Idealism Pt. 2: Kant’s Transcendental Idealism

Author: Addison Ellis
Category: Historical Philosophy
Word Count: 1000

Editor’s Note: This essay is the second of three in a series authored by Addison on the topic of philosophical idealism. Part 1 on Berkeley’s Subjective Idealism can be read here.

In the 18th Century, what has become known as the empiricist picture of knowledge took the mind to have a very specific relationship with the world. The mind, empiricists such as John Locke and David Hume thought, was largely passive, conforming to the world around it. Thus, for me to gain knowledge of the world is to have my mind shaped by the world as it interacts with my senses. There is a problem with this, however. Gaining knowledge is a rule-governed enterprise. Our minds sort and categorize incoming information according to mental standards of some sort. However, if all knowledge is gained entirely passively from the world through the senses, then the rules or norms of thinking are also gained this way. But this cannot be true. If the norms of thought are derived from sense experience, then we would have to judge whether the norms we learned were the right ones, whether they fit our experience. But in order to make such judgments, we must already possess a set of norms for thinking! Thus, the mind itself must contribute at least one element to knowledge. The mind must therefore be, in part, spontaneous – that is, not merely determined by an outside force.1

The Copernican Turn

Immanuel Kant, an 18th-Century German philosopher, was dissatisfied with both the empiricists and the rationalists of his time – the former of which believed that all knowledge was rooted in sensory experience, the latter of which believed that knowledge could come from inner reflection alone. He agreed with the empiricists that knowledge required some element given to us from sense experience. And he agreed with the rationalists that there could be knowledge that transcends sense experience, such as mathematical and moral knowledge. However, he disagreed that knowledge was simply a passive transaction between world and knower. Rather, Kant believed that the knower herself contributes a great deal to what she knows.

Consider the following rule of inference:

If P, then Q

Therefore, Q

I know that this rule is valid, but it seems unlikely that I have this knowledge as a result of various acts of sense perception. It isn’t as though I saw many positive instances of it, then finally gathered enough evidence to conclude that it must be a valid inference rule. Instead, I know it is a valid inference rule simply by reflecting on it. Therefore, the normative rules for thinking are not given to me in experience; they are given by me through acts of spontaneity.

The upshot of this crucial Kantian insight is that the standard model of knowledge is backwards. When we know something, the world does not simply mold the mind. Instead, Kant believed, the world must “conform to” the mind. In other words, the world as we know it is always, in some way, determined by the way our minds work. It is in this sense that Kant is an idealist—mental structuring activity is necessary for and antecedent to the sensory world we perceive.2 Kant’s reversal of the traditional picture is what he calls his “Copernican revolution” in philosophy, because it is akin to Copernicus’s proof that the solar system is heliocentric rather than geocentric.3

In the Critique of Pure Reason (CPR), Kant tells us that knowledge has two sources: (1) the receptive faculty of sensibility (what we receive from the senses), and (2) the spontaneous faculty of the understanding (what the intellect does with what is received from the senses). In order to have knowledge, then, Kant believes that we must subsume what is given by the senses under innate rules for thinking. What we know about is essentially related to how we are equipped to know the world.

Some Worries

One immediate concern about Kant’s project is that it may lead to at least two vicious forms of skepticism. First, if what we can know is necessarily constrained by our spontaneous application of norms of thought, then is it possible that there are different norms of thought for different people? If so, which norms are correct, and how could we possibly know?4

A second skeptical worry is that Kant’s picture entails that we cannot ever know the world as it truly is, but only as we are equipped to know it. Indeed, Kant himself states that we cannot know “things in themselves” (i.e., things that are not related to our way of thinking), but only things “as they appear to us.” Many commentators take this to mean that all we can ever know are our internal representations of the world, but never the world itself.

A Kantian Reply

Despite these worries, Kant may have a way out. First, Kant believes that we are universally equipped with the same basic rules for thinking. If he is correct then all properly functioning, e.g., not severely mentally handicapped, etc., human beings are capable of attaining the same knowledge about the world. To be entitled to this claim would take some argumentation that we do not have space for here, but one thing we can point out is that the basic rules of thought are just the rules of logic, and most philosophers will agree that the rules of logic are universally applicable.

Second, while it is tempting to read Kant to be claiming that we can only ever know our internal representations, Kant does state throughout the CPR that “appearances” are not internal to our minds. There are alternative, more charitable readings of what Kant is thinking. One such reading says that to know things “as they appear to us” is simply to know things that are apt to appear to us when we encounter them. The things that cannot appear to us, because experience of them is ruled out by the structure of the mind, are, as Kant says, “nothing to us.” If so, then arguably we couldn’t hope for a better understanding of reality than the one we already have, because reality is simply all the things that are apt to appear to us when we encounter them.


1The receptive faculty is the faculty of sense experience (the sensibility). The spontaneous faculty is the faculty of the understanding, according to Kant in CPR.

2Kant uses the term “transcendental” to refer to innate cognitive structures (or the norms of thought) that make our knowledge possible. Thus, Kant’s idealism is a transcendental idealism, since the world-to-mind conformity relation is due to these transcendental structures.

3That is, in Kant’s Copernican revolution, as in Copernicus’s own revolution, we ought to experience a kind of gestalt shift so that we see that we’ve been looking at the picture backwards all along.

4According to Hegel, who we will cover in the final essay of this series, our innate conceptual structuring activities are deeply historical and depend on social factors. If so, then one might read Hegel to be claiming that it is indeed possible that some of us will have different ways of carving up the world.


Kant, Immanuel. Critique of Pure Reason. Trans. Paul Guyer and Allen W. Wood. Cambridge: Cambridge UP, 1998.

Addison is a doctoral student at the University of Illinois at Urbana-Champaign. He holds an M.A. in Philosophy from the University of Colorado at Boulder and a B.A. in Philosophy and Psychology from the University of Illinois at Urbana-Champaign. He is currently interested in philosophy of mind (especially problems of intentionality), epistemology (especially the role of philosophical intuitions in philosophical practice), Kant, and post-Kantian philosophy. Apart from philosophy, he is interested in playing good music, hanging out with his dog, Chessie, and watching/thinking about movies. Website: