The Epistemology of
                                 Niels Bohr
                                      &
                               Albert Einstein
                                     ...
                            Two forms of realism
                                     ...


                                      By   
                                Jørgen Krogh
                       Institute for History of Science
                               Aarhus University
                                    Denmark

                                krogh@dfi.aau.dk

                           (received: March 31, 1995)

 Introduction:
   When people look at the epistemology and world views of Albert
   Einstein and Niels Bohr, many sees a disagreement that leads to a
   discussion between two totally different philosophical positions,
   realism and anti-realism, but in my opinion this is an exaggeration.
   I would rather think that it is a discussion between two alternative
   forms of realism.
   There is no doubt that we shall call Einstein's position:
   classical realism. A position where phenomena are the consequence
   of the behavior of objects having properties corresponding to the
   terms used to characterize physical systems in classical mecha-
   nics.
   But when we come to Bohr it is quite a problem to tell which
   position he belongs to. Some have characterized him as a instru-
   mentalist and others as a phenomenalist and others again will call
   him a subjectivist, which all are anti-realistic positions one
   way or the other.
   I will during this session try to describe Bohr as a realist, but
   not as a standard realist, we have to make a special kind of
   realism for him; it could maybe be called complementarity
   realism, but I choose to call it holistic realism for reasons I
   will return to later. 

Bohr:
  When we are looking at things that are small enough, the usual
   mechanical description doesn't give the right answers anymore;
   and as if that was not enough, the description that is able to
   deal with these small sizes, doesn't obey the causal-laws. One
   of the reasons for this, is that we no longer are able to
   differentiate the physical object from its interaction with our
   perception instruments; this means that there are problems with
   one of the most central concepts in physics, observation. This
   brings Bohr to the comment he makes in 'Kausalität und Komplemen-
   tarität' [Bohr, 1937, page 204]:
   
        "Diser Umstand stellt uns in der Tat vor eine in der
        Physik ganz neue Situation bezüglich der Analyse und
        Synthese von Erfahrungen, die uns dazu zwingt, das
        Kausalitätsideal durch einen allgemeineren Gesichts-
        punkt zu ersetzen, den man 'Komplementarität' zu nennen
        pflegt."

   However, the word 'complementarity' doesn't belong to the
   everyday concepts, and certainly doesn't help very much in
   visualizing objects in atomic situations; it only expose the
   total new situation in what we can realize.
   If we in the physical description of the very small objects use
   quantum mechanics, as Bohr suggested, we have a problem, because
   quantum mechanical experiments give a statistical result, which
   leads to that the data of an experiment not are direct reproduci-
   al, which implies a missing objectivity in the classical sense.
   But according to Bohr the objectivity is a vital assumption for
   the scientists acknowledgment, and he does not agree that
   quantum mechanics leads to a missing objectivity. Instead he
   argues that what we need is a new definition of the concept
   'objectivity'. 
   In 'Quantum Physics and Philosophy - Causality and Complementari-
   ty'[Bohr, 1958, page 3] he writes:

        "The description of atomic phenomena has in these
        respects a perfectly objective character, in the sense
        that no explicit reference is made to any individual
        observer and that therefore, with proper regard to
        relativistic exigencies, no ambiguity is involved in
        the communication of information."

   We see here that Bohr's epistemological objectivity concept is
   based on experience, that unambiguously can be communicated to
   others, but what is important to notice is: That there is no
   reference to the observing and communicating subject. Bohr
   formulate this in 'Quantum Physics and Philosophy - Causality and
   Complementarity' [Bohr, 1958, page 7]:

       "Still, the decisive point is that in neither case
        [relativity and complementarity] does the appropriate
        widening of our conceptual framework imply any appeal
        to the observing subject, which would hinder unambi-
        guous communication of experience."

   In 'The Unity of Human Knowledge'[Bohr 1960, page 9] he makes a
   extended formulation:

       "The extension of physical experience in our days has,
        however, necessitated a radical revision of the
        foundation for the unambiguous use of our most elemen-
        tary concepts, and has changed our attitude to the aim
        of physical science. Indeed, from our present stand-
        point, physics is to be regarded not so much as the
        study of something a priori given, but rather as the
        development of methods for ordering and surveying human
        experience. In this respect our task must be to account
        for such experience in a manner independent of in-
        dividual subjective judgment and therefore objective
        in the sense that it can be unambiguously communicated
        in the common human language."

