Niels Bohr on Causality

The following is a fairly comprehensive set of citations — from the four-volume philosophical writings — where Bohr discusses causality (or conservation laws). We omit detailed commentary, which can be found in the corresponding article “Niels Bohr on the Causal Ideal”.

As for page numbers, we use the original articles, which are collected in Niels Bohr Collected Works.

In particular, for impacts in which the time of collision is short compared to the natural periods of the atom and for which very simple results are to be expected according to the usual mechanical ideas, the postulate of stationary states would seem to be irreconcilable with any description of the collision in space and time based on the accepted ideas of atomic structure. (Bohr 1925 p 848)

That such a procedure actually leads to a self-contained theory sufficiently analogous to classical mechanics depends essentially on the fact that, as Born and Jordan were able to show, there exists in Heisenberg’s quantum mechanics a conservation theorem analogous to the energy law of classical mechanics. (Bohr 1925 p 852)

Notwithstandig the difficulties which hence are involved in the formulation of the quantum theory, it seems, as we shall see, that its essence may be expressed in the so-called qnantum postulate, which to any atomic process attributes an essential discontinuity or rather individuality, completely foreign to the classical theories and symbolised by Planck’s quantum of action. This postulate implies a renunciation as regards the causal space-time co-ordination of atomic processes. (Bohr 1928 p 566)

On the other hand, if in order to make observation possible we permit certain interactions with suitable means of measurement, not belonging to the system, an unambiguous definition of its state is naturally no longer possible, and there can be no question of causality in the ordinary sense of the word. The very nature of the quantum theory thus forces us to regard the space-time co-ordination and the claim of causality, the union of which characterises the classical theories, as complementary biit exclusive features of the description, syniholising the idealisation of observation and definition respectively. (Bohr 1928 p 567)

Just as the relativity theory has tgught us that the convenience of sharply distinguishing between space and time solely rests on the smallness of the velocities ordinarily met with compared to the velocity of light, we learn from the quantum theory that the appropriateness of our usual causal space-time description depends entirely upon the small value of the quantum of action as compared to the actions involved in ordinary sense perceptions. (Bohr 1928 p 567)

Nevertheless the conservation of energy and momentum during the interaction between radiation and matter, as evident in the photoelectric and Compton effect, finds its adequate expression just in the light quantum idea put forward by Einstein. As is well known, the doubts regarding the validity of the superposition principle on one hand and of the conservation laws on the other, which were suggested by this apparent contradiction, have been definitely disproved through direct experiments. This situation would seem clearly to indicate the impossibility of a causal space-time description of the light phenomena. On one hand, in attempting to trace the laws of the time-spatial propagation of light we are according to the quantum postulate confined to statistical considerations. On the other hand, the fulfilment of the claim of causality for the individual light processes characterized by the quantum of action entails a renunciation as regards the space time description. (Bohr 1928 p 567-568)

In the language of the relativity theory the content of the relations (2) may be summarized in the statement that according to the quantum theory a general reciprocal relation exists between the maximum sharpness of definition of the space-time and energy-momentum vectors associated with the individuals. This circumstance may be regarded as a simple symbolical expression for the complementary nature of the space-time description and the claims of causality. At the same time, however, the general character of this relation makes it possible to a certain extent to reconcile the conservation laws with the space-time co-ordination of observations, the idea of a coincidence of well-defined events in space-time points being replaced by that of unnharply defined individuals within finite space-time regions. (Bohr 1928 p 571)

Here this postulate does indeed represent the idea of the individuality of the particles which, transcending the space-time description, meets the claim of causality. (Bohr 1928 p 572)

Moreover the conception of a stationary state involves, strictly speaking, the exclusion of all interactions with individuals not belonging to the system. The fact that such a closed system is associated with a particular energy value, may be considered as an immediate expression for the claim of causality contained in the theorem of conservation of energy. (Bohr 1928 p 582)

Thus we see that no causal connection can be obtained between observations leading to the fixation of a stationary state and earlier observations on the behaviour of the separate particles in the atom. (Bohr 1928 p 587)

In both cases we are concerned with a demand of causality complementary to the space-time description, the adequate application of which is limited only by the restricted possibilities of definition and observation. (Bohr 1928 p 587)

