Bohr's Como paper made very little impact upon its audience. It was
not a paper on physics and presented no new empirically testable consequences
for quantum theory. However, the Como paper did make it clear that Bohr
now regarded quantum theory in a certain sense as complete. This surprised,
puzzled and left unimpressed a great many of Bohr's colleagues.
Bohr had intended his Como paper as a general outline of his new framework
to be worked out in greater detail, and in the years following 1927 his
attention turned to refining his concepts of complementarity. The application
to quantum physics was always uppermost in Bohr's mind, but as early as
1929 it was obvious that he had a strong interest in carrying complementarity
as an epistemological lesson to other fields, eventually developing it
as a generalization of the classical framework over all empirical knowledge.
His thoughts did not develop as he had originally planned, however.
Instead, they were sharply influenced by the criticisms of Einstein, and
a great deal of his thoughts were manifested in specific reply to challenges
put forward by Einstein. Although the differences between Bohr and Einstein
reflected a real disagreement about the nature of scientifically describing
physical reality, there were also some very serious misunderstandings on
both sides. Bohr probably weakened his own cause by not stating at the
beginning that what was at stake was no less than a proposed revision of
our understanding of our relationship between physical reality and the
concepts we use to describe it. In a way, Einstein realized this more clearly
than Bohr himself.
The Bohr-Einstein Debates
About one month after Bohr's presentation of his Como paper, he met
Albert Einstein at the Solvay Congress in Brussels. Einstein opposed Bohr's
complementarity view and proposed the first of a series of thought experiments
designed to get around the uncertainty principle so that the classical
notion of the state of system could be retained. In 1927, Bohr had little
difficulty in showing that Einstein's experiment failed to provide more
empirical information about the system than permitted by the quantum postulate.
But at the Solvay Congress of 1930, Einstein presented a particularly difficult
Bohr spent a sleepless night pondering it, then demonstrated that Einstein
had failed to take into consideration the effects of his own general relativity
theory, which when considered, gave the exact degree of uncertainty that
was compatible with Heisenberg's principle. Then, in 1935, in conjunction
with B. Podolsky and N. Rosen, Einstein made his final attempt at a Gedanken-Experiment
show that the physical system had simultaneous properties that quantum
theory could not explain, demonstrating that the theory was incomplete.
This become commonly known as the EPR experiment, or paradox.
This experiment brought to light the opposition between two conceptions
of physical reality. Bohr concluded that complementarity is a "new feature
of natural philosophy [which] means a radical revision of our attitude
as regards physical reality...". Bohr felt that the difference between
complementarity and the classical framework was such that there cannot
be an "experimentum crucis" demonstrating the correctness of one
view or another. In other words, from the point of view of complementarity,
Bohr's answer was completely consistent, while from the point of view of
the classical framework, Einstein's objections were equally consistent.
Of course, Einstein's rejection allowed him to continue to use the classical
framework, while Bohr's acceptance of complementarity forced him to adopt
a new framework.
I am not interested in exploring the actual experiments themselves.
If the reader wishes, they may purchase "Atomic Theory and the Description
of Nature; Philosophical Writings of Niels Bohr Series, Vol. 1" and read
Bohr's comments personally. But I do want to stress that most of Bohr's
writings after 1927 were designed primarily to win Einstein over to the
framework of complementarity. This changed Bohr's original focus on complementarity
as a "description of nature" into a use as a "conceptual framework" for
a detailed analysis of the "physical" situation in numerous imaginary experiments.
This application to quantum theory gave way to the "Copenhagen Interpretation"
and Bohr's original concern with a fundamental revision of the framework
of science was largely lost from sight.
In his last interview, Bohr was asked, "Einstein was always apparently
asking for a definition or a clarification, a precise formulation of what
is the principle complementarity. Could you give that?" Bohr replied, "He
also got it, but he did not like it."  Einstein seemed to think of complementarity
as a principle in a physical sense, and so was frustrated by Bohr's
failure (from Einstein's viewpoint) to provide a clear formulation which
would have testable consequences, like Einstein's own principle of relativity.
