If the world finds this book difficult, it may be because of Kant’s method of work. He wrote to Moses Mendelssohn (August 16, 1783) that though the volume was “the result of reflection which occupied me for at least twelve years, I brought it to completion in the greatest haste within four or five months, giving the closest attention to the contents, but with little thought of the exposition, or of rendering it easy of comprehension by the reader—a decision which I have never regretted, since otherwise, had I longer delayed and sought to give it a more popular form, the work would probably never have been completed at all.”14 Clarity takes time, and Kant was not sure that he had the time. He deliberately omitted illustrative examples lest they swell his book; “these are necessary only from a popular point of view, and this work can never be made suitable for popular consumption.”15 So he wrote for the trade, and trusted to others to dilute him into digestibility. Though Christian von Wolff had preceded him in writing philosophy in German, that language was still crude in phrasing shades of thought, and it had not established a technical terminology. At almost every step Kant had to invent a German translation of a Latin term, and in many cases even Latin lacked terms for the distinctions and subtleties he wished to express. He confused his readers by giving new meanings to old words, and sometimes forgetting his redefinitions. The first hundred pages are tolerably clear; the rest is a philosophical conflagration in which the untutored reader will see nothing but smoke.

The title itself needed clarification. Who could have known that Kritik der reinen Vernunft meant a critical and judicial examination of reason as independent of experience? Kritik meant not only analysis and exposition, it also meant judgment, as in its Greek parent krinein, to judge. Kant proposed to describe sensation, perception, idea, and reason, and to set, for each of these, its proper bounds and jurisdiction. Further, he hoped to show that reason can give us knowledge independently of any confirmatory experience, as when we know that six times six equals thirty-six, or that an effect must have a cause. These are examples of “pure reason”—i.e., of a priori knowledge—i.e., of knowledge requiring no experiential proof. “The faculty of knowledge from a priori principles may be called pure reason, and the general investigation of its possibility and bounds [constitutes] the critique of pure reason.”16 Kant believed that such an investigation would involve all the problems of metaphysics, and he was confident that “there is not a single metaphysical problem which has not been solved, or for the solution of which the key at least has not been supplied” in this Critique.17 He thought that his only danger was “not that of being refuted, but of not being understood.”18

What had drawn him into so heroic an adventure? One might have supposed that the exaltation of reason by the French Enlightenment—the assumption of the philosophes that faith must submit to reason—and the havoc so inflicted upon Christian theology, had been the provocative cause of Kant’s determination to study the origin, operation, and limits of reason. That motive played its part, as stated in Kant’s preface to the second edition;19 but that same preface made it clear that his chosen enemy was all “dogmatism” whatever—i.e., all systems of thought, orthodox or heretical, evolved by an unscrutinized reason. He named as “the greatest of all dogmatical philosophers” Christian von Wolff, who had undertaken to prove the doctrines of Christianity, and the philosophy of Leibniz, by reason alone. All attempts to demonstrate the truth or falsity of religion by pure reason were, to Kant, forms of dogmatism; and he condemned as “the dogmatism of metaphysics” any system of science or philosophy or theology that had not first submitted to a critical examination of reason itself.

He accused his own thinking, till 1770, as guilty of such dogmatism. From such unscrutinized speculations, he tells us, he was awakened by reading Hume—probably the Enquiry concerning the Human Understanding, of which a German translation had appeared in 1755. Hume had argued that all reasoning depended upon the notion of cause; that in actual experience we perceive not causation but only sequence; and that therefore all science, philosophy, and theology rest on an idea—cause—which turns out to be an intellectual supposition, not a perceived reality. “I freely admit,” wrote Kant, “it was David Hume’s remark that first, many years ago, interrupted my dogmatic slumber and gave a completely different direction to my inquiries in the field of speculative philosophy.”20 How could the concept of cause be rescued from the lowly status of uncertain supposition in which Hume had left it? Only, said Kant, by showing that it is a priori, independent of experience, one of those categories, or forms of thought, which, though not necessarily innate, are part of the inherent structure of the mind.* So he set himself to overcome both the dogmatism of Wolff and the skepticism of Hume by a criticism—a critical examination—that would at once describe, delimit, and restore the authority of reason. These three stages—dogmatism, skepticism, criticism—were, in Kant’s view, the three ascending phases in the evolution of modern philosophy.

Loving definitions, distinctions, and classifications, using long words to shorten speech, Kant divided all knowledge into empirical (dependent upon experience) and transcendental (independent of, and therefore transcending, experience). He agreed that all knowledge begins with experience, in the sense that some sensation must precede and arouse the operations of thought; but he believed that the moment experience begins it is molded by the structure of the mind through its inherent forms of “intuition” (perception) or conception. The inherent forms of “intuition” are the universal forms that experience takes in our outward sensation as space, and in our inward sensibility as time.