In particular, any attempt at an ordering in space-time leads to a break in the causal chain, since such an attempt is boundup with anessential exchangeof momentum and energy between the individuals and the measuring rods and clocks used for observation; and just this exchange cannot be taken into account if the measuring instruments are to fulfil their purpose. Conversely, any conclusion, based in an unambiguous manner upon the strict conservation of energy and momentum, with regard to the dynamical behaviour of the individual units obviously necessitates a complete renunciation of following their course in space and time. In general, we may say that the suitableness of the causal space-time mode of description for the ordering of our usual experiences depends only upon the smallness of the quantum of action relative to the actions with which we are concerned in ordinary phenomena. (Bohr 1934b p 98)

It might still be permitted here briefly to refer to the relation which exists between the regularities in the domain of psychology and the problem of the causality of physical phenomena. (Bohr 1934b p 100)

we have learned, by the discovery of the quantum of action, that a detailed causal tracing of atomic processes is impossible and that any attempt to acquire a knowledge of such processes involves a funda- mentally uncontrollable interference with their course. (Bohr 1934b p 100)

This principle symbolizes, as it were, the peculiar reciprocal symmetry relation between the space-time description and the laws of the conservation of energy and momentum, the great fruitfulness of which, already in classical physics, depends upon the fact that one may extensively apply them without following the course of the phenomena in space and time. (Bohr 1934b p 94)

any conclusion, based in an unambiguous manner upon the strict conservation of energy and momentum, with regard to the dynamical behaviour of the individual units obviously necessitates a complete renunciation of following their course in space and time. (Bohr 1934b p 98)

Indeed, only by a conscious resignation of our usual demands for visualization and causality was it possible to make Planck’s discovery fruitful in explaining the properties of the elements on the basis of our knowledge of the building stones of atoms. (Bohr 1934a p 108)

the continuous, causal description is replaced by a fundamentally statistical mode of description. (Bohr 1934a p 110)

any determination of the energy and the momentum of the particles demands that we renounce their exact co-ordination in time and space. In both cases, the invocation of classical ideas, necessitated by the very nature of measurement, is, beforehand, tantamount to a renunciation of a strictly causal description. (Bohr 1934a p 114)

The resignation as regards visualization and causality, to which we are thus forced in our description of atomic phenomena, might well be regarded as a frustration of the hopes which formed the starting-point of the atomic conceptions. Nevertheless, from the present standpoint of the atomic theory, we must consider this very renunciation as an essential advance in our understanding. (Bohr 1934a p 115)

any observation necessitates an interference with the course of the phenomena, which is of such a nature that it deprives us of the foundation underlying the causal mode of description. (Bohr 1934a p 115)

Planck’s discovery has led us to recognize that the adequacy of our whole customary attitude, which is characterized by the demand for causality, depends solely upon the smallness of the quantum of action in comparison with the actions with which we are concerned in ordinary phenomena. (Bohr 1934a p 116)

Just as the freedom of the will is an experiential category of our psychic life, causality may be considered as a mode of perception by which we reduce our sense impressions to order. At the same time, however, we are concerned in both cases with idealizations whose natural limitations are open to investigation and which depend upon one another in the sense that the feeling of volition and the demand for causality are equally indispensable elements in the relation between subject and object which forms the core of the problem of knowledge. (Bohr 1934a p 116-117)

A causal description in the classical sense is possible only in such cases where the action involved is large compared with the quantum of action, and where, therefore, a subdivision of the phenomena is possible without disturbing them essentially. If this condition is not fulfilled, we cannot disregard the interaction between the measuring instruments and the object under investigation, and we must especially take into consideration that the various measurements required for a complete mechanical description may only be made with mutually exclusive experimental arrangements. (Bohr 1933 p 422)

Any question of a return to a mode of description consistent with the principle of causality was, however, …excluded by unambiguous experience of the most varied kind. (Bohr 1938a p 74)

The view-point of ‘complementarity’ does, indeed, in no way mean an arbitrary renunciation as regards the analysis of atomic phenomena, but is on the contrary the expression of a rational synthesis of the wealth of experience in this field, which exceeds the limits to which the application of the concept of causality is naturally confined. (Bohr 1938a p 75)

Indeed, the impossibility in psychical experience to distinguish between the phenomena themselves and their conscious perception clearly demands a renunciation of a simple causal description on the model of classical physics, and the very way in which words like ‘thoughts’ and ‘feelings’ are used to describe such experience reminds one most suggestively of the complementarity encountered in atomic physics. (Bohr 1938a p 78)

