Original Source
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Complementarity and the Nature of Empirical Knowledge

While Bohr's failure to convert Einstein to his point of view was a significant disappointment to him personally, the nature of their disagreement could be put to profitable issues, as both men spoke the language of physics. However, the failure of complementarity to draw any appreciable audience among philosophers was for Bohr a cause of bewilderment and frustration. This was because Bohr and the philosophers hardly spoke the same language. Bohr's statement in his last interview: "I think that it would be reasonable to say that no man who is called a philosopher really understands what is meant by the complementarity description" really says it all.
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It would perhaps be helpful to consider in general the central issue in philosophy over which communications between Bohr and the "philosophers" broke down. This issue divides those who hold scientific theories are attempts to describe the phenomena we experience as the empirical evidence of the behavior of the objects behind these phenomena and those who don't. The former hold the view called "realism" while their opponents are called "anti-realists". However, it is hardly the case that all defenders of realism in science defend the same form of realism. Indeed, the issues that separated Bohr and Einstein were between two alternative forms of realism. Therefore, it is convenient to distinguish "classical realism" from that of "complementarity realism".
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The classical realist holds that the success of the theories in classical physics legitimizes the claim that phenomena are a consequence of the objects having properties corresponding to the terms used to characterize physical systems in classical mechanics. From the classical realist point of view, the concept of the state of the system defined in terms of classical parameters may be regarded as referring to properties possessed by an independently real object, which causes the phenomena that confirm theoretical representations of the system. Bohr's complementarity realism denies this claim of an independently real object, but offers the possibility of an alternative form of knowledge of the domain lying behind the phenomena.
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Just as there are various forms of realism, so are there various forms of anti-realism. The dominant one during Bohr's life was that of "instrumentalism", the view that theoretical terms serve only as constructs enabling predictions concerning phenomena observed in specific circumstances. Another form of anti-realism was known as "phenomenalism", the assertion that the only reality with any content is that of phenomena, and therefore statements about a reality lying behind the phenomena are meaningless. Both these views have been incorrectly imputed to Bohr.
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Bohr did not see the issue between himself and the philosophers as an ontological question about the nature of physical reality. It was important for Bohr to discover whether or not complementarity provided an "objective" description of phenomena. In his last interview, Bohr complained that the philosophers "did not see that it [complementarity] was an objective description, and it was the only possible description. So therefore the relationship between scientists and philosophers is a very curious one".
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Bohr saw the misunderstanding between himself and the philosophers as an epistemological issue of the requirements for an "objective description". Complementarity, as a conceptual framework for describing nature, is meant to be a rational generalization of the framework of classical mechanics. Since classical mechanics is not an epistemology per se, by analogy neither is complementarity. Nevertheless, the advent and subsequent dominance of mechanism had repercussions in epistemology, and the success of classical mechanics was considered a major datum for the traditions of rationalism and empiricism, as well as Kant's critical philosophy. Therefore it is natural to ask, if complementarity is a general framework by which we are able to better understand the description of nature in natural science, what does it tell us about the nature of scientific knowledge?
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The Relationship Between Complementarity and Epistemology

Classical mechanism provides a directive to look for theories and descriptions of a certain kind, which can be accessed as adequate or acceptable descriptions. Mechanism provides an "ideal" for the description of nature by stating which accounts of phenomena can be considered as acceptable claims of scientific knowledge. This classical ideal gives a standard against which prospective theoretical descriptions can be measured. Mechanism not only provides a framework of concepts such that they have specific meaning in the description of nature, but it also serves a directive function orientating the progress of science towards a projected goal of describing an observed phenomenon in accord with its ideal of description.
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The development of quantum mechanics was conditioned by the ideal of classical physics. Since these standards were, which Einstein said, not to be regarded as changeable, quantum theory was necessarily incomplete. These standards imply that a completely objective description is one that determines properties possessed by an independent reality by representing the object as a physical system isolated from any other object. If these properties are represented by classical parameters, then it follows that the inability of quantum theory to represent the object of its descriptions in well-defined classical states means that it does not fulfill what the classical framework considers acceptable scientific method.
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Bohr developed his complementarity to change the standards for this "ideal" description so that quantum theory would be evaluated as presenting an acceptable, consistent and complete description of atomic systems. Therefore, the switch from mechanism to complementarity implies a change of the standards of what defines acceptable science. Complementarity changes the significance of the terms of description of objectivity from what Einstein demanded.
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Bohr's epistemological lesson begins with the recognition that empirical knowledge must ultimately be be based upon experiences which are the property of a perceiving subject, but in scientific knowledge we proceed from this subjective starting point to an objective account of experienced phenomena. For Bohr, "objectivity" is a property of the descriptions of phenomena which science provides. However, the common perception of classical mechanics as a program for eliminating purposive descriptions of natural phenomena is a consequence that such descriptions were the result of importing subjective categories into the description. They provide terms for an "objective description" because they refer to what is directly observable to any normal experiencing subject and can be communicated by unambiguous measurements expressed in the language of mathematics.
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Classically, it is possible to hold that unambiguous descriptions which these terms made possible referred to a reality behind the experienced phenomena. To a classical realist, the "objectivity" of mechanical descriptions implied much more than that they were unambiguously communicable. Objectivity could be grounded in a reality behind the phenomena because the terms defining the state of the system could be consistently held to refer to the properties of an independent reality. If one assumed some sort of causal link between the mental and the physical, it was possible to consider the properties of the observed phenomena as the mechanical causal effects of properties possessed by an independent reality interacting with the perceiving subject's sense organs.
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Subjectivity and the Description of Experience

