Critique of Pure Reason
The Critique of Pure Reason is a book by the German philosopher Immanuel Kant, in which the author seeks to determine the limits and scope of metaphysics. Also referred to as Kant's "First Critique", it was followed by his Critique of Practical Reason and Critique of Judgment. In the preface to the first edition, Kant explains that by a "critique of pure reason" he means a critique "of the faculty of reason in general, in respect of all knowledge after which it may strive independently of all experience" and that he aims to decide on "the possibility or impossibility of metaphysics".
Kant builds on the work of empiricist philosophers such as John Locke and David Hume, as well as rationalist philosophers such as René Descartes, Gottfried Wilhelm Leibniz and Christian Wolff. He expounds new ideas on the nature of space and time, and tries to provide solutions to the skepticism of Hume regarding knowledge of the relation of cause and effect and that of René Descartes regarding knowledge of the external world. This is argued through the transcendental idealism of objects and their form of appearance. Kant regards the former "as mere representations and not as things in themselves", and the latter as "only sensible forms of our intuition, but not determinations given for themselves or conditions of objects as things in themselves". This grants the possibility of a priori knowledge, since objects as appearance "must conform to our cognition...which is to establish something about objects before they are given to us." Knowledge independent of experience Kant calls "a priori" knowledge, while knowledge obtained through experience is termed "a posteriori". According to Kant, a proposition is a priori if it is necessary and universal. A proposition is necessary if it is not false in any case and so cannot be rejected; rejection is contradiction. A proposition is universal if it is true in all cases, and so does not admit of any exceptions. Knowledge gained a posteriori through the senses, Kant argues, never imparts absolute necessity and universality, because it is possible that we might encounter an exception.
Kant further elaborates on the distinction between "analytic" and "synthetic" judgments. A proposition is analytic if the content of the predicate-concept of the proposition is already contained within the subject-concept of that proposition. For example, Kant considers the proposition "All bodies are extended" analytic, since the predicate-concept is already contained within—or "thought in"—the subject-concept of the sentence. The distinctive character of analytic judgments was therefore that they can be known to be true simply by an analysis of the concepts contained in them; they are true by definition. In synthetic propositions, on the other hand, the predicate-concept is not already contained within the subject-concept. For example, Kant considers the proposition "All bodies are heavy" synthetic, since the concept 'body' does not already contain within it the concept 'weight'. Synthetic judgments therefore add something to a concept, whereas analytic judgments only explain what is already contained in the concept.
Before Kant, philosophers held that all a priori knowledge must be analytic. Kant, however, argues that our knowledge of mathematics, of the first principles of natural science, and of metaphysics, is both a priori and synthetic. The peculiar nature of this knowledge cries out for explanation. The central problem of the Critique is therefore to answer the question: "How are synthetic a priori judgments possible?" It is a "matter of life and death" to metaphysics and to human reason, Kant argues, that the grounds of this kind of knowledge be explained.
Though it received little attention when it was first published, the Critique later attracted attacks from both empiricist and rationalist critics, and became a source of controversy. It has exerted an enduring influence on Western philosophy, and helped bring about the development of German idealism. The book is considered a culmination of several centuries of early modern philosophy and an inauguration of late modern philosophy.
Background
Early rationalism
Before Kant, it was generally held that truths of reason must be analytic, meaning that what is stated in the predicate must already be present in the subject. In either case, the judgment is analytic because it is ascertained by analyzing the subject. It was thought that all truths of reason, or necessary truths, are of this kind: that in all of them there is a predicate that is only part of the subject of which it is asserted. If this were so, attempting to deny anything that could be known a priori would involve a contradiction. It was therefore thought that the law of contradiction is sufficient to establish all a priori knowledge.David Hume at first accepted the general view of rationalism about a priori knowledge. However, upon closer examination of the subject, Hume discovered that some judgments thought to be analytic, especially those related to cause and effect, were actually synthetic. They thus depend exclusively upon experience and are therefore a posteriori.
Kant's rejection of Hume's empiricism
Before Hume, rationalists had held that effect could be deduced from cause; Hume argued that it could not and from this inferred that nothing at all could be known a priori in relation to cause and effect. Kant, who was brought up under the auspices of rationalism, was deeply impressed by Hume's skepticism. "I freely admit that it was the remembrance of David Hume which, many years ago, first interrupted my dogmatic slumber and gave my investigations in the field of speculative philosophy a completely different direction."Kant decided to find an answer and spent at least twelve years thinking about the subject. Although the Critique of Pure Reason was set down in written form in just four to five months, while Kant was also lecturing and teaching, the work is a summation of the development of Kant's philosophy throughout those twelve years.
Kant's work was stimulated by his decision to take seriously Hume's skeptical conclusions about such basic principles as cause and effect, which had implications for Kant's grounding in rationalism. In Kant's view, Hume's skepticism rested on the premise that all ideas are presentations of sensory experience. The problem that Hume identified was that basic principles such as causality cannot be derived from sense experience only: experience shows only that one event regularly succeeds another, not that it is caused by it.