How radical a change in our attitude towards the description of nature this development of atomic physics has brought about is perhaps most clearly illustrated by the fact that even the principle of causality, so far considered as the unquestioned foundation for all interpretation of natural phenomena, has proved too narrow a frame to embrace the peculiar regularities governing individual atomic processes. Certainly everyone will understand that physicists have needed very cogent reasons to renounce the ideal of causality itself; but in the study of atomic phenomena we have repeatedly been taught that questions which were believed to have received long ago their final answers had most unexpected surprises in store for us. (Bohr 1939a p 88)

On the contrary, we have to do with a rational development of our means of classifying and comprehending new ex- perience which, due to its very character, finds no place within the frame of causal description that is only suited to account for the behaviour of objects as long as this behaviour is independent of the means of observation. Far from containing any mysticism contrary to the spirit of science, the view-point of "complementarity" forms indeed a consistent generalization of the ideal of causality. (Bohr 1939a p 90)

The question at issue has been whether the renunciation of a causal mode of description of atomic processes involved in the endeavours to cope with the situation should be regarded as a temporary departure from ideals to be ultimately revived or whether we are faced with an irrevocable step towards obtaining the proper harmony between analysis and synthesis of physical phenomena. (Bohr 1949 p 202)

Yet, a certain difference in attitude and outlook remained, since, with his mastery for co-ordinating apparently contrasting experience without abandoning continuity and causality, Einstein was perhaps more reluctant to renounce such ideals than someone for whom renunciation in this respect appeared to be the only way open to proceed with the immediate task of co-ordinating the multifarious evidence regarding atomic phenomena, which accumulated from day to day in the exploration of this new field of knowledge. (Bohr 1949 p 206)

Thus, a sentence like “we cannot know both the momentum and the position of an atomic object” raises at once questions as to the physical reality of two such attributes of the object, which can be answered only by referring to the conditions for the unambiguous use of space-time concepts, on the one hand, and dynamical conservation laws, on the other hand. While the combination of these concepts into a single picture of a causal chain of events is the essence of classical mechanics, room for regularities beyond the grasp of such a description is just afforded by the circumstance that the study of the complementary phenomena demands mutually exclusive experimental arrangements. (Bohr 1949 p 211)

The trend of the whole argumentation presented in the Como lecture was to show that the viewpoint of complementarity may be regarded as a rational generalization of the very ideal of causality. (Bohr 1949 p 211)

Both in relativity and in quantum theory we are concerned with new aspects of scientific analysis and synthesis and, in this connection, it is interesting to note that, even in the great epoch of critical philosophy in the former century, there was only question to what extent a priori arguments could be given for the adequacy of space-time co-ordination and causal connection of experience, but never question of rational generalizations or inherent limitations of such categories of human thinking. (Bohr 1949 p 239)

This phenomenon afforded, as is well known, a most direct proof of the adequacy of Einstein’s view regarding the transfer of energy and momentum in radiative processes; at the same time, it was equally clear that no simple picture of a corpuscular collision could offer an exhaustive description of the phenomenon. Under the impact of such difficulties, doubts were for a time entertained even regarding the conservation of energy and momentum in the individual radiation processesj a view, however, which very soon had to be abandoned in face of more refined experiments bringing out the correlation between the deflection of the photon and the corresponding electron recoil. (Bohr 1949 pp 206-207)

As regards the quantum-mechanical description, we have to deal here with a two-body system consisting of the diaphragm as well as of the particle, and it is just with an explicit application of conservation laws to such a system that we are concerned in the Compton effect where, for instance, the observation of the recoil of the electron by means of a cloud chamber allows us to predict in what direction the scattered photon will eventually be observed. (Bohr 1949 p 216)

The complementary relationship between energy-momentum conservation and time-space co-ordination is most strikingly exhibited in the well-known paradox of particle penetration through potential barriers. (Bohr 1949 p 231)

Notwithstanding refinements of terminology due to accumulation of experimental evidence and developments of theoretical conceptions, all account of physical experience is, of course, ultimately based on common language, adapted to orientation in our surroundings and to tracing relationships between cause and effect. (Bohr 1958 p 308)

In Newtonian mechanics, where the state of a system of material bodies is defined by their instantaneous positions and velocities, it proved possible, by the well-known simple principles, to derive, solely from the knowledge of the state of the system at a given time and of the forces acting upon the bodies, the state of the system at any other time. A description of this kind, which evidently represents an ideal form of causal relationships, expressed by the notion of determinism, was found to have still wider scope. (Bohr 1958 p 308)

