Immanuel Kant’s Critique of Pure Reason is an established classic in the history of epistemology. First published in 1781 and then revised in 1787, it is the fruit of Kant’s later years and, as such, clearly reflects the insight and wisdom of a mature mind. It is a work in which the author attempted to conciliate two conflicting theories of knowledge current at his time: British empiricism as represented by John Locke, George Berkeley, and David Hume, and continental rationalism as represented by René Descartes, Gottfried Wilhelm Leibniz, and Christian von Wolff. The latter theory maintained that important truths about the natural and the supernatural worlds are knowable by pure reason alone, independently of perceptual experience, whereas the former held that perceptual experience is the source of all our legitimate concepts and truths of the world. Kant believed that both these doctrines were wrong, and he tried in Critique of Pure Reason to correct the pretensions of each while saving what was sound in each.

Metaphysics as Science

Kant began his inquiry by asking why metaphysics had not kept pace with mathematics and natural science in the discovery of facts about our world. Celestial mechanics had been developed by German astronomer Johannes Kepler at the beginning of the seventeenth century and terrestrial mechanics by Italian mathematician and physicist Galileo later in the same century, and the two theories were soon united into one by English physicist and mathematician Sir Isaac Newton. These developments represented astonishing progress in natural science, but Kant could detect no parallel progress in metaphysics. Indeed, in metaphysics he saw only interminable squabbling with no apparent method for settling differences. He therefore asked whether it is at all possible for metaphysics to be a science.

Metaphysics can be a science, Kant reasoned, only if there exists a class of truths different in kind either from the straightforward synthetic truths of nature discoverable through sense experience or from the straightforward analytic truths that owe their validity to the fact that the predicate term is contained in the subject term of such judgments—in other words, to the fact that they are true by virtue of the meanings of their terms, true by definition. This distinction is illustrated by the statements “Peaceful resistance is effective” (synthetic) and “Peaceful resisters shun violence” (analytic). This distinction had been recognized by Hume, who regarded it as exhausting the kinds of statements that can be true or false. However, Kant believed that there are statements neither empirical nor analytic in character—synthetic a priori statements. These are statements that are true neither by definition nor because of facts discoverable through sense experience. Rather, they can be seen to be true independently of sense experience; in this sense, they are a priori and necessarily true, because no sense experience can possibly confute them. Kant believed that all mathematical statements are of this sort: for example, “Seven plus five equals twelve.” He also believed that synthetic a priori truths constitute the framework of Newtonian science. However, if such truths exist, Kant next asked himself, how are they possible?

A Priori Bases for Human Knowledge

They are possible, he said, if it can be shown that human knowledge is dependent upon certain concepts which are not empirical in origin but have their origin in human understanding. However, even before he revealed the existence of such concepts, Kant attempted to show in the first major division of Critique of Pure Reason, entitled the “Transcendental Esthetic,” that a priori considerations form the basis even of human perception or sensibility. This view was important to Kant, for in his proposed Copernican revolution in epistemology, the two sources of knowledge are sensibility and understanding working in inseparable harness together. He had already written in the introduction to Critique of Pure Reason that all knowledge begins with experience, but it does not necessarily arise out of experience.

What are these a priori foundations of sensibility? According to Kant, they are space and time. He reasoned that all objects of perception are necessarily located in space and time. Such objects may vary over a period of time in color, shape, size, and so on and still be perceptible objects, but they cannot be deprived of space and time and still remain perceptible. Even to establish ourselves as perceivers, and objects in our environment as objects of perception, requires the use of spatial and temporal terms—hence, the concepts of space and time. As percipients, we regard perceived objects as separate from or distant from us, and we realize that our perceptions themselves, whether of external objects or of our own thoughts and feelings, succeed one another in time. We cannot represent them otherwise and still sensibly preserve the meaning of the terms “perceiver” and “object of perception.” In this sense, space and time deserve recognition as presuppositions of sense experience. All our empirical, descriptive characterizations of perceptible objects take for granted their fundamental nature as objects in space and time. That is why Kant calls space and time “forms of intuition,” in order to distinguish them from the “contents” of sense experience. To be sure, portions of space and moments of time can be perceived, but such parts must always be understood as forming parts of an underlying continuum of space and time. (British phenomenalists such as Berkeley and Hume were not in agreement with this interpretation of space and time.)

Categories of Human Understanding

Believing that he had already exhibited the dependency of human knowledge upon conditions prior to immediate sense experience, Kant next proceeded to a consideration of the a priori conditions of human understanding. In Kant’s view, all knowledge is the product of human understanding applied to sense experience. Does the understanding organize the contents of sense experience according to its own rules—rules that must originate elsewhere than in sense experience if their function is to categorize it? Such rules exist indeed, declared Kant, and he called them the “categories of the understanding.” He argued that there are twelve such categories and that they can be discovered and classified by careful scrutiny of the logical forms of the judgments we characteristically make about the world. For example, if we look at our categorical judgments, we see that they contain a referring expression that we call the grammatical subject and a characterizing expression that we call the predicate. In “Beethoven was a great composer,” the referring or subject term is “Beethoven,” and our characterizing or predicate term is “great composer.” Now a tremendous number of the factual claims we normally make are of this same basic form—substance and predicated property—and for Kant, therefore, the concept of substance deserves the status of a category of knowledge. Under it are subsumed all the substance-words in our conceptual scheme of things—“table,” “tree,” “moon,” “nail,” and so on—which denote material objects in our environment. It is thus a familylike concept denoting all those objects that have substantiality in common, something that none of the individual terms in this category does.

