Critique of Pure Reason

• 1. Introduction

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  Immanuel Kant spent at least twelve years thinking about a single question before he finally sat down to write. Then, while still lecturing and teaching, he put his answer on paper in just four to five months. The result, published in 1781, was the Critique of Pure Reason. In his own preface, Kant described it as a critique of the faculty of reason itself, in respect of all knowledge it might seek to acquire independently of all experience. What he wanted to decide was nothing less than the possibility or impossibility of metaphysics as a discipline.

  Kant had grown up in the tradition of European rationalism, shaped by thinkers such as René Descartes, Gottfried Wilhelm Leibniz, and Christian Wolff. But the Scottish philosopher David Hume had done something to him that Kant later described in blunt terms: the remembrance of Hume, he wrote, first interrupted his dogmatic slumber and gave his investigations in speculative philosophy a completely different direction. What was it about Hume that had that effect, and what had Kant done with those twelve years of thought? The answers trace the contours of one of the most ambitious arguments in the history of Western philosophy.

• 2. The Problem Hume Posed

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  David Hume had started from a position that most rationalists of his day would have recognized. Before Kant, it was generally accepted that all truths of reason must be analytic, meaning the predicate was already contained within the subject. To deny such a truth was to contradict yourself. Hume accepted this view at first.

  But on closer examination Hume discovered something troubling. Some judgments that looked analytic turned out not to be. Statements about cause and effect, in particular, could not be confirmed by analyzing the subject-concept alone. Experience showed only that one event regularly follows another, not that the first actually causes the second. From this, Hume drew a sweeping conclusion: nothing at all could be known a priori about cause and effect.

  Kant found this conclusion impossible to ignore. In section VI of the Critique's introduction, he argues that Hume stopped short at precisely the crucial point. Hume never considered that a synthetic judgment, one where the predicate adds something not already contained in the subject, might be made a priori. That gap is where Kant decided to build his entire system. The central question of the Critique became: how are synthetic a priori judgments possible at all?

• 3. Analytic and Synthetic

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  Kant draws a sharp line between two types of judgment. He uses the proposition "All bodies are extended" as his example of an analytic judgment: the concept of extension is already contained within the concept of body, so nothing new is being added. You are only unpacking what was already there.

  "All bodies are heavy" is different. The concept of weight is not already contained within the concept of body. You are adding something, which makes the judgment synthetic. Before Kant, philosophers had assumed that all a priori knowledge must be analytic, since necessary truths were thought to rest entirely on the law of contradiction.

  Kant's claim was that mathematics, the first principles of natural science, and metaphysics itself all involve knowledge that is both a priori and synthetic. He uses the equation 7 + 5 = 12 to make the case. No analysis of the number 7 or the number 5, he argues, will ever produce 12, because an infinite number of pairs of numbers exist that sum to 12. The sum must be arrived at synthetically. This conclusion pushed Kant toward a new problem: if pure mathematics is synthetic a priori, how is that possible? And if it is possible, could metaphysics be grounded on similar footing?

• 4. Space, Time, and the Mind

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  The Transcendental Aesthetic, the first major section of the Critique, deals with what Kant calls all principles of a priori sensibility. His revolutionary claim is that space and time are not features of the world that exist independently of observers. They are pure forms of intuition that the faculty of sensation contributes to cognition.

  Kant sets his view against both Isaac Newton and Leibniz. Newton held that space and time are real existences in themselves. Leibniz held that they are relations or determinations of things, but relations that belong to those things even when they are not being sensed. Both answers agree that space and time exist independently of the subject's awareness. Kant denies this.

  He gives two expositions of space and time, a metaphysical one and a transcendental one. In the transcendental exposition, he invites the reader to take the proposition that two straight lines can neither contain any space nor form a figure, and then to try to derive that result from the bare concepts of a straight line and the number two. He concludes it is simply impossible to do so analytically. The judgment must draw on a pure a priori intuition of space. Geometry does not proceed by measurement but by demonstration, and that is only possible because space is a form the mind brings to experience rather than a property read off from the world.

  Time receives a parallel treatment. Kant argues that time makes it possible to say that A and not-A are at the same spatial location, provided one considers them at different times with a sufficient change between states. This means time is not a concept subject to formal logical analysis but a pure form of sensible intuition. The result is that the thing in itself, independent of these forms, remains unknowable.

• 5. Categories and the Understanding

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  The Transcendental Logic shifts attention from sensibility to the understanding, which Kant defines as the faculty of mind that deals with concepts. Where the Transcendental Aesthetic asked how objects are given to us through sensation, the Transcendental Logic asks how objects are thought.

