In this blog post, drawing on Heisenberg’s insights and various philosophical discussions, we will examine how quantum mechanics transforms traditional language and concepts of causality, and how it influences the way we form concepts.
The Problem of Language and Reference
One of the primary purposes of language is to refer to the objects around us. For example, when we hear the word “cloud,” images such as white clouds floating in the sky or dark, heavy storm clouds come to mind. In this way, a certain connection is established between a word and the object it refers to, and it is through this connection that we are able to communicate with others.
A philosophical debate has long raged over whether this connection is a necessary relationship or an arbitrary one based on social convention. Some traditions have held that there is a necessary relationship between words and reality, while others have argued that the relationship is arbitrary. Twentieth-century linguistics emphasized, through the concepts of the signifier and the signified, that the relationship between a word’s form and its meaning can be essentially arbitrary.
What is important to note here is the very fact that, regardless of the language we use, we associate specific sound forms or symbols with certain realities. Of course, not every sound form refers to a reality. However, language contains a set of nouns that refer to reality, and we use this set to classify and describe the world.
Meanwhile, while phonetic forms change historically, the objects we refer to are not fixed in space and time either. The object we call a “cloud” has no clear boundaries and constantly changes in shape and size over time. Nevertheless, we group all clouds—regardless of their varied appearances—into the same category and call them “clouds,” clearly distinguishing them from airplanes or the moon. Asking how such classification is possible is precisely the more fundamental question regarding language and concept formation.
Concept Formation and the Premise of Causality
The ancient Stoics believed that impressions derived from the senses exist first, and that the mind conceptualizes those impressions through language. In other words, as we experience various impressions coming from the outside world, we extract common properties to form a single concept and assign a word to that concept. This perspective suggests that concepts are subjective and exist on a different plane from reality.
If we rephrase this question more fundamentally, it becomes, “How can we form a concept?” In response, Kant posited that the thing-in-itself—that is, reality as it truly is—cannot be directly perceived by humans. He argued that judgment and concept formation are only possible when we reconstruct the data received through the senses within the cognitive framework of space, time, and the law of causality.
Kant’s central argument is that absolute space, time, and the principle of causality are the a priori conditions of the cognitive subject. He argued that without a framework to situate our sensory experiences within time and space and to link them as cause and effect, experience itself could not exist. In other words, the premise of causality is essential to the process of conceptualization.
A similar issue can be found in ancient Indian philosophy. Nāgārjuna viewed everything we express through language as a conceptual reconstruction. Even the word “cloud” does not accurately represent the object as it is, but is rather a category created by our minds to describe constantly changing objects.
The Abhidharma tradition prior to Nāgārjuna also held that the reality we refer to is in a state of constant change and therefore lacks a fixed inherent nature (svabhāva). Nāgārjuna went further, asserting that no element possesses an independent essence and that everything arises within the relationship of cause and effect. Ultimately, there is a point of convergence with Kant in that linguistic expression and conceptualization are possible only through the premise of causality.
Of course, differences between the two philosophies also exist. While Kant established space, time, and the law of causality—the premises of cognition—as a priori conditions, Nāgārjuna viewed everything as “emptiness” (śūnyatā) on an ontological level. Nevertheless, both traditions share the commonality of establishing causality as a core element of conceptual formation.
Quantum Mechanics, the Evolution of Causality, and the Reconstruction of Language
Classical physics, built upon the a priori principle of causality established by Kant, provided a stable foundation for explaining physical phenomena through the 19th century and into the early 20th century. Although the theory of relativity revised the concept of absolute spacetime, causality itself remained a fundamental principle of physics. Many physicists, including Albert Einstein, believed that physics could describe an objective world and predict the future, and this belief was expressed as a strongly deterministic worldview.
However, the emergence of quantum mechanics in the early 20th century raised fundamental questions about this causality-centered worldview. In the microscopic world, particles such as electrons exhibited phenomena that were difficult to explain using classical intuition alone. For example, phenomena such as quantum tunneling or the influence of measurement processes on the state of a system were difficult to fully explain using traditional laws of causality alone.