   There are a number of things of importance in these quotations:
     1) Bohr points out that criteria of objectivity is that
        something can unambiguously be communicated in a common
        human language.
     2) Physics is not the study of a priori given things, but
        methods of describing human experience.
   Summed up, Bohr does not mean that the concept of 'objectivity'
   refers to a correspondence between physical concepts and objects,
   in the way done by the classical realists, where objects exists
   independent of our knowledge.
   The reason why Bohr doesn't like the idea of talking about atomic
   objects in themselves, that is independent of human description,
   is not that he does not believe in their existence. The reason
   is rather that talking about atomic objects independent of human
   description would require making a strict distinction between
   subject and object, which Bohr thought was impossible to make.
   If you want to talk about an object, you have to talk about it in
   a context in which you would have a chance of experiencing it.
   For example, if it is an atomic object you want to talk about, you
   would have to talk about the entirety of object and the instru-
   ments you use percepting the object. In 'Quantum Physics and
   Philosophy' [Bohr, 1958, page 4] he writes:

       "While, within the scope of classical physics, the
        interaction between objects and apparatus can be
        neglected or, if necessary, compensated for, in quantum
        physics this interaction thus forms an inseparable part
        of the phenomena. Accordingly, the unambiguous account
        of proper quantum phenomena must, in principle, include
        a description of all relevant features of the ex-
        perimental arrangement."

   In other words you can't talk about the object in it self, the
   only thing you can do is to describe the interaction between
   object and the measuring instruments, and this can only be done
   in a classical mechanical language. However from the quantum
   formalism is it impossible to derive a classical mechanical state
   of the system, that would enable us to picture the object apart
   from the observation. Therefore we now recognize the theoretical
   representation of the isolated system as an abstraction, not a
   'picture' of a concrete real object, this he points out in 'The
   Unity of Human Knowledge' [Bohr, 1960, page 12]:

        "The fact that in atomic physics, where we are con-
        cerned with regularities of unsurpassed exactness,
        objective description can be achieved only by including
        in the account of the phenomena explicit reference to
        the experimental conditions, emphasizes in a novel
        manner the inseparability of knowledge and our possi-
        bilities of inquiry. We are here concerned with a
        general epistemological lesson illuminating our
        position in many other fields of human interest."

   If we return to the well known complementarity phenomena (The
   wave/particle dualism for the electron), with Bohr's holistic
   ideas of description and measurement in memory, there is no
   longer a problem in the duality, because it is only a choice of
   measurement-instruments, and thereby a choice of description. In
   this example we can't talk about the electron as a wave or a
   particle in it self, we have to talk about it in a experimental
   context, where the choice of context determine as what we
   describe the electron.
   This, where the only thing we can talk about, is the interaction
   between object and measuring-instruments, could sound like a
   kind of phenomenalism or operationalism, but I don't think it is,
   because, we have all the time to remember that what Bohr talked
   about, was an interaction between measuring-instruments and a
   specific existing object. In this description we have to make,
   every time we make a new experiment or look at a old one, a
   distinction or section between subject and object.
   Not only in quantum physics is this distinction vital, but in
   "any attempt at exhaustive description of the richness of
   conscious life demands in various situations a different placing
   of the section between subject and object." 'The Unity of Human 
   Knowledge" [Bohr, 1960], page 13.
   This position where both the object perception instrument and
   sometimes even the subject is under consideration when talking
   about something, is a kind of holistic view point, and still,
   because it is talking about an existing object, it ought to have
   realism as a prefix; so I would choose to call it a holistic
   realism.
   However, in a position like this, it is of vital importance to
   remember, that although the complementarity, a quantum mechanical
   statement, including the uncertainty principle, is every thing
   that are to be said about the nature of a physical system.

Einstein:
   If we turn to Einstein, the last statement, where a quantum 
   mechanical statement is every thing that are to be said about
   the nature of a physical system, was a pure metaphysical postulate,
   which obstructs a total unambiguous understanding of the nature 
   of a physical system. In the first section of the paper 'Can Quantum-
   Mechanical Description of Physical Reality Be Considered Complete?'
   [Einstein, Podolsky and Rosen, 1935, page 777] he writes:

       "Any serious consideration of a physical theory must
        take into account the distinction between the objective
        reality, which is independent of any theory, and the
        physical concepts with which the theory operates. These
        concepts are intended to correspond with the objective
        reality, and by means of these concepts we picture this
        reality to ourselves."

