Kant’s Transcendental Idealism
Hume’s explanation of the mind, along with his analysis of induction and causation, woke German philosopher Immanuel Kant (1724-1804) from a “dogmatic slumber.” He feared that Hume’s philosophy might discourage scientific progress. Hume threatened the idea of scientific progress by suggesting that reality is a flux of events without provable causal processes and that induction gives no rational power to predict the future.
But how, thought Kant, could we experience such an orderly and causally structured world if Hume is right? Kant became interested in the limits and structure of our experience of the world and found fundamental and necessary conditions for it.
Kant cannot imagine an experience of something outside time and space. Space and time are necessary conditions for experience. These conditions are forms according to which experience is necessarily structured. Kant calls them a priori forms because they are conditions prior to having any experience.
Kant also finds a priori conditions of understanding (he calls them categories of understanding) with respect to human experience. One example is causality. We cannot make sense of experience without it. Causal understanding structures experience. Another example is substance—roughly the idea that things persist over time. We don’t learn from experience that things persist, but we naturally see them that way.
Could Kant’s conditions—how we find ourselves in a causally structured, space/ time world—be given a neuroscientific explanation? Kant would think not. Brain biology exists in our world of experience—what Kant calls the phenomenal world— but the a priori conditions are not found there. A priori conditions enable the world of phenomena but are not part of the phenomena they determine.
Kant also pondered an ultimate reality behind the world of phenomena—the noumenal world—but realized that we are cognitively excluded from grasping its nature. For Kant, the world of experience is a constructed world. We cannot know reality beyond what we perceive within this construction. Kant expresses this by saying we cannot know the “thing in itself.”
Human knowledge is possible only in a constructed reality whose conditions we know. A common metaphor that has been used to explain how we can think about experience, in Kant’s view, is that of wearing magical spectacles. In his view, the phenomenal world conforms to the a priori forms of experience and the categories of understanding. These conditions work like magical spectacles that determine what we see. The spectacles construct reality as it is for us. Furthermore, we can never take them off and we have no way of finding out how our experience might correspond to ultimate reality. We can only know the conditions the spectacles impose on experience and, consequently, whatever phenomenal reality they support. Kant’s world of experience is a system that we cannot extricate ourselves from, but which we can know and within which we can know what happens.
Why does Kant postulate an ultimate reality if we can say nothing about it? Kant needs an ultimate reality to ground objectivity upon. Even if we don’t know any ultimate reality, we are metaphorically looking at one and the same ultimate reality through our magical spectacles.
Kantian thinking shaped philosophy and science throughout the nineteenth century. However, with the progress and expansion of science around the turn of the twentieth century, the idea of an unknowable ultimate reality seemed passe. During the first two decades of the twentieth century, Albert Einstein (1879-1955) demonstrated how to think of space and time scientifically, with physical—not a priori—concepts. Kant’s space was Euclidean and a priori; Einstein’s was nonEuclidean and scientifically corroborated. Moreover, Einstein believed that “time and space are modes by which we think and not conditions in which we live” (Forsee 1963, p. 81) and held that time and space are “free creations of the human mind,” as are all other physical concepts:
Physical concepts are free creations of the human mind, and are not, however it may seem, uniquely determined by the external world. (Einstein and Infeld 1938, p. 152)
No one can rightfully claim to have arrived at the nature of space and time as perfect concepts. One can only offer alternative ways of thinking about reality. Einstein’s open mind recognizes the epistemological distance between concepts and reality:
In our endeavor to understand reality we are somewhat like a man trying to understand the mechanism of a closed watch. He sees the face and the moving hands, even hears its ticking, but he has no way of opening the case. If he is ingenious he may form some picture of a mechanism which could be responsible for all the things he observes, but he may never be quite sure his picture is the only one which could explain his observations. He will never be able to compare his picture with the real mechanism and he cannot even imagine the possibility of the meaning of such a comparison. (Einstein and Infeld 1938)
We cannot verify the correspondence of our concepts with reality—we are conceptually trapped. Unable to step out of our concepts, unable to perceive reality directly, we do our best to understand what we can, but we may never look at our concepts and reality from the side to see how we are doing in absolute and certain terms. Yet, for Einstein, reality is comprehensible via physical concepts, however elusive it may seem.
