The Critique of Practical Reason Analysis

Immanuel Kant


(Student Guide to World Philosophy)

None of Immanuel Kant’s writings can be understood without a clear recognition of the “Copernican revolution” in philosophy effected by his first critique, Kritik der reinen Vernunft (1781; The Critique of Pure Reason, 1838). Previously, the predominant rational tradition in Western philosophy was founded on the assumption of reason’s capacity for discovering the forms or essential structures characterizing all things. Whether the form of “treeness” was an innate aspect of every existent tree (as Greek philosopher Aristotle believed) or a transcendent form in which each existent tree participated (as Greek philosopher Plato held), the capacity of reason for perceiving such forms was not doubted. The medieval controversy over “universals” centered not in reason’s ability for such perception but in the nature of this rational activity.

From the first questioning of the nominalists, however, through the break between self and “exterior world” in the philosophy of René Descartes, doubt as to the precise authority of rational apprehension increased. Human error and empirical deception began to be seen as intervening between perceiver and perceived, thus raising powerfully the question of the criteria for truth. The Aristotelians, especially from the time of Saint Thomas Aquinas on, affirmed that knowledge begins with sense perception; however, because of reason’s capacity for extracting forms, human knowledge not only possessed the qualities of necessity and universality but also made possible an inductive knowledge of trans-empirical realities.

It was the empiricists, especially David Hume, who provided the most serious challenge to this rationalist claim. Centering his attack on the problem of universal causality (cause and effect as universally operative), Hume raised the question of necessity. On what grounds, he asked, can one insist that, of necessity, all “effects” have causes and, similarly, that such causes necessarily produce identical effects? Hume’s conclusion was that the category of causality, like all human ideas, is derived from sense impressions, having the status simply of a habitual assumption and expectation; human ideas are forever bereft of necessity.

Answering the Empiricists

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It was Kant who saw the seriousness of this empiricist challenge. Reason was bankrupt as an agent of knowledge if it could no longer claim necessity, and thus universality, for its findings. Humanity and the world had been severed, and skepticism seemed the inevitable result.

The answer provided by Kant’s first critique was a revolution, a complete reversal of the previous conception of the knowing process. If human knowledge cannot claim a necessity that is resident within the empirical world itself, it is possible, nevertheless, to claim universality for it if the locus of necessity is within the universal operations of human reason. With this new conception of rational necessity and universality, Kant proceeded to exhibit what he conceived to be the necessary operations of rational apprehension, the manner in which the understanding, by its very structure, has perceived and organized, and of necessity will always perceive and organize, whatever realities encounter it.

As Kant interpreted it, Hume’s error was in seeing subjective necessity as grounded only in habit instead of being a result of the a priori structure of reason. If the latter is the case, rational necessity and universality are guaranteed, although on a far different basis from before. For Kant, the forms perceived through sense experience are the product of the categories of the human mind, but now the externality so encountered is never known as it is in itself (as noumenon), but only in its relation to humanity (as...

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Pure vs. Practical Reason

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The Critique of Practical Reason is of major importance not only as the attempt to create a purely rational ethic but also as a defense of a nondiscursive mode of apprehension, as an insistence that the “rational” is not restricted in meaning to the “cognitive.” It is this point that Kant develops further in the third critique, Kritik der Urteilskraft (1790; The Critique of Judgment, 1892), in terms of beauty and the purposiveness of nature. In order to understand these points, one must beware the misleading title of the second critique. In distinguishing between pure reason and practical reason, Kant is not speaking of two human agents or loci of activity; in both critiques he is speaking of pure reason as such, but in the first he is concerned with its theoretical or speculative function, in the second with its practical or ethical function. For Kant, this second function is the activity known as will. It is his purpose to show that will is not divorced from reason, controlled internally by drives or impulses, or externally by pleasure stimuli. In its fulfilled operation, it is a purely rational enterprise; it is pure reason in its practical operation which must control drives and determine external ends.

Likewise, in this realm it was Hume who haunted Kant, for Hume understood reason as being the pawn of passions, and morality as being rooted in subjective feeling. Just as Kant’s answer in the cognitive realm depended on exhibiting the a priori or categorical laws of human cognitive activity, so his answer in the second critique depended on discovering the a priori or categorical laws of the rational will. Morality could claim objectivity and universality only by being founded not on experience but on pure reason itself. The task of the second critique, then, is to discover the a priori or necessary principles of the practical reason.

The Problem of Freedom

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At the heart of the problem of ethics is the problem of freedom; without freedom, morality is an impossibility. However, according to the first critique, since all things are seen, of necessity, under the category of causality, all things are seen as determined. Yet, Kant insists, the same noumenon-phenomenon distinction applying to the object of such knowledge applies to the subject as well. It is humanity as phenomenon who is seen under the category of necessity, but the nature of the noumenal person remains unknown. Although the speculative function of reason strives for an understanding of the human “soul,” the antinomies, as we have seen, left the matter of freedom for the noumenal self as “problematic but not impossible.” If Kant can exhibit the will as free, he believes, he can also show the capacity of pure reason to determine the will’s total activity.