Werner Heisenberg’s work also reflected this shift in the dimensions of language and methodology. While classical mechanics allowed the position and velocity of a particle to be expressed as continuous trajectories, Heisenberg’s approach emphasized the relationships between observable physical quantities and their probabilistic nature. The uncertainty principle mathematically demonstrated that position and momentum cannot be determined simultaneously with arbitrary precision, revealing that classical causal, continuous descriptions do not apply directly to the microscopic world.
This shift goes beyond a mere change in the mathematical form of the theory. It demands that we rethink the very language used to describe the world. While classical physics explained the world through causal descriptions, quantum mechanics explains it by focusing on probability, observation, and mathematical descriptions of states. Therefore, the language of quantum mechanics no longer presupposes only a direct correspondence with classical objects; it must also reflect the fact that the act of observation and the process of measurement intervene in the formation of concepts.
In summary, the cognitive framework of causality and spacetime underlay our ability to use a single word—“cloud”—to group diverse phenomena into a single category. Quantum mechanics has shown that this framework does not apply directly to the microscopic world; as a result, the language used to describe the world has moved beyond simple denotation and now requires new conceptual frameworks and modes of expression. This prompts us to reconsider the deep connection between the concepts of physics and the way we understand and describe the world.
The Formation of the Concept of the Electron and the Realm of Observation
To explain this point in a bit more detail, let’s examine the word “electron” instead of “cloud.” Unlike a cloud, an electron is not an object we can directly experience through our senses. Nevertheless, we came to use the word “electron” because we were able to form the concept of the electron through specific observational processes. These observations take place entirely within the realm of classical physics—that is, within the world of our experience as presupposed by Kantian epistemology. Therefore, the formation of the concept of the electron also occurred largely within the framework of everyday language and observation.
Even after Joseph John Thomson’s cathode-ray experiments, various experimental results—such as the particle trajectories observed in the cloud chamber—played a crucial role in forming the concept of the “electron.” However, the behavior of electrons around the atomic nucleus could not be fully explained by classical physics alone. After Max Planck proposed the quantum hypothesis, numerous researchers worked to resolve this problem, and through the work of Niels Bohr, Werner Heisenberg, Erwin Schrödinger, and Max Born, the foundations of modern quantum mechanics were established.
Heisenberg fundamentally reexamined the classical concept of an electron’s “orbit” and proposed the uncertainty principle. The core of this principle is that it is impossible to measure both the position and momentum of an electron simultaneously with arbitrary precision, and that there is a lower limit—defined by quantum mechanics—on the measurement uncertainty of these two physical quantities. This means that the classical physics concepts of position and velocity cannot be applied directly to the microscopic world.
To understand this difference intuitively, let’s consider the example of an airplane. To predict where an airplane will be in 3 seconds, we can measure its current position and velocity simultaneously with sufficient accuracy. Therefore, we can also predict its future position with high accuracy. However, in the case of an electron, since its position and momentum cannot be known precisely at the same time, its future position cannot be determined in a classical mechanical manner based solely on its current state. In this respect, it is difficult to apply the classical concept of cause and effect directly to the microscopic world.
Heisenberg documented the philosophical debates surrounding this issue during his time in Leipzig. At that time, the philosopher Grete Hermann, who was visiting the university, engaged in discussions with Heisenberg and his student Karl Friedrich von Weizsäcker, drawing on the philosophy of Kant. Hermann argued that since the natural sciences are disciplines based on experience, quantum mechanics—which fundamentally undermines the principle of causality—cannot be regarded as a natural science in the strict sense.
In response, Heisenberg and von Weizsäcker presented a different perspective. Hermann’s argument starts from the assumption that “the electron actually exists.” According to Kantian analysis of experience, we form the concept of an “electron” based on observational results, and through those observations, we can speak of the electron’s existence. However, in quantum mechanics, observational results emerge that do not allow classical causality to apply directly; therefore, it is difficult to assert the existence of a specific reality in the same way as before. To borrow Heisenberg’s phrasing, in this context, the electron can be understood not as an independent entity but as a conceptual construct used to describe the observational situation. In other words, depending on the method of observation, the electron may or may not be described as a single “entity.” The emergence of quantum mechanics revealed a situation in which it is difficult to fully objectify observational results, as was possible in classical physics.