Bohr's complementarity was vitally concerned with how to reconcile the empiricist conviction that, ultimately, individual subjective experiences are the foundation of all knowledge of nature with the professed goal of objectively describing those experiences. Classical physics had an answer, but it was based on a presupposition that the quantum postulate required discarding. Therefore, a reanalysis of the concept of objectivity and a radical redefinition of this criterion of scientific knowledge form the philosophical heart of complementarity.
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Bohr was very much aware that the requirement of objectivity imposed on the scientific description of nature was in paradoxical contrast to the subjective status on which such scientific description was based. He writes: "Yet occasionally just this 'objectivity' of physical observations becomes particularly suited to emphasize the subjective character of all experience." [1] Here Bohr clearly regards a "physical observation" as an objectively given datum. However, to count as an observation, the experienced phenomena of that interaction must be described unambiguously. The basis for this observation is the experimenter's experience of the phenomena. Since that experience is an everyday experience, it must be described by the usual terms for describing everyday objects. In this way, "objectivity" of "physical observations" emphasizes the "subjective character of all experience".
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Unfortunately, this use of "subjectivity" by Bohr was reckless, as he came to realize from the persistent attempts of critics to interpret complementarity subjectively. Thus, in later years, every essay contains a disclaimer of any subjective intentions, for example: ..
...the decisive point is that in neither case [i.e., in neither quantum physics nor in relativity] does the approximate widening of our conceptual framework imply any appeal to an observing subject, which would hinder unambiguous communication of experience. [2]
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Nevertheless, the classical tendency to regard experience as subjective leads to reading complementarity as endorsing a subjective foundations for science. Bohr was very aware of this temptation: In view of the influence of the mechanical conception of nature on philosophical thinking, it is understandable that one has sometimes seen in the notion of complementarity a reference to the subjective observer, incompatible with the objectivity of scientific description. Of course in every field of experience we must retain a sharp distinction between the observer and the content of the observations, but we must realize that the discovery of the quantum of action has thrown new light on the very foundation of the description of nature and revealed hitherto un-noticed presuppositions to the rational use of the concepts on which the communication of experience rests. In quantum physics, as we have seen, an account of the functioning of the measuring instruments is indispensable to the definition of the phenomena and we must, so to speak, distinguish between subject and object in such a way that each single case secures the unambiguous application of the elementary physical concepts used in that description. [3] ..
Bohr believed this shifting of the distinction between subject and object also caused problems in psychology and biology, as noted in part 6 of this review. In the essay from which the above paragraph was taken he continues by pointing out: While in the mechanical conception of nature the subject - object distinction was fixed, room is provided for a wider description through the recognition that the consequent use of our concepts requires different placings of such a separation. [4] ..
These comments indicate that Bohr believed the task of securing objectivity of empirical knowledge is presumed to refer to the description of experience, or in other words, what happens after the subject has the experience rather than how experience originates. Bohr avoids grounding objectivity in either the objects contribution or the subjects contribution to the formation of experience. His view was that objectivity is secured by understanding the proper use of descriptive concepts and so it is a matter of how we describe an experienced phenomena rather than how such experience originates. Whether consciousness is regarded as reducible to a physical process or as a mental presentation of ideas is irrelevant to complementarity. What concerned Bohr was not the interplay of distinct ontological orders, but the necessity of describing experienced phenomena unambiguously.
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Observation, for Bohr, did not involve any mysterious mental/physical interactions. In order to describe an observation in physics, both systems which interact to produce the observation must be capable of being described as purely physical systems. Bohr is emphatic in his claim that there is no need for new observational language in quantum physics. Since classical descriptive concepts have unambiguous empirical meaning, quantum experimental language presents no need to be described in any fashion other than that of classical physics.
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When speaking of "observing" an atomic system, if Bohr's use of "observing" is confused with his use of "experiencing" it may be philosophically misleading. An observer "experiences" the phenomena that confirms the theory. Some of these phenomena may be described as observations of atomic systems. Classical descriptive terms have unambiguous reference only for describing phenomenal objects, and since "atomic system" cannot refer to a phenomenal object, classical descriptive terms cannot be used to describe atomic systems. Therefore, if any description of the atomic system is at all possible, such a description cannot be communicated unambiguously.
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Bohr's complementarity dramatically alters the classical description of nature. "Observer" and "object" become categories of description having precise meaning only in the context of a particular description of an experiential event.
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The Ideal of Objectivity

