Bohr's intellectual journey began with the recognition that empirical phenomena implied the breakdown of classical mechanics in the atomic domain; this, in turn, led to his adoption of the 'quantum postulate' that justifies the 'stationary states' of his atomic model of 1913. His endeavor to develop a wider conceptual framework harmonizing both classical and quantum descriptions led to his proposal of the new methodological goals and standards of complementarity. Bohr's claim that an empirical discovery can demand methodological revision justifies regarding his epistemological lesson as supporting a naturalistic epistemology.
Bohr's intellectual journey began with the recognition that empirical phenomena implied the breakdown of classical mechanics in the atomic domain; this, in turn, led to his adoption of the 'quantum postulate' that justifies the 'stationary states' of his atomic model of 1913. His endeavor to develop a wider conceptual framework harmonizing both classical and quantum descriptions led to his proposal of the new methodological goals and standards of complementarity. Bohr's claim that an empirical discovery can demand methodological revision justifies regarding his epistemological lesson as supporting a naturalistic epistemology.
What Niels Bohr called the `epistemological lesson' of `complementarity' was the result of reasoning analogically from the classical conception of a mechanical state to a new quantum mechanical conception of an `object' in a mechanical state. Bohr proposed to redefine the `objectivity' essential for scientific description in terms of the epistemological demand for unambiguously communicable descriptions of observational results, a move which has profound consequences for how we can understand the concept of the quantum mechanical state and the nature of the `system' which is `in' this state. Here it is argued that the old notion of the `object' which is in a classical mechanical state is drawn from a substance/property ontology derived from Aristotle's analysis of categorical propositions. In moving to describing a system in a quantum mechanical state, the system that is `in' such a state can no longer be so regarded as a substance possessing properties. Bohr argues that the concept refers to an interaction which has a feature of wholeness or `individuality' that implies that the distinction between `object system' and `observing system' is relative to the context of the description. This conclusion, in turn, implies the need for a combination of complementary modes of description; however, because of his reticence in making ontological claims, he failed to develop this dimension of his new framework of complementarity.
This article explores Bohr's argument, advanced under the heading of 'complementarity,' concerning quantum phenomena and quantum mechanics, and its physical and philosophical implications.In Bohr, the term complementarity designates both a particular concept and an overall interpretation of quantum phenomena and quantum mechanics, in part grounded in this concept.While the argument of this article is primarily philosophical, it will also address, historically, the development and transformations of Bohr's thinking, under the impact of the development of quantum theory and Bohr's confrontation with Einstein, especially their exchange concerning the EPR experiment, proposed by Einstein, Podolsky and Rosen in 1935.Bohr's interpretation was progressively characterized by a more radical epistemology, in its ultimate form, which was developed in the 1930s and with which I shall be especially concerned here, defined by his new concepts of phenomenon and atomicity.According to this epistemology, quantum objects are seen as indescribable and possibly even as inconceivable, and as manifesting their existence only in the effects of their interactions with measuring instruments upon those instruments, effects that define phenomena in Bohr's sense.The absence of causality is an automatic consequence of this epistemology.I shall also consider how probability and statistics work under these epistemological conditions.
I argue here that progress in understanding the lessons of quantum physics has been hindered by the tendency to cast Niels Bohr as a villain. Building on the work of Favrholdt, Faye, and Howard, I present a more accurate view of Bohr's proposal for the "epistemological lesson" of quantum physics. I then argue that several interpretive programs -- often presented as alternatives to Copenhagen -- are, after substantial conceptual work, arriving at a view that is notably similar to Bohr's. Using Maudlin’s formulation of the measurement problem as a foil, I argue that it relies on a notion of wave-function completeness that is inappropriate for a genuinely indeterministic theory, and one that Bohr consistently rejected. On Bohr's view, quantum theory permits concrete agents, situated within specific experimental contexts, to make objective descriptions, while refusing the demand for a single, context-free "description from nowhere".