Although, of course, the classical description of the experimental arrangement and the irreversibility of the recordings concerning the atomic objects ensure a sequence of cause and effect conforming with elementary demands of causality, the irrevocable abandonment of the ideal of determinism finds striking expression in the complementary relationship governing the unambiguous use of the fundamental concepts on whose unrestricted combination the classical physical description rests. (Bohr 1958 p 312)

The decisive point, however, is that in this connection there is no question of reverting to a mode of description which fulfils to a higher degree the accustomed demands regarding pictorial representation of the relationship between cause and effect. (Bohr 1958 p 313)

Far from involving any arbitrary renunciation of the ideal of causality, the wider frame of complementarity directly expresses our position as regards the account of fundamental properties of matter presupposed in classical physical description, but outside its scope. (Bohr 1958 pp 313-314)

In this context, we are of course not concerned with a, restriction as to the accuracy of measurements, but with a limi- tation of the well-defined application of space-time concepts and dynamical conservation laws, entailed by the necessary distinction between measuring instruments and atomic objects. (Bohr 1958 p 312)

Above all the explanation of the orbital motion of the planets in our solar system, based on simple mechanical principles and the law of universal gravitation, deeply influenced the general philosophical attitude in the following centuries and strengthened the view that space and time as well as cause and effect had to be taken as a priori categories for the comprehension of all knowledge. (Bohr 1961b p 63)

New fundamental aspects of the observational problem, entailing a revision of the very foundation for the analysis of phenomena in terms of cause and effect, were to be uncovered by the development initiated by Planck’s discovery of the universal quantum of action in the first year of this century. This discovery proved that the wide applicability of so-called classical physics rests entirely on the circumstance that the action involved in any phenomena on the ordinary scale is so large that the quantum can be completely neglected. In atomic processes, however, we meet with regularities of a novel kind, defying causal pictorial description but nevertheless responsible for the peculiar stability of atomic systems on which all properties of matter ultimately depend. (Bohr 1961b p 64)

The description of ordinary experience presupposes the unrestricted divisibility of the course of the phenomena in space and time and the linking of all steps in an unbroken chain in terms of cause and effect. Ultimately, this viewpoint rests on the fineness of our senses which for perception demands an interaction with the objects under investigation so small that in ordinary circumstances it is without appreciable influence on the course of events. In the edifice of classical physics, this situation finds its idealized expression in the assumption that the interaction between the object and the tools of observation can be neglected or, at any rate, compensated for. (Bohr 1961a p 1105)

A crucial point, irrevocably excluding the possibility of reverting to causal pictorial description, was the recognition that the scope of unambiguous application of the general conservation laws of momentum and energy is inherently limited by the circumstance that any experimental arrangement, allowing the location of atomic objects in space and time, implies a transfer, uncontrollable in principle, of momentum and energy to the fixed scales and regulated clocks indispensable for the definition of the reference frame. (Bohr 1961a pp 1106-07)

Ellis’ demonstration of the continuous spectral distribution of the electrons directly emitted from the nucleus raised a puzzling question about energy conservation, which was eventually answered by Pauli’s bold hypothesis of the simultaneous emission of a neutrino, affording the basis for Fermi’s ingenious theory of β-decay. (Bohr 1961a p 1112)

Notwithstanding the radical departure from deterministic pictorial description, with which we are here concerned, basic features of customary ideas of causality are upheld in the correspondence approach by referring the competing individual processes to a simple superposition of wave functions defined within a common space-time-extension. (Bohr 1962 p 35)

Evidence on the spins of the nuclei involved in the process seemed contradictory to the conservation of angular momentum. It was, in fact, to evade such difficulties that Pauli introduced the bold idea, which should be most fruitful for the later development, that a very penetrating radiation, consisting of particles with vanishing rest mass and spin one-half, the so-called neutrinos, were emitted in β-decay together with the electrons. (Bohr 1962 p 33)

The essential indeterminacy in question must therefore not be taken to imply a one-sided departure from the ideal of causality underlying any account of natural phenomena. The use of energy conservation in connexion with the idea of stationary states, for instance, means an upholding of causality particularly striking when we realise that the very idea of motion, on which the classical definition of kinetic energy rests, has become ambiguous in the field of atomic constitution. (Bohr 1932 p 376)

As I have stressed by the argumentation mentioned, space time co-ordination and dynamical conservation laws may be considered as two complementary aspects of ordinary causality which in this field exclude one another to a certain extent, although neither of them has lost its intrinsic validity. (Bohr 1932 p 376)