Epistemology addresses the fundamental question of how to formulate a description of experience to meet the criterion of objectivity. Classically, a scientific description was held to be "objective" because it was believed that it determined properties possessed by the object as it exists independently apart from any observational determination of those properties. The methodology of science was built around the goal of developing the means for determining such properties and at the same time eliminating any element that arises from the role of the observing system in acquiring that empirical knowledge necessary to confirm scientific theory.
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Bohr's argument was that this understanding of objectivity of a scientific description of nature must be revised. For Bohr, the objectivity of scientific description is grounded in the use of concepts used for describing the experience. These concepts secure that objectivity not by playing any role in the origin or formation of experience, but by guaranteeing that communications expressed in terms of such concepts are unambiguous. Bohr writes: ..
Every scientist, however, is constantly confronted with the problem of objective description of experience, by which we mean unambiguous communication. Our basic tool is, of course, plain language. ...we shall not be concerned here with the origin of such language, but with its scope in scientific communication, and especially with the problem of how objectivity may be retained during the growth of experience beyond the event of daily life.
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The main point to realize is that all knowledge presents itself within a conceptual framework adapted to account for previous experience and that any such frame may prove too narrow to comprehend new experiences... [5]
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As Bohr understood natural science, the objectivity of a description cannot be secured by separating the "objective" properties possessed by an independent reality from the "subjective" properties which exist only in relation to an experiencing subject. In complementarity, the definition of "objectivity" requires the scientists goal to be development of a conceptual scheme or framework for describing the phenomena in a way which can be communicated unambiguously.
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Once the quantum postulate is accepted, it is necessary to realize that any description of a phenomena is a description of a physical interaction in which the distinction between observed object and observing system is made for the purpose of unambiguously describing the interaction as an observation of a specific phenomenal object. To secure unambiguous communication of the result of this observation, it is necessary that the description includes a precise specification of the whole physical situation in which the observation occurs.
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Bohr's epistemological lesson teaches that on the presuppositions of the classical framework, descriptions expressed by those classical mechanical concepts can be considered unambiguous. But the exploration of the atomic system dictates a need to adopt a new presupposition about these phenomena to that expressed by the quantum postulate. This change then renders descriptions expressed in classical terms ambiguous, consequently a new, more general framework is needed to restore unambiguity.
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Because it is necessary to describe the observation as a causal process, which must be done if that observation is to be interpreted as an observation of an object system, it is also necessary to apply classical dynamical conservation principles. And to do this, there must be a theoretical representation of the system isolated from the interaction. But in quantum formalism it is impossible to derive a classical mechanical state of the system that would enable us to "picture" the object apart from the observation. Therefore it must be recognized that the theoretical representation of the atomic system is an abstraction, not a picture of a concrete object.
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The generalization of the classical mechanical framework which Bohr advocated does much more than eliminate the ambiguities that result from quantum formalism. Ultimately, in proposing to revise the basis for objective scientific descriptions, complementarity alters the very concept of physical reality. In Bohr's complementarity, the distinction between the subject that experiences and the phenomena that is experienced is made within the description of the phenomena as an observational interaction, whereas in classical mechanics the distinction is made between the experiencer and the experienced. Bohr writes: ..
The notion of complementarity does in no way involve a departure from our notion as detached observers of nature, but must be regarded as the logical expression of our situation as regard objective description in this field of experience. The recognition that the interaction between the measuring tools and the physical system under investigation constitutes an integral part of the quantum phenomena has not only revealed an unsuspected limitation of the mechanical conception of nature, as characterized by the attribution of separate properties in physical systems, but has forced us, in the ordering of experience, to pay proper attention to the conditions of observation. [6]
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Bohr's claim was challenged by his student and friendly critic, Wolfgang Pauli. Upon reading Bohr's first draft of the essay from which the above quote is taken, Pauli replied to Bohr in his style which was overladen with sarcasm and irony, for which he become infamous for: ..
Under your great influence it was indeed getting more and more difficult for me to find something on which I have a different opinion than you. To a certain extent I am therefore glad, that eventually I found something: the definition and the use of the expression "detached observer". ...According to my own point of view, the degree of this "detachment" is gradually lessened in our theoretical explanation of nature and I am expecting further steps in this direction. ...it seems quite appropriate to call the conceptual description of nature in classical physics, which Einstein so emphatically wishes to retain, "the ideal of the detached observer". To put it drastically the observer has according to this ideal to disappear entirely in a discrete manner as a hidden spectator, never as actor, nature being left alone in a predetermined course of events, independent of the way in which the phenomena are observed. "Like the moon has a definite position" Einstein said to me last winter, "whether or not we look at the moon, the same must also hold for atomic objects, as there are no sharp distinctions between these and macroscopic objects. Observation cannot create an element of reality like a position, there must be something contained in the complete description which corresponds to the possibility of observing a position, already before the observation has been made." ...I considered the impredictable [sic] change of the state by a single observation in spite of the objective character of the result of every observation and notwithstanding the statistical laws for the frequencies of repeated observation under equal conditions - to be an abandonment of the idea of the isolation (detachment) of the observer from the course of physical events outside himself. [7] ..
It is apparent that, even though Pauli worked with Bohr on his Como paper and always regarded himself as an advocate of complementarity, both he and Einstein confused the distinction made in the description of phenomena as an interaction between the observing system and the atomic object with which it interacts with the distinction between the subject that experiences the whole phenomena and the phenomena the subject experiences. Bohr did not realize the phrase "detached observer" confused the issue by tending to identify the experiencing subject, the "observer" in classical physics, with the "observing system", which, according to Bohr's view, is treated in the description of the phenomena as a physical system partially causing the phenomena given to the subject. Bohr replied to Pauli like this: ..
As always, you touch on a very central point. A phrase like "detached observer" ... used in connection with the phrase: "objective description" ... had to me a very definite meaning. In all unambiguous account it is indeed a primary demand that the separation between the observing subject and the objective content of communication [i.e., the phenomena to be described] is clearly defined and agreed upon. ... this condition is indispensable in all scientific knowledge. ... It appears that what we have really learned in physics is how to eliminate subjective elements in the account of experience, and it is rather this recognition which in turn offers guidance as regards objective description in other fields of science. To my mind this situation is well described by the phrase "detached observer". [8]
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Bohr insisted that the description of a phenomena must include a description of the whole interaction between observing system and observed object, all of which forms part of the whole phenomena to be described by an adequate theory. Pauli was certainly misunderstanding Bohr's intention in the phrase "detached observer". While there need not be a separation between observer and observed on a physical level, and indeed there cannot be if there is to be an observation at all, by requiring a description of a "detached observer", objectivity demands that subjective elements in the accounting of experience are eliminated. Thus, the "observer", in this sense, is detached from the observation.
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For further reading, see Humberto R. Maturana, Reality; The Search for Objectivity or the Quest for a Compelling Argument.
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Complementarity and Kantian Epistemology