In the actual discussion, the radioactive decay of a radium-B atom was presented as a prime example. Since radioactive decay cannot, in principle, be predicted deterministically for an individual atom—only described probabilistically—it was used as a representative example illustrating the difference between classical causality and the quantum mechanical method of description. This example demonstrates that the classical conceptual framework, which is based on the premise of causality, may not apply directly to the microscopic world, and it clearly illustrates why the debate between Heisenberg and Hermann holds such philosophical significance.
The Electron as Language, Metaphor, and Symbol
Let us now take another close look at the difference between the linguistic expressions “cloud” and “electron.”
The object referred to by the verbal form “cloud” changes in shape and size from moment to moment, but the reason we can recognize and call it a single “cloud” is that we can causally link its changing states. Because we can understand the cloud’s previous and subsequent states as a single continuous process, we are able to form the stable concept of “cloud.”
On the other hand, if we assume the existence of a reality referred to by the verbal form “electron,” that reality cannot be described in the microscopic world as a continuous trajectory in the classical sense. In quantum mechanics, according to the uncertainty principle, it is impossible to know both the current position and momentum of an electron simultaneously with arbitrary precision; therefore, it is impossible to predict the next state based solely on the state at a single moment according to classical causal relationships. Therefore, the concept of an “electron” differs in nature from concepts in everyday life that directly refer to specific entities, such as a cloud, a rock, or a tree. Rather, it is closer to the spirit of quantum mechanics to understand it as a conceptual and symbolic expression used to describe specific observational situations and experimental results.
This is also why we feel perplexed when trying to understand quantum mechanics. We tend to try to understand concepts from the microscopic world, such as electrons, in the same way we understand nouns that refer to everyday objects like clouds or ping-pong balls. Because we are so accustomed to the experience that a ping-pong ball cannot pass through a wall, the quantum tunneling phenomenon—where an electron “passes through a barrier”—is difficult to accept intuitively. In this way, we experience confusion by interpreting the term “electron” in the same way we interpret words that refer to everyday objects.
However, terms used in quantum mechanics—such as “electron,” “positron,” “proton,” and “neutron”—are fundamentally different in nature from concepts formed through everyday experience. These are scientific concepts constructed on the basis of experiments and mathematical theories, and they cannot be understood in exactly the same way as everyday language. The pioneers of quantum mechanics, including Bohr and Heisenberg, were well aware of this fact. To explain the microscopic world within the framework of our everyday experience, we have no choice but to use everyday language; however, metaphors and symbols inevitably come into play in this process.
Bohr viewed the so-called wave-particle duality as a term that emerged in the process of explaining quantum phenomena using everyday language. In reality, quantum mechanics consistently describes the microscopic world within a single mathematical framework, and the terms “particle” and “wave” are closer to figurative language used to explain that framework in terms of our experienced world. When we say, “An electron is a particle and simultaneously exhibits wave-like properties,” three nouns—electron, particle, and wave—appear. However, as we examined earlier, “electron” is a concept constructed through observation and theory, and “particle” and “wave” can also be understood as metaphorical expressions borrowed from concepts formed in classical physics and everyday experience.
Ultimately, the language used in the microscopic world is fundamentally different in nature from that used in everyday experience and classical physics, and underlying this difference is the distinct way in which the principle of causality applies. Language formed through everyday experience, based on the principle of causality, can serve as a starting point for exploring philosophical problems such as Kant’s “thing-in-itself” or Nagarjuna’s “emptiness.” However, concepts in quantum mechanics, such as “electron,” are difficult to understand in this way. This is because they are theoretical constructs designed to explain observational situations and measurement results. Even the classical concepts of “particles” and “waves” are used in quantum mechanics merely as limited analogies.
In conclusion, to properly understand quantum mechanics, one must first accept the fact that its language is not the same as that of classical physics. The moment one attempts to understand the microscopic world using only the intuitions of everyday language, confusion is bound to recur. Only when one clearly understands the nature of the language used in quantum mechanics can one more accurately grasp the concepts and phenomena presented by the theory and naturally embrace a new worldview that differs from that of classical physics.
