Immanuel Kant and the Critique of Pure Reason
Immanuel Kant was one of the most distinguished Western philosophers, on a short list of influencers alongside Plato, Aristotle, and Hegel. We’ll see today that Kant commenced his philosophical efforts with the aim of responding to Hume in one of the landmark works of Western philosophical thought, his Critique of Pure Reason. To accomplish this, he had to transform the theory of knowledge forever. In this lecture, we will explore the transcendental analytic of knowledge; you may not yet know what those terms entail, but you will. In the subsequent lecture, we will examine another facet of Kant’s Critique of Pure Reason, the transcendental dialectic. For now, however, we will address the foundational material.
Born in what was then East Prussia, Kant undertook several tutoring positions before eventually securing a teaching post at the University of Königsberg. He was essentially a mathematical physicist for the first half of his life and made significant contributions to the nebular theory of stellar formation. He was famously punctual, and the residents of Königsberg were reputedly able to set their watches by his daily constitutional—an event he allegedly missed only once, on the day he read Rousseau. Kant authored numerous philosophical treatises in different subfields, yet his most renowned works are his three critiques, voluminous texts that formulate a comprehensive system of reason: the Critique of Pure Reason (our focus today, concerning the theory of knowledge), the Critique of Practical Reason (pertaining to ethics), and the Critique of Judgment (addressing aesthetics, the theory of art, and teleology—or the concept of purpose in nature). Together, these comprise his vision of what the intellectual faculties of the human mind can achieve.
Confronting Hume’s Challenge
Our interest today lies exclusively in Kant’s theory of knowledge. Let’s recall Hume: Kant remarked that upon reading Hume, he realized that if Hume were correct, science stood on precarious ground. Kant’s first objective, then, was to discover a rebuttal to Hume’s critique of causality and scientific knowledge, for in the absence of such a response, neither science nor philosophy could lay claim to necessarily true knowledge of the world. Achieving necessarily true knowledge is precisely Kant’s ambition. He fulfilled this in one of the most influential philosophical treatises ever composed, his 1781 Critique of Pure Reason. The central question of that work is: How can we know necessary truths about reality? To see how Kant tackles this, we must look back to Hume.
Hume’s Initial Distinction
You may recall that Hume’s skeptical critique of nearly all our knowledge was consistently anchored in one pivotal distinction: the division of knowledge into relations of ideas and matters of fact. Kant now sets out to dissect the criteria Hume employed to define these two categories, thereby creating a third category of knowledge where Hume identified only two. Let’s see how he does that: for Hume, relations of ideas are both a priori—meaning they are justified or valid independent of experience—and are also true by definition alone. These two notions—that relations of ideas are true by definition and necessarily valid outside of experience—Hume intertwined. By contrast, in his category of matters of fact, truths are a posteriori (known by experience; one must observe them) and not true by definition. Again, Hume merged these two characteristics.
Analytic versus Synthetic
What Kant does—drawing partly from Leibniz—is coin a term for truth by definition: analyticity. In an analytic judgment—such as “All bachelors are unmarried,” an example we used earlier—the predicate is contained within the subject, making the statement true by definition. If I say, “All bachelors are unmarried,” the word “bachelor” logically includes “unmarried,” so the predicate is embedded in the subject. Accordingly, it must be true by definition. Conversely, synthetic statements, as Kant employs the term, are those that aren’t analytic; they involve predicates imparting new information not found in the subject. For instance, “There are bachelors in this room,” which might be true or false, or “Bachelors are lonely,” which also depends on empirical verification, cannot be validated by definition alone—we must consult facts to evaluate their truth.
A New Matrix of Knowledge
Kant thus introduces two sets of distinctions where Hume had only one. In Kant’s terminology, Hume’s relations of ideas are both a priori and analytic, while Hume’s matters of fact are a posteriori and synthetic (not true by definition). Up to this point, there is no conflict; the difference emerges now, leading us to examine a diagram included with the written materials for this course. Imagine two columns: the left column labeled “analytic knowledge” and the right column “synthetic knowledge,” then two rows: the top row labeled “a posteriori knowledge” and the bottom row labeled “a priori knowledge.” These four possibilities yield four cells of potential knowledge. Could analytic a posteriori knowledge exist? Kant argued no, because “a posteriori” implies knowledge from experience, whereas “analytic” implies truth by definition—so that combination is impossible. That leaves synthetic a posteriori knowledge, which corresponds to Hume’s matters of fact; they are established by experience, and their predicates aren’t contained within the subject. Next is analytic a priori knowledge, matching Hume’s relations of ideas.
We’ve covered three of the four possible cells, so no surprises arise yet, since Kant is merely categorizing Hume’s argument. The real question is whether knowledge could exist in the fourth cell: synthetic a priori. By that, Kant asks: can there be knowledge that is a priori (not requiring empirical validation) and yet not merely true by definition (like “All bachelors are unmarried”)—that is, knowledge whose predicate adds something new not implicit in the subject? Hume would say no; only those two categories exist, relations of ideas and matters of fact. Hume would be right—if Kant had not introduced his greatest theoretical innovation to demonstrate the possibility of synthetic a priori knowledge.
