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Last Updated on August 5, 2019, by eNotes Editorial. Word Count: 600

In Immanuel Kant's Critique of Pure Reason is a reconstruction of the then predominant school of skeptical empiricism, which typified almost all of seventeenth-century European philosophy from the works of Renee Descartes to George Berkley to David Hume. Most thinkers of Kant’s day believed that ideas in the mind took the “shape” of the objects in the external world, which were revealed to them via the sense perceptions. The mind, these philosophers argued, was something like a repository for ideas, and that upon introduction to external objects, the mind modified itself in order to best understand them. Critique of Pure Reason inverted this perspective. Instead of causing a fundamental modification of the mind, Kant argued that the external objects in the world rather conform to the mind; the ways in which human beings are able to perceive their environment, he maintained, is a direct consequence of the ability of the mind to act on this environment and convert it into something meaningful to the observer.

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Kant further argued that, in order for the mind to accomplish this manipulation of the external world, it must have possessed certain categories of understanding that existed outside of sense experience. These categories of thought, he said, existed a priori, or prior to observation, of the object in question. Kant argued that “space” and “time” were the two primary a priori categories that dictated the way the mind operates. These criteria must be presupposed to exist in order for an individual to be capable of having sensibility of the external world at all. Kant maintained that this was because, though space and time are not “objects” in the literal sense, they provide the internal logic by which the human mind operates in order to imbue objects in the external world with any degree of coherency, organization, and meaning. It is impossible to conceive of, say, a book without first recognizing that as a body it both extends outwards (space) and acts and reacts to other extended objects in space temporally (time). Thus, while space and time are not necessarily contained within the formal definition that one would associate with book, they are nevertheless required in order to make this definition meaningful.

In this way, Kant drew a distinction between what he called the “phenomenal world,” or the world that our mind creates for us, and the “noumenal world,” or the world as it really is, apart from our experience of it. Kant’s enduring contribution to metaphysical philosophy comes from his denial of pure knowledge. Human beings are incapable of understanding the essence of a thing outside of the way their minds modify it within the boundaries set by a priori categories. Thus, the essences of things, what Kant called “things-in-themselves” (dinge an sich) in German, cannot be arrived at through human reason alone. Instead, the “ideas” of reason, which Kant classified as God, the world, and the self, help unify (but not absolutely define) knowledge, and can point to possibilities in the noumenal world.

Critique of Pure Reason is a notoriously long and convoluted book. Kant wrote it for a highly specialized academic audience (a sixteenth-century academic audience, no less) in prose that is almost impossible to follow. His colleagues at the University of Konigsberg suggested to him that he write a condensed version of the book so that it could be made available to more of a mainstream audience. If you are really pressed for time, this 70-ish page book is a good summation of Kan’t overarching argument. It is called Prolegomena to any Future Metaphysics, and is widely available online.

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Last Updated on May 5, 2015, by eNotes Editorial. Word Count: 1515

Immanuel Kant’s Critique of Pure Reason is a masterpiece in metaphysics designed to explore the possibility of synthetic a priori judgments. A synthetic judgment is one whose predicate is not contained in the subject; an a priori judgment is one whose truth can be known independently of experience. Kant therefore in effect questioned how it is that statements in which the idea of the subject does not involve the idea of the predicate can nevertheless be true and can also be known to be true without recourse to experience.

To make the question clearer, Kant offered examples of analytic and synthetic judgments. The statement that “All bodies are extended” is offered as an analytic judgment because it would be impossible to think of a body, that is, a physical object, that was not spread out in space; the statement “All bodies are heavy” is offered as a synthetic judgment, because Kant believed that it is possible to conceive of a body without supposing that it has weight.

The judgment that “All red apples are apples” is analytic because it would be impossible to conceive that something that was red and an apple could possibly not be an apple; the predicate is, in this case, included in the subject. The judgment “All apples are red,” however, is synthetic, because it is possible to think of an apple without supposing it to be red; in fact, some apples are green. Synthetic judgments can be false, but analytic judgments are never false.

A priori knowledge is knowledge “absolutely independent of all experience,” whereas a posteriori knowledge is empirical knowledge, that is, knowledge possible only through experience. Human beings can know a priori that all red apples are apples (and that they are red), but to know that a particular apple has a worm in it is something that can be known only a posteriori.

The question whether synthetic a priori judgments are possible concerns judgments that must be true—because they are a priori and can be known to be true without reference to experience—even though, being synthetic, their predicates are not conceived in thinking of their subjects. As an example of a synthetic a priori judgment Kant offers the statement “Everything that happens has its cause.” He argues that he can think of something happening without considering whether it has a cause; the judgment is, therefore, not analytic. Yet he supposes that it is necessarily the case that everything that happens has a cause, even though his experience is not sufficient to support that claim. The judgment must be a priori. How are such synthetic a priori judgments possible?