After Kant and the decline of German idealism, a monist, physicalist view of the universe ensued, as given by physics. Kant’s dualist ontology—with empirical reality on the one hand and ultimate reality on the other—faded. Even so, Kantian metaphysics remained stimulating to Werner Heisenberg (1901-1976), Niels Bohr (1885-1962), Einstein, and other physicists of the twentieth century. Kant made a deep impression on Einstein, as did Hume. Einstein absorbed from them what he thought was true:
I am reading Kant’s Prolegomena here, among other things, and am beginning to comprehend the enormous suggestive power that emanated from the fellow and still does. Once you concede to him merely the existence of synthetic a priori judgments, you are trapped. I have to water down the “a priori” to “conventional,” so as not to have to contradict him, but even then the details do not fit. Anyway, it is very nice to read, even if it is not as good as his predecessor Hume’s work. Hume also had a far sounder instinct. (Born 1969, p. 25)
Einstein finds Kant’s metaphysics problematic because we can never justify our concepts as being perfect tools for explaining reality; they are always, to some degree, “conventional.” In his commentary chapter in The Philosophy of Bertrand Russell, Einstein clarifies this point and why he values Hume’s philosophy:
Hume saw that concepts which we must regard as essential, such as, for example, causal connection, cannot be gained from material given to us by the senses. . . . Man has an intense desire for assured knowledge. That is why Hume’s clear message seemed crushing: the sensory raw material, the only source of our knowledge, through habit may lead us to belief and expectation but not to the knowledge and still less to the understanding of lawful relations. (Einstein and Seelig 1960, p. 21)
Einstein values Hume’s demonstration of the failure of philosophy as a way of deriving perfect physical concepts of, for example, cause and effect. He also values Kant’s explication of what it would mean to have assured knowledge:
Then Kant took the stage with an idea which, though certainly untenable in the form in which he put it, signified a step towards the solution of Hume’s dilemma: whatever in knowledge is of empirical origin is never certain (Hume). If, therefore, we have definitely assured knowledge, it must be grounded in reason itself. This is held to be the case, for example, in the propositions of geometry and in the principle of causality. These and certain other types of knowledge are, so to speak, a part of the implements of thinking and therefore do not previously have to be gained from sense data (i.e., they are a priori knowledge). (Einstein and Seelig 1960, p. 22)
Einstein takes Kant to have demonstrated that assured (certain) knowledge must derive from reason itself (the implements of thinking). At the same time, however, he points out the arbitrary nature of Kantian metaphysics:
Today everyone knows, of course, that the mentioned concepts contain nothing of the certainty, of the inherent necessity, which Kant had attributed to them. (Einstein and Seelig 1960)
Kantian metaphysics fails, but something remains correct:
The following, however, appears to me to be correct in Kant’s statement of the problem: in thinking we use, with a certain “right,” concepts to which there is no access from the materials of sensory experience, if the situation is viewed from the logical point of view. (Einstein and Seelig 1960)
From a logical point of view, we must use concepts to understand reality. Our knowledge claims operate within our ways of thinking, which structure reality for us. This is how the world makes sense. Nevertheless, how this sense making happens is, for Einstein, beyond human understanding:
The very fact that the totality of our sense experiences is such that by means of thinking (operations with concepts, and the creation and use of definite functional relations between them, and the coordination of sense experiences to these concepts) it can be put in order, this fact is one which leaves us in awe, but which we shall never understand. One may say “the eternal mystery of the world is its comprehensibility.” (Einstein and Seelig 1960, p. 292)
The lesson from Kant is that the very concept of an external world (an empirical world of research and inquiry) demands comprehensibility:
It is one of the great realizations of Immanuel Kant that the setting up of a real external world would be senseless without this comprehensibility. (Einstein and Seelig 1960)
The power of Hume’s skepticism ceases when we realize this fact. Even if we cannot disprove his skepticism, we must realize the consequences of endorsing radical skepticism. We can accept Hume’s view, but then we find ourselves lost— literally beyond comprehension. Einstein picked the former alternative. What remained was a picture of a single reality to be studied and understood by science.
-  Kant does this under the heading “The Transcendental Deduction of the Categories” in TheCritique of Pure Reason (Kant et al. 1998).