‘The new wave mechanics gave rise to the hope that an account of atomic phenomena might be obtained which would not differ essentially from that afforded by the classical theories of electricity and magnetism. Unfortunately, Bohr's statement in the following communication of the principles underlying the description of atomic phenomena gives little, if any, encouragement in this direction.’ This comment is to be found in the brief note prefaced by the editors of the British journal Nature to Niels Bohr's paper ‘The Quantum Postulate and the Recent Development of Atomic Theory’ in the supplement of 14 April 1928. The article in question is the famous paper which first introduced and defined the concept of ‘complementarity’ and outlined the basic points of what was to become known as the Copenhagen interpretation of quantum mechanics. In the paper, Bohr returned to the arguments contained in the paper discussed at the International Congress of Physicists held at Como in September 1927 to mark the centenary of the death of Alessandro Volta and published in the proceedings of the same congress. It was probably his conviction that the new viewpoint adopted in the description of nature fully expressed the theoretical and cognitive content of quantum physics coupled with his enthusiasm at having achieved ‘after many years of struggling in the dark […] the fulfilment of the old hopes’ that induced him to give his work broader and more prestigious circulation.
Niels Bohr thought that what quantum mechanics has taught us is not only to understand something new, but also a new meaning of the term “to understand”. Bohr wanted to free himself from ontology by putting emphasis on epistemology; i.e. on certain conditions for observation and description. He believed that the epistemological lesson of quantum mechanics was a crucial one. Basically it has two important aspects. 1. Bohr's philosophy leads to a break with the so-called correspondence theory for truth and meaning. It states that true propositions are descriptions of a world that is independent of our observations; i.e. that on a regular basis one can compare with reality, that is, in compare the two areas against each other. Bohr believed that this did not make sense, since in many situations cannot add meaning to a world that has a sich structure (the thing-in-itself), which in turn can be depicted when observed. 2. All knowledge is attained under certain conditions for description. Bohr thought that the long-lasting criterion for valid knowledge is that can communicate it unambiguously to each other. When Bohr emphasised that we are suspended in language in the sense that all knowledge determines what can say about the world in an understandable way, and not what the world actually is, this must be understood in the sense that only know the world as recognised; i.e., structured on the basis of the conditions for description, to which are, as part of the world, necessarily are subjected to. Bohr's emphasis on conditions for descriptions invalidates the well-known distinction between analytic and synthetic propositions. An important feature of the descriptive use of ordinary language, as well as in classical physics, is that description is based on a dividing line between subject and object. This results in idealism and materialism not being tenable positions. Furthermore, it is such that cannot use the designation of things independently of the designation of time and space. Logical principles are only meaningfully applied in situations relating to our conditions for observation. So based on inspiration from Bohr, Favrholdt developed a more adequate and comprehensive philosophy, where he, among other things, states that all humans possess a fundamental language, underlying all languages. This constitutes a number of concepts – the core – where the correct use is dictated by the structure of the world, which humans learn through sensory perception and action.
Recent high-precision experimental confirmations of quantum complementarity have revitalized foundational debates about measurement, description, and realism. This article argues that complementarity is most productively interpreted as an epistemic principle--constraining what can be simultaneously accessed and represented--rather than as an ontological claim about quantum reality. Reexamining the Einstein-Bohr debate through this lens reveals a persistent tension between descriptive completeness and contextual meaning, a tension experiments clarify but do not dissolve. Building on this analysis, we introduce cognitive complementarity as a structural principle governing reasoning under non-classical uncertainty, where mutually constraining representations cannot be jointly optimized. Within this framework, we propose quantum intuition as a testable cognitive capacity: the ability to sustain representational plurality, regulate commitment timing, and resolve perspective-incompatibilities in a context-sensitive manner. Formulated as a naturalistic construct grounded in shared informational constraints, quantum intuition offers a principled bridge between quantum measurement theory and cognition. This work reframes the historical debate, extends epistemic lessons from quantum foundations into cognitive science, and outlines empirical pathways for studying decision-making in contexts of irreducible uncertainty.