In section VI of the introduction to the Critique of Pure Reason, Kant explains that Hume stopped short of considering that a synthetic judgment could be made 'a priori'. Kant's goal was to find some way to derive cause and effect without relying on empirical knowledge. Kant rejects analytical methods for this, arguing that analytic reasoning cannot tell us anything that is not already self-evident, so his goal was to find a way to demonstrate how the synthetic a priori is possible.
To accomplish this goal, Kant argued that it would be necessary to use synthetic reasoning. However, this posed a new problem: how is it possible to have synthetic knowledge that is not based on empirical observation; that is, how are synthetic a priori truths possible? This question is exceedingly important, Kant maintains, because he contends that all important metaphysical knowledge is of synthetic a priori propositions. If it is impossible to determine which synthetic a priori propositions are true, he argues, then metaphysics as a discipline is impossible.
Synthetic ''a priori'' judgments
Kant argues that there are synthetic judgments such as the connection of cause and effect where no analysis of the subject will produce the predicate. Kant reasons that statements such as those found in geometry and Newtonian physics are synthetic judgments. Kant uses the classical example of 7 + 5 = 12. No amount of analysis will find 12 in either 7 or 5 and vice versa, since an infinite number of two numbers exist that will give the sum 12. Thus Kant concludes that all pure mathematics is synthetic though a priori; the number 7 is seven and the number 5 is five and the number 12 is twelve and the same principle applies to other numerals; in other words, they are universal and necessary. For Kant then, mathematics is synthetic judgment a priori. Conventional reasoning would have regarded such an equation to be analytic a priori by considering both 7 and 5 to be part of one subject being analyzed, however Kant looked upon 7 and 5 as two separate values, with the value of five being applied to that of 7 and synthetically arriving at the logical conclusion that they equal 12. This conclusion led Kant into a new problem as he wanted to establish how this could be possible: How is pure mathematics possible? This also led him to inquire whether it could be possible to ground synthetic a priori knowledge for a study of metaphysics, because most of the principles of metaphysics from Plato through to Kant's immediate predecessors made assertions about the world or about God or about the soul that were not self-evident but which could not be derived from empirical observation. For Kant, all post-Cartesian metaphysics is mistaken from its very beginning: the empiricists are mistaken because they assert that it is not possible to go beyond experience and the dogmatists are mistaken because they assert that it is possible to go beyond experience through theoretical reason.Therefore, Kant proposes a new basis for a science of metaphysics, posing the question: how is a science of metaphysics possible, if at all? According to Kant, only practical reason, the faculty of moral consciousness, the moral law of which everyone is immediately aware, makes it possible to know things as they are. This led to his most influential contribution to metaphysics: the abandonment of the quest to try to know the world as it is "in itself" independent of sense experience. He demonstrated this with a thought experiment, showing that it is not possible to meaningfully conceive of an object that exists outside of time and has no spatial components and is not structured following the categories of the understanding, such as substance and causality. Although such an object cannot be conceived, Kant argues, there is no way of showing that such an object does not exist. Therefore, Kant says, the science of metaphysics must not attempt to reach beyond the limits of possible experience but must discuss only those limits, thus furthering the understanding of ourselves as thinking beings. The human mind is incapable of going beyond experience so as to obtain a knowledge of ultimate reality, because no direct advance can be made from pure ideas to objective existence.
Kant writes: "Since, then, the receptivity of the subject, its capacity to be affected by objects, must necessarily precede all intuitions of these objects, it can readily be understood how the form of all appearances can be given prior to all actual perceptions, and so exist in the mind a priori". Appearance is then, via the faculty of transcendental imagination, grounded systematically in accordance with the categories of the understanding. Kant's metaphysical system, which focuses on the operations of cognitive faculties, places substantial limits on knowledge not found in the forms of sensibility. Thus it sees the error of metaphysical systems prior to the Critique as failing to first take into consideration the limitations of the human capacity for knowledge. Transcendental imagination is described in the first edition of the Critique of Pure Reason but Kant omits it from the second edition of 1787.
It is because he takes into account the role of people's cognitive faculties in structuring the known and knowable world that in the second preface to the Critique of Pure Reason Kant compares his critical philosophy to Copernicus' revolution in astronomy. Kant writes:
Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge.Kant's view is that in explaining the movement of celestial bodies, Copernicus rejected the idea that the movement is only in the stars in order to allow that such movement is also due to the motion of ourselves as spectators. Thus, the Copernican revolution in astronomy shifted our understanding of the universe from one that is geocentric, without reference to the motion of ourselves as spectators, to one that is heliocentric with reference to the motion of ourselves as spectators. Likewise, Kant aims to shift metaphysics from one that requires our understanding to conform to the nature of objects to one that requires the objects of experience to conform to the necessary conditions of our knowledge. Consequently, knowledge does not depend solely on the object of knowledge but also on the capacity of the knower.