Preface to the Second Edition (1787)

Kant identifies that metaphysics, unlike the sciences, has failed to achieve scientific progress and certainty. Practitioners disagree fundamentally about method and reach contradictory conclusions, suggesting the discipline has not yet found the secure path of science.

Kant establishes that logic has achieved certainty by confining itself to a narrow field dealing with the formal laws of thought alone, without regard to objects or their content.

Kant argues that logic's success stems from its limited scope—it merely forms a vestibule to the sciences rather than producing substantive knowledge of objects themselves.

Kant distinguishes between theoretical rational cognition, which determines conceptions of objects, and practical rational cognition, which establishes their reality. Both must begin with the pure or a priori element.

Kant argues that mathematics underwent a revolutionary transformation when someone discovered that knowledge of geometric properties requires constructing them a priori rather than merely contemplating given figures.

Kant identifies that physics entered the sure path of science when thinkers like Galileo and Bacon realized that reason must approach nature actively, posing questions to it according to rational principles rather than passively observing.

Kant observes that metaphysics, despite being the oldest science, has failed to achieve the secure path of science and exhibits endless disagreement among practitioners.

Kant raises the fundamental question: is the certain path impossible for metaphysics, or has it simply been missed? If nature has given us the restless aspiration toward it, there must be hope of finding it.

Kant proposes reversing the traditional assumption: instead of our cognition conforming to objects, objects must conform to our cognition. This parallels Copernicus's methodological reversal regarding celestial motion.

Kant explains that if intuitions must conform to the nature of objects, a priori knowledge is impossible; but if objects conform to our faculty of intuition, a priori knowledge becomes conceivable.

Kant argues that we must assume objects conform to our conceptions and thus to experience, which requires understanding. This allows a priori cognition to be possible before objects are given to us.

Kant notes that reason necessarily thinks certain objects that cannot be given in experience, and the new method will test itself against these cases where thought extends beyond possible experience.

Kant explains that since pure reason's propositions cannot be tested empirically, we must view conceptions from two perspectives—as objects of experience and as objects of mere thought—and check for contradictions.

Kant claims that the new method succeeds in explaining the possibility of a priori cognition and demonstrating the a priori laws underlying nature as the sum of objects of experience.

Kant observes that while the method succeeds for phenomena, it reveals that our faculty of cognition cannot transcend the limits of possible experience, seemingly contradicting metaphysics's traditional ambitions.

Kant explains that reason's impulse to transcend experience stems from seeking the unconditioned to complete the series of conditions, but this leads to contradictions under the old assumption.

Kant argues that by assuming objects conform to our representation, the contradiction disappears: the unconditioned lies not in things as we know them but in things-in-themselves beyond our cognition.

Kant compares his method to the chemist's synthetic process: analysis separates cognition into phenomena and things-in-themselves, and dialectic reunites them while confirming the necessity of the distinction.

Kant suggests that while speculative reason cannot know things-in-themselves, practical cognition may provide data to determine transcendent conceptions and satisfy metaphysics's higher aims.

Kant notes that Copernicus's hypothesis was vindicated when it revealed hidden forces and made predictions, suggesting Kant's metaphysical hypothesis will similarly be proven through its explanatory power.

Kant clarifies that the Critique of Pure Reason presents a new method for metaphysics rather than a complete metaphysical system, defining the bounds and structure of metaphysics itself.

Kant argues that metaphysics has a unique advantage: once placed on the sure path of criticism, it can complete itself fully since it deals only with principles and their limitations, not with infinite empirical objects.

Kant acknowledges that the Critique appears merely negative, warning against transcending experience with speculative reason, but argues this has positive implications.

Kant argues that by confining speculative reason to phenomena, criticism removes obstacles to practical reason's necessary use, particularly in morality, which must transcend sensibility.

Kant compares criticism to a system of police: though seemingly negative in preventing harm, it provides the positive benefit of allowing each person to pursue their vocation in security.

Kant summarizes the main conclusions: space and time are forms of sensible intuition; we have no conceptions except with corresponding intuitions; we can know only phenomena, not things-in-themselves.

Kant distinguishes between cognizing (which is limited to phenomena) and thinking (which can extend to things-in-themselves without contradiction, preserving necessary presuppositions).

Kant demonstrates that the distinction between phenomena and things-in-themselves resolves the apparent contradiction between free will and natural causality by allowing the will to be unfree as phenomenon but free as thing-in-itself.

Kant shows that with the critical distinction, freedom becomes thinkable (involving no contradiction) even though it cannot be cognized, thereby protecting moral agency.

Kant acknowledges that speculative reason cannot cognize the soul as a thing-in-itself and thus cannot cognize liberty as a real property, yet the critical method preserves the thinkability of freedom.

Kant argues that if morality necessarily presupposes liberty and reason contains practical a priori principles requiring it, then without criticism, natural mechanism would destroy morality.

