We can imagine the context in which Albert Einstein’s Gedankenexperimente (thought experiments) were born: the study of philosophy. To dismantle the idea that space and time were the same for everyone, the young physicist had to engage in an intellectual duel with two giants of thought: Immanuel Kant and David Hume.
Kant’s legacy: space as an objective necessity
From an early age, Einstein had studied Kant in depth, and from him he learned a fundamental concept: we are not passive spectators of the world; our mind does not simply record reality, but actively processes it. Kant argued that space and time are internal structures of the mind, the necessary conditions for having any experience at all. See also my post Kant and Space.
This idea was crucial for Einstein because it helped him understand that science is not merely a collection of data, but a construction of reason. However, the limit of Kant’s thought lay in the claim that these structures must be rigid and identical for every rational being. For Kant, space was only the one described in Euclid’s geometry books, and time was a universal flow, the same for everyone.
Hume’s inspiration: the courage to doubt
When Einstein began working on his thought experiments about light, he realized that Kant’s ‘fixed rules’ no longer worked. To move beyond them, David Hume’s philosophy was certainly of help.
Hume was a radical philosopher who urged people to take nothing for granted. His motto was simple: if a concept cannot be verified by the senses or through a concrete measurement, then it is suspicious. Hume taught Einstein to have the courage to doubt concepts that seem obvious to ‘common sense’. Einstein was aware that no one had ever measured ‘absolute time’, flowing identically for all; it was only a belief grounded in habit, not in physical evidence.
The synthesis: Relativity is born
Without the support of Hume’s skepticism, Einstein might not have had the audacity to question time. He understood that if the speed of light must remain constant, then time and space must be able to change depending on the speed of the observer (the principles of Special Relativity).
In this way, Einstein went beyond Kant using Hume’s method: he retained the Kantian idea that the mind must create categories to read the world, but he showed that these categories are not immutable. Space and time are not unchanging a priori forms, but physical quantities that can contract or stretch.
Einstein was thus a philosopher among physicists. He learned from Kant that the mind must anticipate reality (that is, describe it even before physical observations) with a coherent theoretical model, but he learned from Hume that no idea is untouchable. In this way, he found the courage to understand that if old theories had become full of gaps or incapable of describing the universe in a unified (that is, invariant) way, one had to have the audacity to redesign the very structure of thought itself, even before the facts forced it.