Still, just as the account of those aspects of atomic constitution essential for the explanation of the ordinary physical and chemical properties of matter implies a renunciation of the classical ideal of causality, the features of atomic stability, still deeper-lying, responsible for the existence and the properties of atomic nuclei, may force us to renounce the very idea of energy balance. (Bohr 1932 p 383)

Thus, any attempt to fix the space-time co-ordinates of the constituent particles of an atom would ultimately involve an essentially uncontrollable exchange of energy and momentum with the measuring rods and clocks which prevents an unambiguous correlation of the dynamical behaviour of the atomic particles before the observation with their later behaviour. Inversely, every application of conservation theorems, for instance to the energy balance in atomic reactions, involves an essential renunciation as regards the pursuance in space and time of the individual atomic particles. In other words, the use of the idea of stationary states stands in a mutually exclusive relationship to the applicability of space-time pictures. (Bohr 1932 p 375)

In fact, such measurements of momentum require only an unambiguous application of the classical law of conservation of momentum, applied for instance to a collision process between the diaphragm and some test body, the momentum of which is suitably con- trolled before and after the collision. It is true that such a control will essentially depend on an examination of the space-time course of some process to which the ideas of classical mechanics can be applied; if, however, all spatial dimensions and time intervals are taken sufficiently large, this involves clearly no limitation as regards the accurate control of the momentum of the test bodies, but only a renunciation as regards the accuracy of the control of their space-time coordination. (Bohr 1935 p 698)

By allowing an essentially uncontrollable momentum to pass from the first particle into the mentioned support, however, we have by this procedure cut ourselves off from any future possibility of applying the law of conservation of momentum to the system consisting of the diaphragm and the two particles and therefore have lost our only basis for an unambiguous application of the idea of momentum in predictions regarding the behavior of the second particle. (Bohr 1935 p 700)

The analysis of new experiences is liable to disclose again and again the unrecognized presuppositions for an unambiguous use of our most simple concepts, such as space-time description and causal connection. (Bohr 1937 p 290)

The recognition of the essential dependence of any physical phenomenon on the system of reference of the observer, which forms the characteristic feature of relativity theory, implies, however — as especially Einstein himself has emphasized — no abandonment whatever of the assumption underlying the ideal of causality, that the behavior of a physical object relative to a given system of coordinates is uniquely determined, quite independently of whether it is observed or not. (Bohr 1937 p 290)

There appear new uniformities which cannot be fitted into the frame of the ordinary causal description. (Bohr 1937 p 290)

Indeed this circumstance presents us with a situation concerning the analysis and synthesis of experience which is entirely new in physics and forces us to replace the ideal of causality by a more general viewpoint usually termed “complementarity.” (Bohr 1937 p 291)

We thus see that the impossibility of carrying through a causal representation of quantum phenomena is directly connected with the assumptions underlying the use of the most elementary concepts which come into consideration for the description of experience. (Bohr 1937 p 293)

For the requirement of communicability of the circumstances and results of experiments implies that we can speak of well defined experiences only within the framework of ordinary concepts. In particular it should not be forgotten that the concept of causality underlies the very interpretation of each result of experiment, and that even in the coordination of experience one can never, in the nature of things, have to do with well-defined breaks in the causal chain. (Bohr 1937 p 293)

In particular it should not be forgotten that the concept of causality underlies the very interpretation of each result of experiment, and that even in the coordination of experience one can never, in the nature of things, have to do with well-defined breaks in the causal chain. The renunciation of the ideal of causality in atomic physics which has been forced on us is founded logically only on our not being any longer in a position to speak of the autonomous behavior of a physical object, due to the unavoidable interaction between the object and the measuring instruments which in principle cannot be taken into account, if these instruments according to their purpose shall allow the unambiguous use of the concepts necessary for the description of experience. (Bohr 1937 p 293)

I hope by these remarks to have conveyed the impression that in abandoning the causal description in atomic physics we are not con- cerned with a hasty assertion of the impossibility of comprehending the wealth of phenomena, but with a serious effort to account for the new type of laws here encountered in conformity with the general lesson of philosophy regarding the necessity of a balance between analysis and synthesis. (Bohr 1937 p 294)

Above all, just the impossibility in introspection of sharply distinguishing between subject and object as is essential to the ideal of causality would seem to provide the natural play for the feeling of free will. (Bohr 1937 p 297)