  In the Metaphysical Deduction, Kant derives twelve pure concepts of the understanding, which he calls categories, from the logical forms of judgment. He arranges judgments under four heads: Quantity, Quality, Relation, and Modality. Under each head there are three logical forms. Corresponding to the logical form of hypothetical judgment, for example, there corresponds the category of causality. Kant calls these pure concepts categories, echoing Aristotle's notion of a category as a concept not derived from any more general concept. They are universal and necessary for human thought.

  The Transcendental Deduction then argues that these twelve categories are conditions of all possible experience. The ground of all experience is the self-consciousness of the experiencing subject, whose constitution requires that all thought be rule-governed in accordance with the categories. The categories are thus necessary components in any possible experience whatsoever.

  Bridging the categories to actual sensory appearances requires the schematism. Each category has a schema, a connection through time between the pure a priori concept of the understanding and a phenomenal a posteriori appearance. Time is shared by both the categories and sensed phenomena, so it becomes the schema of the categories generally. Succession is the form of sense impressions and also of the category of causality.

• 6. Reason and Its Illusions

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  The Transcendental Dialectic is what Kant calls a logic of illusion. Its task is to expose the fraudulence of applying reason beyond the limits of experience. The Analytic was a logic of truth; the Dialectic shows how reason, when it strains toward the unconditioned, produces systematic errors that cannot be resolved by evidence.

  The Paralogisms of Pure Reason target the idea of an immortal soul. Kant identifies four paralogisms. The first mistakes the logical subject of every thought, the ever-present "I think," for a permanent real substance. The second mistakes the unity of apperception for an indivisible simple soul. The third takes the consistency of the "I" across time for evidence of an everlasting personal identity. The fourth misidentifies mind and world as wholly separate. In each case, the inference looks valid but rests on a confusion between logical form and real existence. Kant does not deny that these ideas of the soul can guide human behavior; he denies that speculative reason can prove them.

  The Antinomies of Pure Reason go further. Kant presents four pairs of arguments where both the thesis and the antithesis appear to be valid. The world has a beginning in time: provable. The world is infinite in time: also provable. Both conclusions are false, Kant argues, because the world as a whole is not an object of possible experience. Neither claim is based on experience, so neither can be sustained. He notes that Leibniz and Samuel Clarke, Newton's spokesman, had recently conducted a debate on precisely these cosmological questions, a debate that shaped how he framed the antinomies.

  The Ideal of Pure Reason addresses God. Pure reason generates the concept of an ens realissimum, the most real being conceivable, and treats it as the sum total of all reality. Kant traces this to Anselm of Canterbury, who lived from 1033 to 1109 and presented the ontological proof in a short treatise titled Discourse on the existence of God. Kant's counter-argument is that existence is not a predicate. The copula "is" in a declarative sentence merely connects subject to predicate; it does not add a new attribute. To say "God exists" is not to attach a new quality to the concept of God. The idea of ten dollars is different from having ten dollars only in reality, not in conceptual content. No accumulation of predicates will ever move a concept from inner subjectivity to actual existence.

• 7. The Copernican Turn

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  In the second preface to the Critique, Kant compares his critical philosophy to Copernicus's revolution in astronomy. Copernicus rejected the view that all celestial motion belongs to the stars. By allowing that the motion is partly due to the observer's own position, he transformed the picture of the universe from geocentric to heliocentric. Kant aims for an analogous shift in metaphysics.

  Hitherto, Kant writes, all knowledge was assumed to conform to objects. But every attempt to extend knowledge of objects a priori, by establishing something about them through concepts alone, had ended in failure. The Critique proposes the opposite trial: that objects must conform to our knowledge, and specifically to the necessary conditions of the knowing subject. This is the heart of transcendental idealism.

  Kant insists his position differs from George Berkeley's idealism, which denies all extramental existence and turns phenomena into things in themselves. Kant's claim is narrower. Knowledge is limited to phenomena as objects of sensible intuition. The thing in itself is not denied but is declared unknowable. Kant adds the Refutation of Idealism to the second edition of 1787 in direct response to critics who misread him as a Berkeleyan. Self-consciousness, he argues, presupposes external objects. All determinations of the self in time require something permanent in perception, and that permanence cannot be internal. This argument reversed the priority that Descartes had given to inner over outer experience.

  The book received little attention when it first appeared in 1781, but it later drew attacks from empiricist and rationalist critics alike and became a sustained source of controversy. It is now considered a culmination of several centuries of early modern philosophy and the inauguration of what came after it. Kant's two further Critiques, the Critique of Practical Reason in 1788 and the Critique of Judgment in 1790, extend the project into morality and aesthetics, but both take the limits established in 1781 as their starting point.