NOTE: This section discusses Kant’s Critique of Pure Reason and Prolegomena to Any Future Metaphysics. There is also much more detailed separate study guide treatment of Prolegomena to Any Future Metaphysics on SparkNotes that discusses its relationship to Critique of Pure Reason.


Kant published the Critique of Pure Reason in 1781. It is very long and almost unreadable due to its dry prose and complex terminology. Kant tried to ease his readers’ confusion by publishing the Prolegomena to Any Future Metaphysics two years later. While it is hardly a page-turner, Prolegomena to Any Future Metaphysics is much briefer than Critique of Pure Reason and much more accessible in style, making it a valuable entry point to Kant’s metaphysics and epistemology.

Kant’s primary aim is to determine the limits and scope of pure reason. That is, he wants to know what reason alone can determine without the help of the senses or any other faculties. Metaphysicians make grand claims about the nature of reality based on pure reason alone, but these claims often conflict with one another. Furthermore, Kant is prompted by Hume’s skepticism to doubt the very possibility of metaphysics.

Kant draws two important distinctions: between a priori and a posteriori knowledge and between analytic and synthetic judgments. A posteriori knowledge is the particular knowledge we gain from experience, and a priori knowledge is the necessary and universal knowledge we have independent of experience, such as our knowledge of mathematics. In an analytic judgment, the concept in the predicate is contained in the concept in the subject, as, for instance, in the judgment, “a bachelor is an unmarried man.” (In this context, predicate refers to whatever is being said about the subject of the sentence—for instance, “is an unmarried man.”) In a synthetic judgment, the predicate concept contains information not contained in the subject concept, and so a synthetic judgment is informative rather than just definitional. Typically, we associate a posteriori knowledge with synthetic judgments and a priori knowledge with analytic judgments. For instance, the judgment “all swans are white” is synthetic because whiteness is not a part of the concept of “swan” (a black swan would still be a swan even though it isn’t white), but it is also a posteriori because we can only find out if all swans are white from experience.

Kant argues that mathematics and the principles of science contain synthetic a priori knowledge. For example, “7 + 5 = 12” is a priori because it is a necessary and universal truth we know independent of experience, and it is synthetic because the concept of “12” is not contained in the concept of “7 + 5.” Kant argues that the same is true for scientific principles such as, “for every action there is an equal an opposite reaction”: because it is universally applicable, it must be a priori knowledge, since a posteriori knowledge only tells us about particular experiences.

The fact that we are capable of synthetic a priori knowledge suggests that pure reason is capable of knowing important truths. However, Kant does not follow rationalist metaphysics in asserting that pure reason has the power to grasp the mysteries of the universe. Instead, he suggests that much of what we consider to be reality is shaped by the perceiving mind. The mind, according to Kant, does not passively receive information provided by the senses. Rather, it actively shapes and makes sense of that information. If all the events in our experience take place in time, that is because our mind arranges sensory experience in a temporal progression, and if we perceive that some events cause other events, that is because our mind makes sense of events in terms of cause and effect. Kant’s argument has a certain parallel to the fact that a person wearing blue-tinted sunglasses sees everything in a bluish light: according to Kant, the mind wears unremovable time-tinted and causation-tinted sunglasses, so that all our experience necessarily takes place in time and obeys the laws of causation.

Time and space, Kant argues, are pure intuitions of our faculty of sensibility, and concepts of physics such as causation and inertia are pure intuitions of our faculty of understanding. Sensory experience only makes sense because our faculty of sensibility processes it, organizing it according to our intuitions of time and space. These intuitions are the source of mathematics: our number sense comes from our intuition of successive moments in time, and geometry comes from our intuition of space. Events that take place in space and time would still be a meaningless jumble if it were not for our faculty of understanding, which organizes experience according to the concepts, like causation, which form the principles of natural science.

If time and space, among other things, are constructs of the mind, we might wonder what is actually “out there,” independent of our minds. Kant answers that we cannot know for certain. Our senses react to stimuli that come from outside the mind, but we only have knowledge of how they appear to us once they have been processed by our faculties of sensibility and understanding. Kant calls the stimuli “things-in-themselves” and says we can have no certain knowledge about their nature. He distinguishes sharply between the world of noumena, which is the world of things-in-themselves, and the world of phenomena, which is the world as it appears to our minds.

After giving what he considers a satisfactory account of how synthetic a priori knowledge makes mathematics and science possible, Kant turns to metaphysics. Metaphysics relies on the faculty of reason, which does not shape our experience in the way that our faculties of sensibility and understanding do, but rather it helps us reason independent of experience. The mistake metaphysicians typically make is to apply reason to things in themselves and try to understand matters beyond reason’s grasp. Such attempts tend to lead reason into contradiction and confusion. Kant redefines the role of metaphysics as a critique of pure reason. That is, the role of reason is to understand itself, to explore the powers and the limits of reason. We are incapable of knowing anything certain about things-in-themselves, but we can develop a clearer sense of what and how we can know by examining intensively the various faculties and activities of the mind.