Einstein argued that properties corresponding to classical parameters
belong to physical entities independent of observation, which he considered
justified because such parameters have empirical reference to observed
phenomenal properties which are used to confirm theory. Einstein felt that
since, with the classical framework, it is always possible in principle
to define precisely the mechanical state of the system even as it
is being observed, it then follows that if these parameters used to define
the state are interpreted as corresponding to properties possessed by the
system, these systems must "really" always have simultaneousness of such
properties as position and momentum.
The EPR Experiment
Einstein finally gave up trying to disprove quantum mechanics by finding
a way to simultaneously make observational determinations, and instead
focused on demonstrating the incompleteness of quantum theory. And to argue
that a theory is incomplete makes it necessary to stipulate what constitutes
completeness. As a criterion of completeness, Einstein suggested "every
element of physical reality must have a counterpart in physical theory.
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." 
In the EPR experiment, the authors propose a situation in which two
physical systems, the initial states of which are known, are allowed to
interact for a finite period of time. As noted already, quantum theory
says that once the systems interact, it becomes theoretically impossible
to define the states of each system separately. However, given the representations
of the initial states as defined in quantum formalism, it is possible to
define the state of the systems combined, treated in this sense as an individual
interacting whole. What cannot be defined by quantum theory is the state
of each system considered separately either during the interaction or after
that interaction. For that, another observation is needed. As the uncertainty
principle demands, observation cannot determine both parameters
necessary to define precisely the state of either system. However, from
the observation of the first system and theoretical definition of the combined
states of the two systems, by means of unambiguous communication, it is
possible to theoretically predict the interaction of the second
system without in any way interacting with that system.
Bohr did not dispute this conclusion. And indeed, using the assumption
that "position" and "momentum" refer to properties possessed by systems
independent of an observational interaction, then what the observing system
records is the causal effects of said properties. The claim that measuring
the value of a phenomenal observable which is in no way physically interacting
with the observer would seem to demand a mysterious, non-physical communication
between the two systems. This claim does indeed seem unreasonable, as the
EPR experiment was designed to show.
However, from Bohr's point of view, these same theoretical parameters
do not refer to properties of an independent reality, but instead
refer to phenomenal properties. From this point of view, if there
is no interaction to observe, say the position of the system, there is
no phenomenal property to which the position parameter can refer. From
this point of view, Einstein's conclusion seems equally unreasonable. Bohr
believed that although we have "free choice" of which experiment to observe,
that freedom merely indicated a freedom in which phenomenon we choose to
bring about. Bohr writes:
...in the phenomena concerned we are not dealing with an incomplete
description characterized by the arbitrary picking out of different elements
of physical reality at the cost of sacrificing other such elements, but
with a rational discrimination between essentially different experimental
arrangements and procedures which are suited either for an unambiguous
use of the idea of space location, or for a legitimate application of the
conservation theorem of momentum. Any remaining appearance of arbitrariness
concerns merely our freedom of handling the measuring instruments, characteristic
of the very idea of experiment.
In fact, the renunciation in each experimental arrangement of the one
or the other of two aspects of the description of physical phenomena -
the combination of which characterizes the method of classical physics,
and which therefore in this sense may be considered complementary to
one another - depends essentially on the impossibility, in the field of
quantum theory, of accurately controlling the reaction of the object on
the measuring instruments, i.e., the transfer of momentum in the
case of position measurements and the displacement in case of momentum
measurements...we are, in the "freedom of choice" offered by the...[EPR]
arrangement, just concerned with a discrimination between different
experimental procedures which allow of the unambiguous use of complementary
classical concepts. 
Bohr's alternative interpretation of the EPR experiment revealed the difference
between two conceptions of physical reality. The ambiguity that Bohr focused
on was hidden in Einstein's use of "state of the system". Bohr believed
that when we determine the state of the system, the "system" to which we
refer is necessarily a phenomenal object. Einstein, on the other
hand, believed that observable properties of the state of the system is
presumed to be existing independent from the observation. In a paper written
in 1939, four years after the EPR experiment was proposed, Bohr writes:
...the very fact that in quantum phenomena no sharp separation
can be made between an independent behavior of the objects and their interaction
with the measurement instruments, lends indeed to any such phenomena a
novel feature of individuality which evades all attempts at analysis on
classical lines, because every imaginable experimental arrangement aiming
at a subdivision of the phenomena will be incompatible with its appearance
and give rise, within the latitude indicated by the uncertainty relations,
to other phenomena of similar individual character.
