The Critique of Practical Reason Analysis
by Immanuel Kant

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(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

(Student Guide to World Philosophy)

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 phenomenon).

Although reason attempts to complete this knowledge by bringing it into a comprehensive unity, it is barred from success in this speculative operation by certain antinomies, both sides of which are in harmony with a person’s phenomenal knowledge. In...

(The entire section is 3,910 words.)