   Here we see the striking difference between Bohr and Einstein.
   They both think that there are objects 'out there', but, Bohr
   can't talk about these objects in them self. When he talks about
   them, he talks about the whole of object and measuring instru-
   ment, even when he talks about them in theory, he doesn't talk
   about the object in it self, he talks about the interaction they
   make with the instruments. Einstein, on the contrary, talks about
   the object in it self, independent of theory and everything else.
   When Einstein in theory describes an object, he thinks about it as
   a real existing one; as he says in same paper as above [Einstein,
   1935, page 777]:

       "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."

   This we could call a kind of weak reality definition, and it was
   this together with the non-locality demand, which he (together
   with Boris Podolsky and Nathan Rosen) thought would tilt the
   completeness of quantum mechanics.
   We have here seen that what is essential for Einstein is not the
   description, for instance quantum mechanics, it is the reality.
   But what is this reality?  When we look in 'Out Of My Late
   Years', it is easy to get the impression that reality is
   something created by the human mind. F.ex. [Einstein, 1936] page
   60:

       "I believe that the first step in the setting of a
        'real external world' is the formation of the concept
        of bodily objects and of bodily objects of various
        kinds. Out of the multitude of our sense experiences
        we take, mentally and arbitrarily, certain repeatedly
        occurring complexes of sense impression (..), and we
        attribute to them a meaning - the meaning of bodily
        objects.  ...   The second step is to be found in the
        fact that, in our thinking ... , we attribute to this
        concept of the bodily objects a significance, which is
        to a high degree independent of the sense impression
        which originally gives rise to it. This is what we mean
        when we attribute to the bodily object 'a real existen-
        ce'."

   Confronted, by the Irish writer James Murphy, with the inter-
   pretation of his theory, as if he thinks that the outer world is
   a derivative of human consciousness, Einstein responses:

        "No physicist believes that. Otherwise he wouldn't be
        a physicist. ...You must distinguish between what is
        a literary fashion and what is a scientific pronoun-
        cement... Why should anybody go to the trouble of
        gazing  at the stars if he did not believe that the
        stars were really there?... We cannot logically prove
        the existence of the external world, any more than you
        can logically prove that I am talking with you nor that.
        I am here. But you know that I am here and no subjec-
        tive idealist can persuade you to the contrary."

        	(From Epilogue to Max Planck. Where Is Science
        	Going? Allen & Unwin, London, 1933, page 213. 
		Taken from Gibanov [1987, page 114].)

   However, Einstein is perfectly aware that talking about the
   things in them self, can create problems. When we are trying to
   get information about atomic objects, it is impossible with our
   perception instruments (i.e., eye, ear feeling etc.), to avoid
   the use of measuring-instruments, and where is the real object
   in such setups? We find a kind of answer in his 'Reply to
   Criticisms' [Einstein, 1949b, page 673]:

        "We represent the sense-impressions as conditioned by
        an 'objective' and by a 'subjective' factor. For this
        conceptual distinction there also is no logical-
        philosophical justification. But if we reject it, we
        cannot escape solipsism."

   This brings the importance of concepts into the picture. The nature
   of concepts, Einstein points out, is not a metaphysical one.
   [Einstein 1949b, page 673]

        "A basic conceptual distinction, which is a necessary
        prerequisite of scientific and pre-scientific thinking,
        is the distinction between 'sense-impressions' (and the
        recollection of such) on the one hand and mere ideas
        on the other."

   Einstein summes all of this up in what he calls his epistemologi-
   cal credo. This is found in 'Albert Einstein Autobiographical
   Notes' [Einstein, 1949a, page 11]:

        "I see on the one side the totality of sense-experienc-
        es, and, on the other, the totality of the concepts and
        propositions which are laid down in books. The rela-
        tions between the concepts and propositions among
        themselves and each other are of a logical nature, and
        the business of logical thinking is strictly limited
        to the achievement of the connection between concepts
        and propositions among each other according to firmly
        laid down rules, which are the concern of logic. The
        concepts and propositions get 'meaning', viz., 'content',
        only trough their connection with sense-experiences. 
        The connection of the latter with the former is purely 
        intuitive, not itself of a logical nature. The degree of
        certainty with which this connection, viz., intuitive
        combination, can be undertaken, and nothing else, 
        differentiates empty phantasy from scientific 'truth'.
        The system of  concepts is a creation of man together
        with the rules of syntax, which constitute the structure
        of the conceptual systems. Although the conceptual systems
        are logically entirely arbitrary, they are bound by the
        aim to permit the most nearly possible certain (intuitive)
        and complete co-ordination with the totality of sense-
        experiences; secondly they aim at greatest possible
        sparsity of their logically independent elements (basic
        concepts and axioms), i.e., undefined concepts and
        underived propositions.
        A proposition is correct if, within a logical system,
        it is deduced according to the accepted logical rules.
        A system has truth-content according to the certainty
        and completeness of its co-ordination-possibility to
        the totality of experience. A correct proposition
        borrows its 'truth' from the truth-content of the
        system to which it belongs."

