  By rigorously formalizing the Einstein-Podolsky-Rosen (EPR) argument, and
Bohr's 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 Alice's
system can be regarded as not disturbing Bob's 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 Bohr's definition of disturbance prevents the EPR chain of
reasoning from establishing even this. Moreover, I show that Bohr's 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 Bohr's sense) Bob's 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.