When Bohr refused to define how experience originates, he turned his back on the whole representational tradition of epistemology. Although there is a superficial resemblance between complementarity and Kantian epistemology, the apparent similarities are created by equating such terms as "experience" and "objectivity", which change in meaning in the shift from Kantian epistemology to complementarity.
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A true Kantian would never argue, as Bohr does, that a physical description would demand a change in the proper use of concepts which give experience its form. If Bohr were a Kantian, he would argue, as he does, that the classical concepts are indispensable, but then his claim that the quantum postulate demonstrates that the classical framework is no longer tenable would be a complete non sequitur. Bohr's rejection of the Kantian outlook follows from the fact that as he understood Kant, the critical philosophy was designed to show that classical concepts were independent of the content of experience and thus could not be shown to be inadequate by any empirical discovery.
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Nevertheless, Bohr's use of classical concepts in the description of phenomenal objects has a certain Kantian appearance. But that appearance is deceptive, which we see when it is recalled that Bohr's complementarity has nothing to do with how experienced phenomena arises, as Kantian philosophy does, but have only to do with communicating a description of a phenomena as an objective datum already given by experience. For further reading, see Distinguishing the Observer; An Attempt at Interpreting Maturana by Ernst von Glasersfeld and Ontology of Observing; The Biological Foundations of Self Consciousness and the Physical Domain of Existence by Humberto R. Maturana.
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This ends Part 7 of this review. Thanks for reading!
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Other Links

Perceptions of Quality
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Footnotes

[1] Bohr, Atomic Theory and the Description of Nature
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[2] Bohr, Quantum Physics and Philosophy
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[3] Bohr, Unity Of Knowledge
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[4] Ibid.
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[5] Ibid.
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[6] Ibid.
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[7] Letter from Wolfgang Pauli to Niels Bohr
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[8] Letter from Niels Bohr to Wolfgang Pauli



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The Framework of Complementarity

Part 1 - Overview Early Years Bohr Formulates Complementarity
Part 2 - Argument for Complementarity
Part 3 - Comments on Complementarity
Part 4 - Complementarity and the Uncertainty Principle
Part 5 - Refinement of Complementarity
Part 6 - Extension of Complementarity
Part 7 - The Nature of Empirical Knowledge
Part 8 - Complementarity and the Metaphysics of Quality