Likewise there are inherent forms of conception or thought, which are independent of experience and mold it; Kant called them categories, and divided them with fond and suspicious symmetry into four triplets: three categories of quantity—unity, plurality, and totality; three categories of quality—reality, negation, and limitation; three twin categories of relation-substance and quality, cause and effect, activity and passivity; and three twin categories of modality—possibility and impossibility, existence and nonexistence, necessity and contingency. Every perception falls into one or more of these basic forms or molds of thought. Perception is sensation interpreted by the inherent forms of space and time; knowledge is perception transformed by the categories into a judgment or an idea. Experience is not a passive acceptance of objective impressions upon our senses; it is the product of the mind actively working upon the raw material of sensation.

Kant tried to counter Hume’s skepticism of causation by making the cause-and-effect relation not an objective reality but an intrinsic form of thought; as such it is independent of experience, and is not subject to the uncertainty of empirical ideas. Yet it is a necessary part of all experience, since we cannot understand experience without it. Hence “the concept of cause involves the character of necessity, which no experience can yield.”22 Kant supposed that by this léger-de-plume he had saved science from that humiliating limitation to probability to which Hume had condemned it. Indeed, he argued, it is the human mind, and not nature, that establishes the universal “laws of nature,” by endowing some of our generalizations—like those of mathematics—with qualities of universality and necessity not objectively perceived. “We ourselves introduce that order and regularity in the appearance which we entitle ‘nature.’ We could never find them in appearances had we not ourselves, by the nature of our own mind, originally set them there.”23 The “laws of nature” are not objective entities but mental constructs useful in handling experience.

All knowledge takes the form of ideas. In this sense the idealist is right: the world, for us, is merely our ideas. Since we know matter only as and through ideas, materialism is logically impossible, for it attempts to reduce the directly known (ideas) to the unknown or indirectly known. But the idealist is wrong if he believes that nothing exists except our ideas; for we know that ideas can be produced by sensations, and we cannot explain all sensations without assuming, for many of them, an external cause. As our knowledge is limited to phenomena or appearances—i.e., to the form the external cause takes after being molded by our modes of perception and conception—we can never know the objective nature of that external cause;24 it must remain for us a mysteriousDing-an-sich, a thing-in-itself, a “noumenon” conceived but never perceived. The external world exists, but in its ultimate reality it is unknowable.25

The soul too is real but unknowable. We never perceive it as an entity additional to the mental states that we perceive; it too is a noumenon, necessarily conceived as the reality behind the individual self, the moral sense, and the forms and processes of the mind. The sense of self mingles with every mental state, and provides continuity and personal identity. The consciousness of self (“apperception”) is the most intimate of all our experiences; and by no feat of the imagination can we conceive it as material.26 It seems impossible that an immaterial soul should act upon—and be acted upon by—a material body; but we may believe that the unknowable reality behind matter “may not, after all, be so different in character” from that inner thing-in-itself which is the soul.27

We cannot prove by pure or theoretical reason (as Wolff tried to do) that the individual soul is immortal, or that the will is free, or that God exists; but neither can we by pure reason disprove these beliefs (as some skeptics thought to do). Reason and the categories are equipped to deal only with phenomena or appearances, external or internal; we cannot apply them to the thing-in-itself—the reality behind sensations or the soul behind ideas. When we try to prove or disprove the dogmas of faith we fall into “paralogisms” (fallacies) or “antinomies”—inherent contradictions. We end in equal absurdities if we hold that the world had or had not a beginning; that the will is or is not free; or that a necessary or supreme being does or does not exist. Kant expressed with unwonted eloquence the argument from design,28 but he concluded that “the utmost that the argument can prove is an architect … who is always very much hampered by the adaptability of the material in which he works, not a creator … to whose idea everything is subject.”29

And yet how can we rest content with so baffling a conclusion—that free will, immortality, and God can be neither proved nor disproved by pure reason? There is (Kant urges) something in us deeper than reason, and that is our irrefutable consciousness that consciousness, mind, and soul are not material, and that the will is in some measure, however mysteriously and illogically, free; and we cannot be long content to think of the world as a senseless sequence of evolution and dissolution without moral significance or inherent mind. How can we justify our will to believe? Partly (says Kant) by the intellectual usefulness of belief—by its offering us some guidance in the interpretation of phenomena, as well as some philosophical sanity and religious peace.

