Kant and quantum reality

In the previous post of “quantum reality” we mentioned the so called “Copenhagen Interpretation” of Quantum Mechanics. As we mentioned, this new Physics did not evolved smoothly form the Classical Physics² (which was mainly supported by a mechanistic view of the world.) In the first years of this new Physics, the philosophy behind it, was dominated by the views of Niels Bohr³ and Heisenberg¹ which is called the Copenhagen Interpretation.

Copenhagen Interpretation

The synopsis of the view is;

The quantum world can only be examined by constructing machines that perform measurements. Quantum Mechanics is a probability calculus which can be applied to a specific combination of measuring devices and quantum system. The waveform is not “real”; it is a mathematical construction that represents the observer’s knowledge of the system. The description of the world is inherently probabilistic; at some point no deeper level of understanding is possible.



The term  “waveform” refers to mathematics behind quantum mechanics. The key point of the interpretation is that it accepts that  no deeper level of understanding is possible, which means that the “weirdness” of quantum mechanics is an innate characteristic of Nature. Here is where philosophy enters the picture;

Someone can ask; is it Nature that is weird or our limitations on how we experience Nature? Is there an objective view of Nature independent of humans?

For so many years scientists were used to perceive Nature (or our reflection of Nature in humans mind) in a platonic way (Nature is real and eternal like the Ideas in Plato’s world). Now some Physics doubts the basic idea for its existence;  the idea that we can understand Nature as it is in every detail. Physics makes a turn and claims that it can only meet a probabilistic view of Nature.

Bohr Einstein

Bohr Einstein

This was something that a lot of physicists did not like, among them Albert Einstein who fought hard to prove that quantum mechanics is not a complete theory and it cannot describe Nature as a whole; that there is subtle “reality” that we are missing. This missing information leads us to probabilistic solutions, but after we discover it then the weirdness will go away. For that reason we tried to disprove quantum mechanics with thought experiments made to prove the incompleteness of the theory. These experiments were addressed to Bohr the ‘Magus’ of quantum mechanics.

Bohr a subjectivist?

Bohr knew that the probabilistic behavior was the core of the theory and he tried to respond to Einstein’s challenges. The issue is that this specific ‘fight’ between Einstein and Bohr was the first open debate about the meaning of quantum mechanics and the philosophy behind it. Bohr answered to Einstein’s challenges but the heart of the most of the scientists was with Einstein. That is because for them Einstein was defending the ‘heart’ of physics and Bohr was a mysticism that was believing in a ghost-like Nature. That is the reason that Bohr was misunderstood by a lot of scientists and philosophers to be a subjectivist‡.

Bohr was not a subjectivist, he explicitly rejected the idea that the experimental outcome is due to the observer.

“It is certainly not possible for the observer to influence the events which may appear under the conditions he has arranged”


According to Stanford Encyclopedia of philosophy Bohr was undoubtedly infulenced  by Kant;

“Much of Kant’s philosophy can be seen as an attempt to provide satisfactory philosophical grounds for the objective basis of Newton’s mechanics against Humean scepticism. Kant showed that classical mechanics is in accordance with the transcendental conditions for objective knowledge.”

According to Kant, the human being is organized in such a way that, when the things make impressions on his senses, he then brings these impressions into spatial or temporal relationships. Man receives from outside only impressions, sensations. The ordering of these in space and in time, the combining of them into ideas, is his own work. The sensations are also not something that stems from the things. It is not the things that man perceives but only the impressions they make on him, the world of experience is  subjectively produced from within.



Bohr believed that we could have objective knowledge only if the experiential subject uses causal and spatial-temporal concepts for describing the sensorial content, placing phenomena in causal connection in space and time, since it is the causal space-time description of our perceptions that constitutes the criterion of reality for them. In short, we can only talk about an object in an objectively existing reality when we apply the physical properties of the spatial-temporal concepts to the object under experiment; that is “position,” “time,” “momentum,” and “energy”.

Correspondence principle

What is important about Bohr’s Kantian philosophy is that to this philosphy we owe the “Correspondence principle” ;

In short the correspondence principle expresses the need that quantum mechanics and classical physics will give the same results when applied to macroscopic systems.

In a first reading the principle sounds obvious; If you want to develop a theory that will be a superset of an old theory, then a prerequisite for the new theory is to give the same results with the old, when applied in the old scope. There is though a Kantian reason for the correspondence principle; The correspondence rule was based on the epistemological idea that classical concepts were indispensable for our understanding of physical reality, and it is only when classical phenomena and quantum phenomena are described in terms of the same classical concepts that we can compare different physical experiences.


² Quantum Mechanics is actually mathematical formalism that is used to describe (initially at least) the world of subatomic particles, a field that Classical Physics failed to describe.

¹ In fact Bohr and Heisenberg never totally agreed on how to understand the mathematical formalism of quantum mechanics, and none of them ever used the term “the Copenhagen interpretation” as a joint name for their ideas. In fact, Bohr once distanced himself from what he considered to be Heisenberg’s more subjective interpretation (APHK, p.51). The term is rather a label introduced by people opposing Bohr’s idea of complementarity, to identify what they saw as the common features behind the Bohr-Heisenberg interpretation as it emerged in the late 1920s. Today the Copenhagen interpretation is mostly regarded as synonymous with indeterminism, Bohr’s correspondence principle, Born’s statistical interpretation of the wave function, and Bohr’s complementarity interpretation of certain atomic phenomena. (

³ Some disagree on what is called Copenhagen interpretation should be credited to Bohr. See here

† Plato holds to the world of ideas, because he believes that the true being of the world must be eternal, indestructible, unchangeable, and he can ascribe these qualities only to ideas.

‡Karl Popper in his critique on “The Copenhagen Interpretatoin” claimed that Bohr and Heisnberg are subjectivists. (Subjectivism holds that because reality is an aspect of our minds, it is affected by them). Popper has tried to defend the EPR thought experiment and show that contemporary Quantum Mechanics is not complete. Critique of Popper’s view claims that Pooper did not understand the outcome of the ERP experiment and he assumed that Bohr “won” because he was influencial.

[T]he necessity of making an extensive use … of the classical concepts, upon which depends ultimately the interpretation of all experience, gave rise to the formulation of the so-called correspondence principle which expresses our endeavours to utilize all the classical concepts by giving them a suitable quantum-theoretical re-interpretation (ATDN, p. 8)

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