The second book in the Critique, and by far the shorter of the two, attempts to lay out the formal conditions of the complete system of pure reason.
In the Transcendental Dialectic, Kant showed how pure reason is improperly used when it is not related to experience. In the Method of Transcendentalism, he explained the proper use of pure reason.
The Discipline of Pure Reason
In section I, the discipline of pure reason in the sphere of dogmatism, of chapter I, the discipline of pure reason, of Part II, transcendental discipline of method, of the Critique of Pure Reason, Kant enters into the most extensive discussion of the relationship between mathematical theory and philosophy.
Discipline is the restraint, through caution and self-examination, that prevents philosophical pure reason from applying itself beyond the limits of possible sensual experience. Philosophy cannot possess dogmatic certainty. Philosophy, unlike mathematics, cannot have definitions, axioms or demonstrations. All philosophical concepts must be ultimately based on a posteriori, experienced intuition. This is different from algebra and geometry, which use concepts that are derived from a priori intuitions, such as symbolic equations and spatial figures. Kant's basic intention in this section of the text is to describe why reason should not go beyond its already well-established limits. In section I, the discipline of pure reason in the sphere of dogmatism, Kant clearly explains why philosophy cannot do what mathematics can do in spite of their similarities. Kant also explains that when reason goes beyond its own limits, it becomes dogmatic. For Kant, the limits of reason lie in the field of experience as, after all, all knowledge depends on experience. According to Kant, a dogmatic statement would be a statement that reason accepts as true even though it goes beyond the bounds of experience.
Restraint should be exercised in the polemical use of pure reason. Kant defined this polemical use as the defense against dogmatic negations. For example, if it is dogmatically affirmed that God exists or that the soul is immortal, a dogmatic negation could be made that God doesn't exist or that the soul is not immortal. Such dogmatic assertions can't be proved. The statements are not based on possible experience. In section II, the discipline of pure reason in polemics, Kant argues strongly against the polemical use of pure reason. The dogmatic use of reason would be the acceptance as true of a statement that goes beyond the bounds of reason while the polemic use of reason would be the defense of such statement against any attack that could be raised against it. For Kant, then, there cannot possibly be any polemic use of pure reason. Kant argues against the polemic use of pure reason and considers it improper on the grounds that opponents cannot engage in a rational dispute based on a question that goes beyond the bounds of experience.
Kant claimed that adversaries should be freely allowed to speak reason. In return, they should be opposed through reason. Dialectical strife leads to an increase of reason's knowledge. Yet there should be no dogmatic polemical use of reason. The critique of pure reason is the tribunal for all of reason's disputes. It determines the rights of reason in general. We should be able to openly express our thoughts and doubts. This leads to improved insight. We should eliminate polemic in the form of opposed dogmatic assertions that cannot be related to possible experience.
According to Kant, the censorship of reason is the examination and possible rebuke of reason. Such censorship leads to doubt and skepticism. After dogmatism produces opposing assertions, skepticism usually occurs. The doubts of skepticism awaken reason from its dogmatism and bring about an examination of reason's rights and limits. It is necessary to take the next step after dogmatism and skepticism. This is the step to criticism. By criticism, the limits of our knowledge are proved from principles, not from mere personal experience.
If criticism of reason teaches us that we can't know anything unrelated to experience, can we have hypotheses, guesses, or opinions about such matters? We can only imagine a thing that would be a possible object of experience. The hypotheses of God or a soul cannot be dogmatically affirmed or denied, but we have a practical interest in their existence. It is therefore up to an opponent to prove that they don't exist. Such hypotheses can be used to expose the pretensions of dogmatism. Kant explicitly praises Hume on his critique of religion for being beyond the field of natural science. However, Kant goes so far and not further in praising Hume basically because of Hume's skepticism. If only Hume would be critical rather than skeptical, Kant would be all-praises. In concluding that there is no polemical use of pure reason, Kant also concludes there is no skeptical use of pure reason. In section II, the discipline of pure reason in polemics, in a special section, scepticism not a permanent state for human reason, Kant mentions Hume but denies the possibility that skepticism could possibly be the final end of reason or could possibly serve its best interests.
Proofs of transcendental propositions about pure reason (God, soul, free will, causality, simplicity) must first prove whether the concept is valid. Reason should be moderated and not asked to perform beyond its power. The three rules of the proofs of pure reason are: (1) consider the legitimacy of your principles, (2) each proposition can have only one proof because it is based on one concept and its general object, and (3) only direct proofs can be used, never indirect proofs (e.g., a proposition is true because its opposite is false). By attempting to directly prove transcendental assertions, it will become clear that pure reason can gain no speculative knowledge and must restrict itself to practical, moral principles. The dogmatic use of reason is called into question by the skeptical use of reason but skepticism does not present a permanent state for human reason. Kant proposes instead a critique of pure reason by means of which the limitations of reason are clearly established and the field of knowledge is circusmcribed by experience. According to the rationalists and skeptics, there are analytic judgments a priori and synthetic judgments a posteriori. Analytic judgments a posteriori do not really exist. Added to all these rational judgments is Kant's great discovery of the synthetic judgment a priori.