Kant establishes that morality does not require speculative cognition of liberty, only that freedom be thinkable without contradiction and consistent with nature's mechanism, which criticism ensures.

Kant clarifies that while speculative sources cannot establish objective validity for conceptions like freedom, practical sources can, distinguishing mere logical possibility from real possibility.

Kant indicates that the critical method similarly protects conceptions of God and the soul's immortality, allowing them to be thought for practical purposes without speculative pretension.

Kant argues that only by limiting speculative reason's pretensions can one coherently assume God, freedom, and immortality for practical purposes, replacing dogmatic claims with rational belief.

Kant contends that metaphysical dogmatism—advancing in metaphysics without critical inquiry—generates the skepticism that militates against morality by failing to justify its presuppositions.

Kant argues that a criticism-based metaphysics serves reason by replacing random groping with scientific method, benefits youth by directing them to genuine science, and especially benefits morality and religion.

Kant argues that the limitations placed on speculative reason damage only the schools' monopoly, not the actual interests of humanity, since common moral and religious convictions rest on feeling and duty, not subtle speculation.

Kant observes that traditional metaphysical proofs about the soul, freedom, and God have never penetrated public understanding, suggesting that the hope for future life and belief in divine wisdom arise from immediate feeling rather than speculation.

Kant identifies that moral conviction arises from the feeling that the temporal is inadequate to our nature, freedom from the clear exhibition of duty against inclination, and belief in God from nature's order—not from philosophical demonstration.

Kant argues that the Critique appropriately limits the schools to comprehensible, morally sufficient proofs available to common understanding, stripping them of exclusive claim to profound insight on matters of general human concern.

Kant assigns the schools the duty of investigating speculative reason thoroughly through criticism to prevent metaphysical controversies from destabilizing the public and to counter pernicious doctrines.

Kant argues that criticism alone can undermine materialism, fatalism, atheism, fanaticism, and superstition, which harm humanity, as well as idealism and skepticism, which harm the schools.

Kant urges governments to favor critical inquiry over dogmatic school despotism, since criticism alone establishes reason on a firm basis while dogmatism merely destroys inconsequential cobwebs.

Kant clarifies that criticism is not opposed to dogmatic procedure (which pure cognition requires) but to dogmatism (presuming progress without prior critical inquiry into reason's powers).

Kant insists that criticism, while rejecting unsupported dogmatism, also rejects shallow populism and skeptical destruction of metaphysics, instead preparing a rigorous scientific metaphysical system.

Kant praises Wolf for establishing principles for rigorous metaphysics through careful definition and scrutiny but notes that Wolf lacked the critical step of examining reason's own faculties.

Kant attributes Wolf's failure to perceive the need for criticism to the dogmatic philosophical spirit of his age, not to any personal deficiency.

Kant argues that rejecting both Wolf's method and the Critique of Pure Reason results in abandoning scientific rigor, replacing labor with play and philosophy with opinion.

Kant explains that the second edition preserves all propositions and demonstrations unchanged but improves exposition, as the organic structure of pure reason admits no alteration without self-contradiction.

Kant lists the specific obscurities addressed: the aesthetic part regarding time, deductions of understanding's conceptions, principles of pure understanding, and paralogisms of rational psychology.

Kant notes his addition of a new, strict refutation of psychological idealism and demonstration of external intuition's objective reality, addressing the scandal of merely assuming external things.

Kant's refutation argues that consciousness of one's existence in time necessarily implies an external something permanent to ground temporal change, so external experience is inseparable from internal experience.

Kant responds to the objection that we are only conscious of our representations by arguing that consciousness of existence in time is more than mere representation and constitutes empirical, not fictional, knowledge.

Kant argues that determination of one's existence in time depends necessarily on something external, making internal and external experience interdependent; we are as certain of external things related to sense as of our own temporal existence.

Kant clarifies that a permanent external object is distinct from permanent representation; representations change and vary, but must refer to something permanent external to and distinct from them.

Kant notes that he omitted or abridged non-essential passages to increase clarity without expanding the work, relying on readers to compare with the first edition if desired.

Kant expresses pleasure that the spirit of profound and thorough investigation persists in Germany despite fashionable superficiality, and that acute thinkers have mastered the science of pure reason.

Kant announces his decision to abstain from controversy due to advanced age, leaving the task of clarifying obscurities and defending the system to younger talented philosophers.

Kant notes he has reached his 64th year and must economize time to elaborate the metaphysics of nature and morals in confirmation of the Critique's principles, while delegating defense of the work to others.

Kant argues that philosophical systems cannot achieve the complete armor of mathematical treatises, and apparent contradictions between isolated passages often dissolve when understood within the organic whole.

Kant contends that if a theory possesses internal stability, critical action and reaction eventually smooth superficial roughness, and when capable minds attend to it, the system achieves requisite elegance.