In case, however — as in the region of quantum phenomena — this interaction plays an essential role for the appearance of the phenomena themselves, the situation is completely changed, and we are in particular forced to renounce the combination, characteristic of classical physical description, of the space-time coordination of the event with the general conservation theorems of dynamics. For the use of rods and clocks to fix the system of reference makes it by definition impossible to take into account the energy of momentum which might be transferred to them in the course of the phenomenon. Conversely, those quantum laws whose formulation rests essentially on the application of the concept of energy or momentum can appear only under circumstances of investigation from which a detailed account of the space-time behavior of the object is excluded. (Bohr 1937 pp 291–292)

The fundamentally statistical character of quantum mechanics which is expressed in Heisenberg’s uncertainty relations is indeed not a temporary restriction of the analysis of atomic events, but it corresponds in an appropriate manner to the point of view of complementarity, which is more comprehensive than the ideal of causality, and necessary in order to account for the wealth of experiences depending on the existence of the quantum of action. (Bohr 1938b p 320)

Above all one should mention here Gamow’s explanation of the fine structure of α-spectra, which created, with the interpretation of optical spectra as a model, the basis for a more intimate knowledge of the discrete quantum states of nuclei. In the first place this consisted — contrary to the analysis of atomic spectra using correspondence — only of the appropriate use of the classical conservation laws and the quantum postulates. (Bohr 1938b pp 322-323)

Due to the essentially statistical character of the thermodynamical problems which led to the discovery of the quantum of action, it was also not to begin with realized, that the insufficiency of the laws of classical mechanics and electrodynamics in dealing with atomic problems, disclosed by this discovery, implied a shortcoming of the causality ideal itself. (Bohr 1939b pp 11–12)

Just this circumstance obviously excludes any possibility of describing the fate of a single photon as a causal event in space and time. (Bohr 1939b p 12)

These so-called quantum postulates are not only totally foreign to classical mechanical ideas, but imply an explicit renunciation of any causal description of such atomic processes. (Bohr 1939b p 13)

The view-point of complementarity allows us indeed to avoid any futile discussion about an ultimate determinism or indeterminism of physical events, by offeringa straight-forward generalization of the very ideal of causality, which can aim only at the synthesis of phenomena describable in terms of a behaviour of objects independent of the means of observation. (Bohr 1939b p 25)

The causal mode of description has deep roots in the conscious endeavours to utilize experience for the practical adjustment to our environments, and is in this way inherently incorporated in common language. By the guidance which analysis in terms of cause and effect has offered in many fields of human knowledge, the principle of causality has even come to stand as the ideal for scientific explanation. (Bohr 1948 p 312)

However, a wholly new situation in physical science was created through the discovery of the universal quantum of action, which revealed an elementary feature of ‘individuality’ of atomic processes far beyond the old doctrine of the limited divisibility of matter originally introduced as a foundation for a causal explanation of the specific properties of material substances. This novel feature is not only entirely foreign to the classical theories of mechanics and electromagnetism, but is even irreconcilable with the very idea of causality. (Bohr 1948 p 313)

These so-called indeterminacy relations explicitly bear out the limitation of causal analysis, but it is important to recognize that no unambiguous interpretation of such relations can be given in words suited to describe a situation in which physical attributes are objectified in a classical way. (Bohr 1948 p 315)

Recapitulating, the impossibility of subdividing the individual quantum effects and of separating a behaviour of the objects from their interaction with the measuring instruments serving to define the conditions under which thc phenomena appear implies an ambiguity in assigning conventional attributes to atomic objects which calls for a reconsideration of our attitude towards the problem of physical explanation. In this novel situation, even the old question of an ultimate determinacy of natural phenomena has lost its conceptional basis, and it is against this background that the viewpoint of complementarity presents itself as a rational generalization of the very ideal of causality. (Bohr 1948 p 317)

In particular, the place left for the feeling of volition is afforded by the very circumstance that situations where we experience freedom of will are incompatible with psychological situations where causal analysis is reasonably attempted. In other words, when we use the phrase ‘I will’ we renounce explanatory argumentation. (Bohr 1948 p 318)

A sentence like “we cannot know both the momentum and the position of an electron” raises at once questions as to the physical reality of such two attributes, which can be answered only by referring to the mutually exclusive conditions for the unambiguous use of space-time coordination, on the one hand, and dynamical conservation laws, on the other. (Bohr 1948 p 315)