   In this credo he points out that there are two main features:
   Sense-experience and concepts/propositions. Every relation inside
   the concepts and propositions, and relation between them are of
   a logical nature. But, all these concepts and propositions don't
   have any 'content' or 'meaning' in them self, this they get from
   the sense-experience. That is why:

        "Every element of the physical reality must have a
        counterpart in the physical theory." [Einstein (EPR),
        1935 , page 777]

   So, what we have is human made concepts, and propositions made
   of these, by strict logical rules, which all gets their meaning
   from the sense-experience. By getting their meaning, the concepts
   produce some sort of order among the sense impressions, and
   it's this that make the world of our sense experiences 
   comprehensible.
   Now, from the order that is created among the concepts and
   propositions, we can say something about combing sense-experien-
   ces, but, this connection from concepts to sense-experience is
   of a pure intuitive nature. Einstein also formulate this in 'Out
   Of My Late Years' [Einstein, 1936, page 63]:

        "The aim of science is, on the one hand, a comprehen-
        sion, as complete as possible, of the connection
        between the sense experiences in their totality, and,
        on the other hand, the accomplishment of this aim by
        the use of a minimum of primary concepts and rela-
        tions."


Conclusion:
   If I briefly shall sum up the positions of the two, Einstein and
   Bohr. It would be easiest to start where they agree, because that
   is only on a few points.
   They agree on the, in my opinion, most important issue: There is
   a world out there independent of us; this is the reason why I
   will call both of them, realists.
   When talking about these really existing objects the harmony
   disappears, because, Bohr don't want, at all, to talk about them
   in them self, he states that it is impossible to talk about them
   out of their experimental context. If we want to talk about them,
   we have to talk about the whole of object and the perceptions
   instruments which we use. And, even when we do this there often
   are more ways to describe the objects.
   The classical realist, Einstein, allows us to talk about the
   objects in them self. But I see a problem in talking about the
   atomic objects, with the concepts, that inhered their 'meaning'
   from sense-experiences. Sense-experiences that are created in
   interaction with our measuring-instruments, where we are not
   able to determine what comes from the object and what comes from
   the instruments.
   As a final comment, in this essay about what two persons thought
   the world are made of, I will point out that Einstein showed a
   new way inside the classical realism, where the real existing
   objects, not necessarily are material bodily objects; actually
   Einstein thought, on the basis of relativity-theory, that the
   material point no longer could be the basic concept in the
   theory, it would have to be something that continuously covered
   all of space: Fields for instance.


Bibliography:


   Bohr, 1937:      "Kausalitet und Komplementarität" in "Niels
                    Bohr 1885-1962 - Der Kopenhagener Geist in
                    der Physik" ed. K.V. Mayenn et.al., Vieweg
                    & Sohn, Braunschweig, 1985.

   Bohr, 1958:      "Quantum Physics and Philosophy - Causality
                    and Complementarity" in Niels Bohr: "Esays
                    1958/62 on Atom Physics and Human Knowled-
                    ge", Copenhagen., 1963.

   Bohr, 1960:      "The Unity of Human Knowledge" in Niels
                    Bohr: "Esays 1958/62 on Atom Physics and
                    Human Knowledge", Kbh., 1963.

   Einstein, Podolsky & Rosen, 1935:    "Can quantum mechanical des-
                    scription of reality be considered com-
                    plete?", Phys. Rev. 47, p. 777-780.

   Einstein, 1936:  "Physics and Reality" in "Out of My Late
                    Years", Greenwood Press, Connecticut, 1950.

   Einstein, 1949a: "Albert Einstein Autobiographical Notes" in
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                    ed. P.A.Schilpp, Illinois, 1949.

   Einstein, 1949b: "Reply to Criticisms" in "Albert Einstein:
                    Philosopher - Scientist" ed. P.A.Schilpp,
                    Illinois, 1949.

   Gibanov, D.P. 1987:   "Albert Einsteins Philosophical views and
                    theory of relativity", Progress Publischeres,
                    Moscow, 1987.