In the Critique of Pure Reason, Kant achieves a synthesis between the competing traditions of rationalism and empiricism. From rationalism, he draws the idea that pure reason is capable of significant knowledge but rejects the idea that pure reason can tell us anything about things-in-themselves. From empiricism, he draws the idea that knowledge is essentially knowledge from experience but rejects the idea that we can infer no necessary and universal truths from experience, which is Hume’s conclusion. As a result, he avoids the metaphysical speculations of the rationalists, for which any definite proof seems unattainable but maintains the rationalists’ ambitious agenda, which attempts to give some answer to the sorts of questions that inevitably occur when we think philosophically. By locating the answers to metaphysical questions not in the external world but in a critique of human reason, Kant provides clear boundaries for metaphysical speculation and maintains a sensible, empirical approach to our knowledge of the external world.

Kant achieves what he calls a Copernican revolution in philosophy by turning the focus of philosophy from metaphysical speculation about the nature of reality to a critical examination of the nature of the thinking and perceiving mind. In effect, Kant tells us that reality is a joint creation of external reality and the human mind and that it is only regarding the latter that we can acquire any certain knowledge. Kant challenges the assumption that the mind is a blank slate or a neutral receptor of stimuli from the surrounding world. The mind does not simply receive information, according to Kant; it also gives that information shape. Knowledge, then, is not something that exists in the outside world and is then poured into an open mind like milk into a cup. Rather, knowledge is something created by the mind by filtering sensations through our various mental faculties. Because these faculties determine the shape that all knowledge takes, we can only grasp what knowledge, and hence truth, is in its most general form if we grasp how these faculties inform our experience.

The lynchpin to Kant’s critical philosophy is his category of the synthetic a priori. Although distinctions similar to Kant’s a priori/a posteriori distinction and his synthetic/analytic distinction have been made by thinkers such as Hume and Leibniz, Kant is the first to apply two such distinctions to generate a third category for knowledge. Hume, for instance, does not distinguish between what Kant calls the analytic and the a priori and what he calls the synthetic and the a posteriori, so that, for Hume, all synthetic judgments are necessarily a posteriori. Since only a priori truths have the important qualities of being universal and necessary, all general truths about reality—as opposed to particular observations about unconnected events—must be a priori. If our a priori knowledge is limited to definitional analytic judgments, then Hume is right in concluding that rationally justified knowledge of universal and necessary truths is impossible. Kant’s coup comes in determining that synthetic judgments can also be a priori. He shows that mathematics and scientific principles are neither analytic nor a posteriori, and he provides an explanation for the category of the synthetic a priori by arguing that our mental faculties shape our experience.

Kant differs from his rationalist predecessors by claiming that pure reason can discern the form, but not the content, of reality. Rationalists, such as Descartes, Spinoza, and Leibniz, speculated about the nature of time, space, causation, God, and the universe, and they believed at least on some level that they could come up with relatively confident answers through the exercise of pure reason. Kant, who was educated in this tradition, argues that his predecessors have not given any clear grounding for their metaphysical speculation, but that is because they assume that time, space, causation, and the like are the content of an external reality that the mind must reach out and grasp. Kant turns this assumption on its head, suggesting that time, space, and causation are not found in experience but are instead the form the mind gives to experience. We can grasp the nature of time, space, and causation not because pure reason has some insight into the nature of reality but because pure reason has some insight into the nature of our own mental faculties.

Kant has earned the great compliment of having detractors who criticize him with great insight and ingenuity. German idealism, which dominated nineteenth-century philosophy, finds its footing by attacking Kant’s conception of things-in-themselves. Idealists such as Hegel argue that there is something deeply suspicious about these mysterious entities, which Kant claims are the source of our sensations while claiming we can have no direct knowledge of them. Idealism jettisons things-in-themselves and the whole noumenal realm, arguing instead that reality consists primarily of mental phenomena. Analytic philosophy, which is one of the leading schools of twentieth-century philosophy, also gets its start through an attack on Kant.

The logician Gottlob Frege criticizes Kant for basing the analytic–synthetic distinction on the subject-predicate form of grammar, which is not a necessary feature of the logical structure of language or reality. Frege argues that we should base the analytic/synthetic distinction on whether we justify a given judgment by appealing to its logical form or to empirical investigation and that, according to this distinction, the category of the synthetic a priori becomes unnecessary. Kant is only able to argue that geometry, for instance, relies on synthetic a priori knowledge because he fails to distinguish between pure geometry—the stuff of mathematical axioms and proofs—and empirical geometry—the application of geometrical principles to science. Pure geometry is a priori, but it is also analytic, since it is justified according to logical principles alone. Empirical geometry is synthetic, but it is also a posteriori, since we only learn from experience what sort of geometry applies to the real world.

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