Much the same point can be made about the concept of...

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Principles of Newtonian Science

It is chiefly as ordering principles that Kant viewed the categories. What they order or synthesize in his partly phenomenalistic theory of knowledge are the items of experience: colors, shapes, sizes, sounds, tactile impressions, odors, and so on. However, Kant believed that there is a problem in showing how such a priori principles can be applied to empirical data, and he thought that the answer to this problem is to be found in the mediatory power of time, which, as seen above, is an a priori ordering form which is a necessary condition of sense experience. Kant proceeded to relate the categories to the concept of time, and it was this merger of the concepts of substance, causality, and time that paved the way to his discussion of the presuppositions of Newtonian science. Kant believed that there are three such presuppositions: namely, the principles of the conservation of matter, of universal causality, and of the universal interrelation of all things making up the natural world. (In the Newtonian view of the universe, all objects were considered to be made up of material particles governed in their behavior by the universal laws of motion and attraction.)

Such principles are not analytic truths, according to Kant, because their denials are not self-contradictory, nor are they empirical generalizations, because we know them to be necessarily true, and no empirical generalization is ever necessarily true. They must therefore be genuine synthetic a...

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The Transcendental Dialectic

Up to this point Kant’s concern was to explore the foundations of scientific knowledge and to disclose the dependency of such knowledge upon a handful of forms, concepts, and principles. In this exploration, he clashed sometimes head-on, sometimes obliquely, as we have seen, with accounts of human knowledge provided by British empiricists. However, his conclusions thus far were also brewing trouble and embarrassment for continental rationalism as well—for what follows from showing that concepts such as causality and substance are presuppositions of empirical knowledge? It follows, Kant said, that their use independent of sense experience is illegitimate and can result only in conceptual difficulties and empty noise. Kant’s...

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The Limits of Reason

Kant next turned to metaphysical speculation about the universe at large. People have always asked themselves with respect to the universe whether it had a beginning in time or has always existed, whether it is finitely or infinitely extended in space, and whether it was created. Kant showed that no definitive answers to such questions are possible. Indeed, he argued that reasoning can establish with equal cogency alternative answers to such questions. His explanation for such a disconcerting and paradoxical state of affairs in metaphysics was that one cannot regard the universe as a substance or given entity in the way a desk, for example, can be so regarded. It is of course meaningful to ask when a certain desk was made, how it...

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Implications for Later Philosophy

Many philosophers since Kant have appreciated his middle road between rationalism and empiricism, even if they have not been able to accept the details of this reasoning, and they have credited Kant with the rare ability to raise problems worthy of philosophical investigation.

Other philosophers have not been impressed by Kant’s strictures against rationalism and empiricism, and they have borrowed from his meticulous genius (happily wedded to broad vision) what suits their purposes while ignoring what does not. Thus German philosopher Georg Wilhelm Friedrich Hegel, for example, was stimulated by Kant to seize upon pure reason’s dialectical tendencies—so futile in Kant’s view—and erect upon such tendencies a...

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Additional Reading

Allison, Henry E. Idealism and Freedom: Essays on Kant’s Theoretical and Practical Philosophy. New York: Cambridge University Press, 1996. An important interpreter of Immanuel Kant explores relationships between Kant’s theory of knowledge and his moral philosophy.

Bohman, James, and Matthias Lutz-Backmann, eds. Perpetual Peace: Essays on Kant’s Cosmopolitan Ideal. Cambridge, Mass.: MIT Press, 1997. The contributors appraise Kant’s theories about and hopes for a universal rationality that would encourage shared moral understanding and reduce political conflict.

Cassirer, Ernst. Kant’s Life and Thought. Translated by James Hayden. New Haven, Conn.: Yale University Press,...

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(Student Guide to World Philosophy)

Additional Reading

Allison, Henry E. Idealism and Freedom: Essays on Kant’s Theoretical and Practical Philosophy. New York: Cambridge University Press, 1996. An important interpreter of Immanuel Kant explores relationships between Kant’s theory of knowledge and his moral philosophy.

Bohman, James, and Matthias Lutz-Backmann, eds. Perpetual Peace: Essays on Kant’s Cosmopolitan Ideal. Cambridge, Mass.: MIT Press, 1997. The contributors appraise Kant’s theories about and hopes for a universal rationality that would encourage shared moral understanding and reduce political conflict.


(The entire section is 468 words.)