If there is to be an objective ethic, an ethic based on freedom, the only possibility for it can be reason presupposing nothing else but itself, for a rule can be objective and universal only if it is not subject to any contingent, subjective conditions. Thus, moral laws cannot be based on the pleasure principle, for the objects of pleasure and pain can be identified only empirically, thus having no objective necessity. Further, hedonism can make no legitimate distinction between higher and lower pleasures; only if reason is able to determine the will can there be a higher...

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The Categorical Imperative

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For Kant, the fundamental law of the pure practical reason is this: “So act that the maxim of your will could always hold at the same time as a principle establishing universal law.” Such rational control of the will is objective, for the legislation is made in indifference to any contingencies. Yet a distinction must be drawn between a pure will and a holy will; although the moral law is a universal law for all beings with reason and will, because the free person has wants and sensuous motives, that person is capable of maxims that conflict with the moral law. Thus, this law comes to people as a “categorical imperative.” It is categorical because it is unconditioned; it is an imperative because it is experienced as “duty,” as an inner compulsion provided by reason. Holiness is above duty, but in this life it remains the ideal to be striven for, but never reached. Each maxim must strive for unending progress toward this ideal; it is such progress that deserves the name “virtue.”

Kant’s formulation of the moral law is, in effect, a philosophical statement of the Golden Rule. As Kant says, the moral law of universality alone, without the need of any external incentive, arises as duty “to extend the maxim of self-love also to the happiness of others.” Put on a commonsense level, Kant’s moral formula is rooted in the integrity required by reason. It is self-evident that reason, to be rational, must operate in complete self-consistency; since the rational is the universal, reason qua reason must consent to will only that which can consistently be willed universally.

For Kant, the demand of duty is unmistakable and can, without difficulty, be perceived by the simplest person. Where the difficulty arises is in following the imperative. Kant’s estimate of humanity is such that he goes so far as to maintain that the good act is done only when duty and inclination are in conflict. What he really means here is that aversion is a sign that the individual has gone beyond self-interest to real duty. It is necessary to insist, Kant maintains, that satisfaction follows but does not precede awareness of the moral law; there is certainly a “moral feeling” that should be cultivated, but duty cannot be derived from it.

Six Ethical Systems

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Kant’s rejection of all ethical theory but his own formal principle provides a helpful summary of alternative ethical systems. Of the subjective type, there are two kinds: external and internal. In the former, philosophers such as Michel de Montaigne root ethics in education, while others, such as Bernard Mandeville, see its basis in a civil constitution. Of the internal variety, Epicurus sees physical feeling as central, while Francis Hutcheson grounds ethics in “moral feeling.” There are likewise internal and external types within the objective ethical systems. The former is the ethic of perfection, held by Christian von Wolff and the Stoics; the latter is the “will of God” ethic of theological morality. The subjective...

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Pure Rational Faith

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The affirmation of such postulates Kant calls the activity of “pure rational faith,” for although they are objective (necessary), freedom, the soul, and God are not known as they are in themselves. This, he affirms, is in truth the essence of “the Christian principle of morality.” It is from morality that religion springs, for religion is nothing more than “the recognition of all duties as divine commands.”

Because morality involves the moral law, with the form of an action, it follows that no “thing” is good or evil; such designations properly apply only to an acting will. Good and evil are defined only after and by means of the moral law; to reverse this procedure is to develop an empirical, subjective ethic. It is the practical judgment that determines the applicability of a universal maxim to a concrete act. To make an application such as this is very difficult, for it is here that the laws of freedom (the noumenal realm) are applied to the laws of nature (the phenomenal realm). Such a meeting is possible because the moral law is purely formal in relation to natural law. That is, it raises this question: If this proposed act should take place by a law of nature of which you were a part, would your will regard it as possible? The center of the moral act thus rests in one’s intentions, not in consequences. If the right act occurs but not for the sake of the moral law, it is not a moral act. The only incentive that is valid is the moral law itself.

For human beings as they are, their natural feelings of self-love are ever at war with the moral law. The very fact that morality resides in law reveals the severe “limitation” of humanity. The moral law is victorious only if all inclinations and feelings are set aside out of respect for the moral law, in and of itself. An act not performed out of such a sense of duty is inevitably tainted with the self-pride of believing goodness to be a spontaneous reflection of one’s nature.

Problems of Application

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Perhaps the major difficulty in Kant’s ethic is the problem of application. There are few acts that a performer would not defend as universally valid if the hypothetical performer and situation were in every way identical with those of the actual performer. Every evil has been defended by the exigencies of person and circumstance. Kant’s moral formula is designed to eliminate all such individualized decisions. Yet to the degree that the formula is interpreted, not in such a particularized fashion but in an absolutely universal sense, its inadequacy becomes evident. Total truth-telling, total promise-keeping, and the like, all have obvious moral exceptions. Likewise, how is one to resolve conflicts between these objective...

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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.


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