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Bohr answered Einstein's challenge as follows:
<blockquote>[The argument of] Einstein, Podolsky and Rosen contains an ambiguity as regards the meaning of the expression "without in any way disturbing a system." ... [E]ven at this stage [i.e., the measurement of, for example, a particle that is part of an [[quantum entanglement|entangled]] pair], there is essentially the question of an influence on the very conditions which define the possible types of predictions regarding the future behavior of the system. Since these conditions constitute an inherent element of the description of any phenomenon to which the term "physical reality" can be properly attached, we see that the argumentation of the mentioned authors does not justify their conclusion that quantum-mechanical description is essentially incomplete."<ref>{{Cite journal |author=Bohr N |year=1935 |title=Can Quantum-Mechanical Description of Physical Reality be Considered Complete? |journal=Physical Review |volume=48 |issue=8 |pages=700 |url=http://prola.aps.org/pdf/PR/v48/i8/p696_1 |doi=10.1103/physrev.48.696|bibcode = 1935PhRv...48..696B |doi-access=free }}</ref></blockquote>
Bohr is here choosing to define a "physical reality" as limited to a phenomenon that is immediately observable by an arbitrarily chosen and explicitly specified technique, using his own special definition of the term 'phenomenon'. He wrote in 1948:
<blockquote>As a more appropriate way of expression, one may strongly advocate limitation of the use of the word ''phenomenon'' to refer exclusively to observations obtained under specified circumstances, including an account of the whole experiment."<ref>{{cite journal | author = Bohr N. | author-link = Niels Bohr | year = 1948 | title = On the notions of causality and complementarity | journal = Dialectica | volume = 2 | issue = 3–4| pages = 312–319 [317] | doi=10.1111/j.1746-8361.1948.tb00703.x}}</ref><ref>[[Léon Rosenfeld|Rosenfeld, L.]] (). 'Niels Bohr's contribution to epistemology', pp. 522–535 in ''Selected Papers of Léon Rosenfeld'', Cohen, R.S., Stachel, J.J. (editors), D. Riedel, Dordrecht, {{ISBN|978-90-277-0652-2}}, p. 531: "Moreover, the complete definition of the phenomenon must essentially contain the indication of some permanent mark left upon a recording device which is part of the apparatus; only by thus envisaging the phenomenon as a closed event, terminated by a permanent record, can we do justice to the typical wholeness of the quantal processes."</ref></blockquote>
This was, of course, in conflict with the definition used by the EPR paper, as follows:
<blockquote>''If, without in any way disturbing a system, we can predict with certainty (i.e., with probability equal to unity) the value of a physical quantity, then there exists an element of physical reality corresponding to this physical quantity.'' [Italics in original]<ref name="EPR"/></blockquote>
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