One difficulty arises at this point. Critics of Kant have argued that Kant’s examples are not satisfactory. The judgment that everything that happens has a cause is regarded either as being an analytic rather than a synthetic a priori judgment (every event being a cause relative to an immediately subsequent event, and an effect relative to an immediately preceding event) or as being a synthetic a posteriori rather than an a priori judgment (which leaves open the possibility that some events may be uncaused). A great many critics have maintained that Kant’s examples are bound to be unsatisfactory for the obvious reason that no synthetic a priori judgments are possible. The argument is that unless the predicate is involved in the subject, the truth of the judgment is a matter of fact, to be determined only by reference to experience.

Kant’s answer to the problem concerning the possibility of synthetic a priori judgments that pure reason—that is, the faculty of arriving at a priori knowledge—is possible because the human way of knowing determines, to a considerable extent, the character of what is known. Whenever human beings perceive physical objects, they perceive them in time and space; time and space are what Kant calls “modes of intuition,” that is, ways of apprehending the objects of sensation. Because human beings must perceive objects in time and space, the judgment that an object is in time must be a priori but, provided the element of time is no part of the conception of the object, the judgment is also synthetic. It is somewhat as if a world were being considered in which all human beings were compelled to wear green glasses. The judgment that everything seen is somewhat green would be a priori (since nothing could be seen except by means of the green glasses), but it would also be synthetic (since being green is no part of the conception of object).

In Kant’s terminology, a transcendental philosophy is one concerned not so much with objects as with the mode of a priori knowledge, and a critique of pure reason is the science of the sources and the limits of what contains the principles by which human beings know a priori. Space and time are the forms of pure intuition, that is, modes of sensing objects. The science of all principles of a priori sensibility, that is, of those principles that make a priori intuitions (sensations) possible, Kant calls the transcendental aesthetic.

Human beings do more than merely sense or perceive objects; they also think about them. The study of the existence of a priori concepts, as distinguished from intuitions, is called transcendental logic. This study is divided into transcendental analytic, dealing with the principles of the understanding without which no object can be thought, and transcendental dialectic, showing the error of applying the principles of pure thought to objects considered in themselves.

Using Aristotle’s term, Kant calls the pure concepts of the understanding categories. The categories are of quantity (unity, plurality, totality), quality (reality, negation, limitation), relation (substance and accident, cause and effect, reciprocity between agent and patient), and modality (possibility-impossibility, existence-nonexistence, necessity-contingency). According to Kant, everything that is thought is considered according to these categories. It is not a truth about things in themselves that they are one or many, positive or negative, but that all things fall into these categories because the understanding is so constituted that it can think in no other way.

Kant maintained that there are three subjective sources of the knowledge of objects: sense, imagination, and apperception. By its categories, the mind imposes a unity on the manifold of intuition; what would be a mere sequence of appearances, were the mind not involved, makes sense as the appearance of objects.

The principles of pure understanding fall into four classes: axioms of intuition, anticipations of perception, analogies of experience, and postulates of empirical thought in general. The principle of the axioms of intuition is that “All intuitions are extensive magnitudes,” proved by reference to the claim that all intuitions are conditioned by the spatial and temporal mode of intuition.

The principle by which all perception is anticipated is that “the real that is an object of sensation has intensive magnitude, that is, a degree.” It would not be possible for an object to influence the senses to no degree; hence, various objects have different degrees of influence on the senses. The principle of the analogies of experience is that “Experience is possible only through the representation of a necessary connection of perceptions.” Human experience would be meaningless were it not ordered by the supposition that perceptions are of causally related substances that are mutually interacting.

Kant’s postulates of empirical thought in general relate the possibility of things to their satisfying the formal conditions of intuition and of concepts, the actuality of things to their satisfying the material conditions of sensation, and the necessity of things to their being determined “in accordance with universal conditions of experience” in their connection with the actual.

A distinction that is central in Kant’s philosophy is the distinction between the phenomenal and the noumenal. The phenomenal world is the world of appearances, the manifold of sensation as formed spatially and temporally and understood by use of the categories. The noumenal world is the world beyond appearance, the unknown and unknowable, the world of “things-in-themselves.”

In the attempt to unify experience, reason constructs certain ideas—of a soul, of the world, of God. These ideas are, however, transcendental in that they are illegitimately derived from a consideration of the conditions of reason. To rely on them leads to difficulties that Kant’s “Transcendental Dialectic” was designed to expose. The “Paralogisms of Pure Reason” are fallacious syllogisms for which the reason has transcendental grounds; that is, the reason makes sense out of its operations by supposing what, on logical grounds, cannot be admitted. The “Antinomies of Pure Reason” are pairs of contradictory propositions, all capable of proof provided the arguments involve illegitimate applications of the forms and concepts of experience to matters beyond experience.

Kant concludes the Critique of Pure Reason with the suggestion that the ideas of God, freedom, and immortality arise in the attempt to make moral obligation intelligible. This point was developed at greater length in his Grundlegung zur Metaphysik der Sitten (1785; Foundations of the Metaphysics of Morals, 1950) and his Kritik der praktischen Vernunft(1788; Critique of Practical Reason, 1873).

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