The Canon of Pure Reason
The canon of pure reason is a discipline for the limitation of pure reason. The analytic part of logic in general is a canon for the understanding and reason in general. However, the Transcendental Analytic is a canon of the pure understanding for only the pure understanding is able to judge synthetically a priori.
The speculative propositions of God, immortal soul, and free will have no cognitive use but are valuable to our moral interest. In pure philosophy, reason is morally (practically) concerned with what ought to be done if the will is free, if there is a God, and if there is a future world. Yet, in its actual practical employment and use, reason is only concerned with the existence of God and a future life. Basically, the canon of pure reason deals with two questions: Is there a God? Is there a future life? These questions are translated by the canon of pure reason into two criteria: What ought I to do? and What may I hope for? yielding the postulates of God's own existence and a future life, or life in the future.
The greatest advantage of the philosophy of pure reason is negative, the prevention of error. Yet moral reason can provide positive knowledge. There can't be a canon, or system of a priori principles, for the correct use of speculative reason. However, there can be a canon for the practical (moral) use of reason. Reason has three main questions and answers:
- What can I know? We cannot know, through reason, anything that can't be a possible sense experience; ("that all our knowledge begins with experience there can be no doubt")
- What should I do? Do that which will make you deserve happiness;
- What may I hope? We can hope to be happy as far as we have made ourselves deserving of it through our conduct.
Reason tells us that there is a God, the supreme good, who arranges a future life in a moral world. If not, moral laws would be idle fantasies. Our happiness in that intelligible world will exactly depend on how we have made ourselves worthy of being happy. The union of speculative and practical reason occurs when we see God's reason and purpose in nature's unity of design or general system of ends. The speculative extension of reason is severely limited in the transcendental dialectics of the Critique of Pure Reason, which Kant would later fully explore in the Critique of Practical Reason.
In the transcendental use of reason, there can be neither opinion nor knowledge. Reason results in a strong belief in the unity of design and purpose in nature. This unity requires a wise God who provides a future life for the human soul. Such a strong belief rests on moral certainty, not logical certainty. Even if a person has no moral beliefs, the fear of God and a future life acts as a deterrent to evil acts, because no one can prove the non-existence of God and an afterlife. Does all of this philosophy merely lead to two articles of faith, namely, God and the immortal soul? With regard to these essential interests of human nature, the highest philosophy can achieve no more than the guidance, which belongs to the pure understanding. Some would even go so far as to interpret the Transcendental Analytic of the Critique of Pure Reason as a return to the Cartesian epistemological tradition and a search for truth through certainty.
The Architectonic of Pure Reason
All knowledge from pure reason is architectonic in that it is a systematic unity. The entire system of metaphysic consists of: (1.) Ontology—objects in general; (2.) Rational Physiology—given objects; (3.) Rational cosmology—the whole world; (4.) Rational Theology—God. Metaphysic supports religion and curbs the extravagant use of reason beyond possible experience. The components of metaphysic are criticism, metaphysic of nature, and metaphysic of morals. These constitute philosophy in the genuine sense of the word. It uses science to gain wisdom. Metaphysic investigates reason, which is the foundation of science. Its censorship of reason promotes order and harmony in science and maintains metaphysic's main purpose, which is general happiness. In chapter III, the architectonic of pure reason, Kant defines Metaphysics as the critique of pure reason in relation to pure a priori knowledge. Morals, analytics and dialectics for Kant constitute Metaphysics, which is Philosophy and the highest achievement of human reason.
The History of Pure Reason
Metaphysics began with the study of the knowledge of God and the nature of a future world. It was concluded early that good conduct would result in happiness in another world as arranged by God. The object of rational knowledge was investigated by sensualists (Epicurus), and intellectualists (Plato). Sensualists claimed that only the objects of the senses are real. Intellectualists asserted that true objects are known only by the understanding mind. Aristotle and Locke thought that the pure concepts of reason are derived only from experience. Plato and Leibniz contended that they come from reason, not sense experience, which is illusory. Epicurus never speculated beyond the limits of experience. Locke, however, said that the existence of God and the immortality of the soul could be proven. Those who follow the naturalistic method of studying the problems of pure reason use their common, sound, or healthy reason, not scientific speculation. Others, who use the scientific method, are either dogmatists (Wolff) or skeptics (Hume). All of the above methods are faulty. The method of criticism remains as the path toward the completely satisfying answers to the metaphysical questions about God and the future life in another world.
Historically speaking from the point Kant left off, the Critique of Pure Reason led to the great systematic syntheses of German idealism. Hegelians like Bauer, Feuerbach, Marx and even Nietzsche rejected the creative and opted for the destructive potential of the Critique. Neo-Kantians Cohen and Rickert stressed the philosophical justification of science in the Critique. Heidegger and Heimsoeth stressed the ontology and Strawson the limits of reason within the boundaries of sensory experience. In recent times, Arendt and Lyotard stressed the work of orientation of a limited understanding in the field of world history.