Kant’s Analytic-Synthetic Framework:
| Analytic: Predicate contained in Subject | Synthetic (Predicate not contained in subject) | |
| A Posteriori: Dependent on experience | X (Incoherent category) | Synthetic A Posteriori: Hume's Matters of Fact: "Belgium contains bachelors" |
| A Priori: independent of experience | Analytic A Priori (Hume's relations of ideas: "All bachelors of unmarried") | Synthetic A Priori (Kan'ts category of non-trivial knowledge valid independent of experience) |
Kant’s Copernican Revolution
Here is how Kant accomplishes that. He concurs with empiricists that all knowledge undoubtedly begins with experience; the infant mind begins devoid of ideas or knowledge. However, he insists that this does not necessarily mean all knowledge derives from experience. He termed this insight his Copernican revolution: instead of imagining that our cognition simply adapts to external objects, suppose that external objects must adapt to our cognition. In other words, what if our cognitive apparatus lacks innate content but inherently and unavoidably imposes a structure or organization on every experience? In essence, what if the mind actively fashions what we perceive, so that each experience is always the confluence of external sensations and the mind’s own transcendental organizing principles?
In that sense, from the moment we open our eyes each morning, our experience of the room is produced by two factors: the raw data from external objects and our mind’s active ordering of those sensations into categories enabling us to have objective knowledge of how the world appears. Hence, cognition is active rather than passive. Historically, philosophers often compared the mind to soft wax receiving impressions, but Kant holds that the mind dynamically grasps and orchestrates experience.
If Kant is correct—if this Copernican revolution inverts our usual perspective just as Copernicus refuted the assumption that the Sun revolves around the Earth—then synthetic a priori knowledge emerges. We can know certain facts about tomorrow’s experiences, not because we are confident the external world remains the same, but because our cognitive faculties will continue to enforce the same structural rules on any future experience. A crude but illuminating analogy: if I have rose-colored glasses permanently affixed to my face, I know with certainty that everything I see tomorrow will appear rose-hued. If human perception has invariant characteristics, we can anticipate how phenomena will present themselves in advance.
Mathematics, Physics, and the Parallel Postulate
Kant suggested we can observe this kind of active structuring in all mathematics—particularly arithmetic and geometry—and in some foundational principles of physics. One key example is the parallel postulate in Euclidean geometry, which states that given a plane and a line in that plane, and a point not on that line, only one line can be drawn through that point that remains parallel to the original line. Most of us, if we visualize the scenario, readily perceive that no more than one such line is possible. If you attempt to draw more than one, one of them either ceases to be parallel or merges with the original. Kant contends we comprehend this truth a priori, so no amount of empirical testing is required. According to Hume, truths are either empirical or definitional; yet the parallel postulate is neither purely empirical nor a matter of definition. Hence, Kant infers that we must already possess an a priori intuition of space, heralding a major transformation in philosophical thought.
Kant’s Cognitive Framework
Let us situate this within Kant’s broader theory. Kant aims to prove that synthetic a priori knowledge exists, effectively refuting the strict dichotomy Hume proposed. By affirming that the mind is active, Kant’s Copernican revolution permits such knowledge to flourish. He divides the mind’s cognitive powers into intuition (or sensibility), Understanding, and Reason. Within intuition, Kant identifies two a priori forms—space and time—indicating that everything we perceive unfolds in space and time; if it did not, we could not perceive it. Meanwhile, the Understanding applies categories (notions like substance and causality) to our sensations. By furnishing these concepts to experience, the mind supplies universally valid truths—this is why, in Kant’s view, we are certain that the future will resemble the past with respect to causal relations.
Kant’s 12 Categories
In the Critique of Pure Reason, Kant enumerates twelve categories of the Understanding, grouped into four classes of three:
These categories constitute the conceptual framework by which the mind structures experience, ensuring a world that appears orderly and governed by laws.
The Bounds of Knowledge
There is, however, a cost to Kant’s position. All our knowledge is confined to what we can experience, and the way we experience it. If you query the nature of the Moon independent of any observer—for instance, before humanity existed—Kant would argue that falls outside the scope of what we can legitimately know. He effectively synthesizes elements of empiricism and rationalism by granting us knowledge that does not originate wholly in experience while simultaneously affirming our inability to access things in themselves. The objects we examine scientifically are phenomena, and regarding noumena (the thing in itself), our knowledge is nil. Moses Mendelssohn famously labeled Kant “the Great Destroyer,” because although Kant validates synthetic a priori knowledge, he also restricts it to the realm of appearances, excluding ultimate reality.
Conclusion
Finally, Kant’s system includes another faculty, Reason, which comes into play in the subsequent part of the Critique, the transcendental dialectic. For now, suffice it to say that Kant giveth and Kant taketh away: he provides a means of asserting necessarily true knowledge about appearances, yet insists we cannot know things in themselves. By doing so, he assigns an active role to the mind—an insight that has influenced philosophy for centuries—while establishing a conceptual boundary between human cognition and ultimate reality.
Blog