In fact the [EPR] paradox finds its complete solution within the framework
of the quantum mechanical formalism, according to which no well-defined
use of the concept of "state" can be made as referring to the object separate
from the body with which it has been in contact, until the external conditions
involved in the definition of this concept are unambiguously fixed by a
further suitable control of the auxiliary body. 
Bohr's point is that there can be no empirical reason for assuming classical
parameters as referring to an "independent reality", and if one does make
such an assumption, quantum theory becomes involved in the contradiction
of asserting the atomic domain system as having incompatible properties
of both particles and waves. As Bohr saw it, these descriptive concepts
are well-defined only in reference to observed phenomenal objects,
From Einstein's point of view, it seemed the "free choice" we have of
which of the two possible measurements to make involves one and the
same system. But from Bohr's point of view, in order to give terms
like "position" and "momentum" empirical significance, "system" must be
interpreted in the sense of that which is observed. And since the experimental
arrangement necessary for alternative observations of position and momentum
are mutually exclusive, he concluded that the two observations refer to
the properties of two distinct phenomenal objects. There was simply
no "same system" in a sense that both position and momentum could be observed.
In short, the EPR experiment, as advanced by Einstein, assumes that
in the interaction necessary to determine the parameter representing the
phenomenal observation, it remains possible to describe the system independently
of the observational interaction. And if the observing apparatus is changed,
for example, if measurement involves momentum instead of position, Einstein
assumed that it will not influence the "property" of the "system". In fact,
Bohr was disheartened by Einstein's reasoning, for he felt it missed the
core of his framework of complementarity and the quantum postulate. And
unfortunately, since all Bohr's arguments were erected on the quantum postulate,
it all by-passed Einstein.
Bohr's Concept of "Phenomenon"
Following 1935, Bohr distinguished his view by emphasizing that the
description of nature is free from ambiguity and metaphysical dogma only
if it is realized that the observational basis of science depends on devising
ways to describe unambiguously an individual interaction between systems.
This point of view was already implicit in his Como paper, however after
1935 there was a real change in Bohr's emphasis and manner of speaking.
In his first philosophical writing after the EPR experiment, in 1937, Bohr
is careful to point out that what we must renounce is the classical attempt
to "visualize" objects of quantum description as possessing "such inherent
attributes as the idealizations of classical physics [i.e., "waves"
and "particles"] would ascribe to the object.
...the fundamental postulate of the indivisibility of the quantum
of action is itself, from the classical point of view, an irrational element
which inevitably requires us to forgo a causal mode of description and
which, because of the coupling between phenomena and their observation,
forces us to adopt a new mode of description designated as complementary
in the sense that any given application of classical concepts precludes
the simultaneous use of other classical concepts which in a different connection
are equally necessary for the elucidation of the phenomena...
...the finite magnitude of the quantum of action prevents altogether
a sharp distinction being made between a phenomenon and the agency by which
it is observed, a distinction which underlies the customary concept
of observation and, therefore, forms the basis of the classical ideas of
From 1939 on, Bohr always used the word "phenomenon" to refer to the
whole observational interaction. Bohr emphasized that the two complementary
measurements of position and momentum are different phenomena and
hence determine the properties of different phenomenal objects. Eventually
he adopted a way of speaking which referred to the complementarity of different
or complementary evidence from different observations.
For this reason, some physicists have seen Bohr's complementarity as
having undergone a radical change during this time and that therefore there
are really two distinct notions of complementarity. Bohr agreed that complementarity
presents a many faceted viewpoint, yet he resolutely maintained that the
complementarity of modes of description and the complementarity of phenomena
are simply two consequences of the quantum postulate. The first indication
of a change in Bohr's way of describing complementarity came in 1938 when
Information regarding the behavior of an atomic object obtained
under definite experimental conditions may...be adequately characterized
as complementary to any information about the same object by some
other experimental arrangement excluding the fulfillment of the first conditions.
Later, Bohr began to de-emphasize the complementarity between space-time
description and applying the conservation principle in favor of defining
the complementary relationship as holding between phenomena.
Although the phenomena in quantum physics can no longer be
combined in the customary manner, they can be said to be complementary
in a sense that only together do they exhaust the evidence regarding the
objects, which is unambiguously definable. 
This sentence, or variations of it, became a standard formula for Bohr's
later work and is repeated over and over again in later essays. He does
continue to use other formulations as well however, for example:
...we are faced with the contrast revealed by the comparison
between observations regarding an atomic system, obtained by means of different
experimental arrangements. Such empirical evidence exhibits a novel relationship,
which has no analogue in classical physics and which may be conveniently
termed "complementarity" in order to stress that in the contrasting phenomena
we have to do with equally essential aspects of all well-defined knowledge
about the objects. 
Once it is recognized that Bohr's talk of complementarity phenomena refers
to the effects observed under different experimental conditions, then the
confusion between complementary aspects of nature and complementary descriptive
modes disappears. All of Bohr's analyses of experiments in his debates
with Einstein were intended to show that determining the spatio-temporal
properties of the observed object is possible only in experimental interactions
which preclude any interaction determining momentum or energy properties
of the object. Thus, to give empirical reference to terms like spatio-temporal
co-ordination and causal descriptions require physical interactions, "phenomena"
in Bohr's later usage, which exclude each other but are complementary.
The Object of Description
His debate with Einstein forced Bohr to realize that there was a fundamental
ambiguity hidden in the notion of "physical reality" as that which is described
in natural science. In order for observation to determine unambiguously
the values of phenomenal objects, the description of the observation must
indicate exactly what part of the whole interaction is considered observing
instruments and what part is considered phenomenal object being observed.
For this reason, Bohr introduced an "inherent arbitrariness" into the description
of any phenomenal object, making what is now part of the observing system
part of the observed or visa versa. Only by making such an arbitrary
separation between what is treated as the means of observation and what
the phenomenal object is can an unambiguous description of what
is observed be communicated.
"Observing system" and "observed object" are terms which are well defined
only in the context of a particular description of interaction.
They must be regarded as descriptive categories invoked for an unambiguous
communication of the results of an observation rather than as referring
to different constituents of nature. The requirement that descriptive terms
must have some empirical reference if they are to describe phenomenal objects
make their attempted use to describe an atomic system apart from its phenomenal
appearances in observational interactions descriptively ambiguous.
Bohr never argued that the notion of object as an independent reality
is "meaningless", but he did argue that because of a certain physical fact,
the individuality of atomic interactions, the classical description of
particles and waves become restricted in their reference to the phenomenal
object. Therefore, the use of these concepts to picture the
state of an isolated system does not refer to an independent reality
but to an abstraction. Bohr's point is that we cannot describe an independent
reality in the manner which was thought possible in the classical framework,
because that framework makes different presuppositions about the nature
An example involving the concept of temperature may help clarify Bohr's
position here. Since it has been empirically discovered that material bodies
can be represented as composed of many molecules, and that the property
of temperature observed in many-molecule bodies can be represented as the
causal effects of the motion of these molecules, these physical facts make
the concept of temperature inapplicable below the molecular level.
In Bohr's epistemological lesson we are taught that at the atomic level,
spatio-temporal concepts are not defined in an unambiguous sense for an
objective description of an independent physical reality.
This ends Part 5 of this review. Thanks for reading!
 Niels Bohr, Can Quantum Mechanical Description of Reality Be Considered
 AHQP, Bohr, Last Interview, page 4
 A. Einstein, B. Podolsky, N. Rosen, Can Quantum Mechanical Description
of Reality Be Considered Complete? Review 48
 Niels Bohr, Can Quantum Mechanical Description of Reality Be Considered
 Niels Bohr, The Causality Problem in Atomic Physics
 Niels Bohr, Atomic Theory and the Description of Nature
 Niels Bohr, Natural Philosophy and Human Culture
 Niels Bohr, Newton's Principles and Modern Atomic Physics
 Niels Bohr, On the Notions of Causality and Complementarity