The things of the world must be viewed as if they received their existence from a highest intelligence. The idea [of God] is thus really a heuristic, not an ostensive, concept [it is an assumption helpful to discovery and understanding, but it is not a demonstration].... In the domain of theology we must view everything as if the sum of all appearances (the sensible world itself) had a single, highest, and all-sufficient ground beyond itself—namely, a self-subsistent, original, creative reason. For it is in the light of this idea of a creative reason that we so guide the empirical employment of our reason as to secure its greatest possible extension. … The only determinate concept which the purely speculative reason gives us of God is, in the strictest sense, deistic; that is, reason does not determine the objective validity of such a concept, but yields only the idea of something which is the ground of the highest and necessary unit of all empirical reality.30

But a more imperative reason for religious belief, in Kant’s view, is that such belief is indispensable to morality. “If there is no primordial being distinct from the world, if the world is … without an Author, if our will is not free, if the soul is … perishable like matter, then moral ideas and principles lose all validity.”31 If moral character and social order are not to depend entirely on fear of the law, we must support religious belief, if only as a regulative principle; we must act as if we knew that there is a God, that our souls are immortal, that our wills are free.32 Moreover, as an aid to thought and morals, “we are justified in representing the cause of the world in terms of a subtle anthropomorphism (without which we could not think anything whatever in regard to it), namely, as a being that has understanding, feelings of pleasure and displeasure, and desires and volitions corresponding to these.”33

So the famous Critique concludes, leaving opposite schools of thought comforted and displeased. The skeptics could argue that Kant had justified agnosticism, and could scorn his reinstatement of God as a supplement to the police. The buffeted theologians reproached him for admitting so much to the infidels, and rejoiced that religion had apparently survived its perilous passage through Kant’s labyrinthine mind. In 1786 Karl Reinhold described the turmoil:

The Critique of Pure Reason has been proclaimed by the dogmatists as the attempt of a skeptic who undermines the certainty of all knowledge; by the skeptics as a piece of arrogant presumption that undertakes to erect a new form of dogmatism upon the ruins of previous systems; by the supernaturalists as a subtly plotted artifice to displace the historical foundations of religion, and to establish naturalism without polemic; by the naturalists as a new prop for the dying philosophy of faith; by the materialists as an idealistic contradiction of the reality of matter; by the spiritualists as an unjustifiable limitation of all knowledge to the corporeal world, concealed under the name of the domain of experience . . .34

Almost all these schools of thought attacked the book, giving it fame if only as a succès de scandale. Even its difficulty exalted it, making it a challenge that every up-to-date mind had to meet. Soon the sesquipedalia verba of Kant were in every learned mouth.

He could not understand why his critics could not understand him. Had he not defined every basic term over and over again? (Yes, and how variously!) In 1783 he answered the attacks by rephrasing the Critique in what he thought was a simpler form; and he defiantly entitled his rejoinder Prolegomena to Every Future Metaphysic That Will Be Able to Appear as Science. Before his Critique, he claimed, there had been no real metaphysics at all, for no system had prefaced itself with a critical scrutiny of its instrument—reason. If some readers could not understand the Kritik, that might be because they were not quite up to it; “in such a case one should apply one’s mental gifts to another object”; after all, “there is no need for everybody to study metaphysics.”35 The old professor had humor and pride, and temper too. As it proceeded, the Prolegomena became as difficult as the Critique.

The controversy continued under the tolerant regime of Frederick the Great. Kant had written in the Critique some eloquent passages on the nobility of reason, and its right to freedom of expression.36 In 1784, still relying on protection by Frederick and Zedlitz, he published an essay entitled Wasist Aufklärung? He defined the Enlightenment as freedom and independence of thought, and took as his motto and counsel Sapere aude— “Dare to know.” He regretted that intellectual liberation was so retarded by the conservatism of the majority. “If we ask whether we live in an enlightened [aufgeklärt] age, the answer is no”; we live only “in an age of enlightening” (Aufklärung) . He hailed Frederick as the embodiment and protector of the German Enlightenment, as the one monarch who had told his subjects, “Reason as you will.”37

This may have been written in the hope that Frederick’s successor would keep to the policy of toleration. But Frederick William II (1786-97) was more interested in the power of the state than in the freedom of the mind. When a second edition of the Critique of Pure Reason was prepared (1787) Kant modified some passages, and tried to soften his heresies with an apologetic preface: “I have found it necessary to deny knowledge [of things in themselves] in order to make room for faith. … Criticism alone can sever the root of materialism, fatalism, atheism, freethinking, fanaticism, and superstition.”38 He had reason for caution. On July 9, 1788, Johann Christian von Wöllner, “minister for the Lutheran Department,” issued a Religionsedikt which explicitly rejected religious toleration as responsible for the loosening of morals, and threatened with expulsion from their pulpits or chairs all preachers or teachers who deviated from orthodox Christianity. It was in this atmosphere of reaction that Kant published his second Critique.

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