for when a measurement on Alices system can be regarded as not disturbing Bobs system. The EPR criteria allow their conclusion (incompletness) to be reached by establishing the physical reality of just a single observable $q$ (not a conjugate pair $q$ and $p$), but I show that Bohrs definition of disturbance prevents the EPR chain of reasoning from establishing even this. Moreover, I show that Bohrs definition is intimately related to the asymmetric concept of quantum discord from quantum information theory: if and only if the joint state has no Alice-discord, she can measure any observable without disturbing (in Bohrs sense) Bobs system. Discord can be present even when systems are unentangled, and this has implications for our understanding of the historical development of notions of quantum nonlocality.
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