Immanuel Kant

Immanuel Kant spent almost his entire life in a single city, Königsberg, on the eastern edge of Prussia, and never travelled more than about a hundred miles from it. He never married. His days were so regular that, according to the legend his first biographers spread, neighbours set their clocks by his afternoon walk. From this quiet life came the most consequential revolution in modern philosophy. Before Kant, philosophy was divided between rationalists who trusted pure reason and empiricists who trusted only experience; after him, almost every school — German Idealism, Romanticism, neo-Kantianism, phenomenology, logical positivism, analytic philosophy, liberal political theory, modern human-rights thinking — defines itself in relation to his three Critiques.

His central idea can be put in a sentence. We do not passively receive the world; the mind actively structures experience through its own forms — space, time and the categories — and therefore can know certain things about any possible experience in advance, with necessity, while being forever unable to know things as they are in themselves. From this flows a new theory of knowledge, a new metaphysics of limits, a morality of autonomy and dignity, an aesthetics of the beautiful and the sublime, and a philosophy of history and perpetual peace.

Life

Königsberg

Kant was born on 22 April 1724 in Königsberg (now Kaliningrad, in a Russian exclave on the Baltic), the fourth of nine children of a harness-maker. His family were Pietists — members of a Lutheran renewal movement that emphasized inner devotion, moral seriousness and plain living. Kant later said of his parents that they “never once” gave him a bad example. The moral earnestness and suspicion of outward religious display stayed with him even as he rejected Pietist doctrine.

He studied at the Collegium Fridericianum and then, from 1740, at the University of Königsberg, where Martin Knutzen introduced him to Newton’s physics and to the rationalist philosophy of Leibniz and Christian Wolff. When his father died in 1746, Kant left the university and worked for nearly a decade as a private tutor to families in the countryside.

The pre-critical Kant

Returning to Königsberg in 1755, he became a Privatdozent — a lecturer paid directly by students — and taught an astonishing range: logic, metaphysics, ethics, mathematics, physics, physical geography, anthropology, natural theology, even fortification and pyrotechnics. He was a popular, witty lecturer and an elegant man about town, fond of cards, billiards and dinner company.

His early works were largely in natural science. In the Universal Natural History and Theory of the Heavens (1755) he proposed that the solar system formed from a rotating cloud of gas — the nebular hypothesis, later developed by Laplace — and speculated, correctly, that some nebulae are distant galaxies (“island universes”). He wrote on earthquakes (after the Lisbon disaster of 1755), on winds, on the rotation of the Earth.

In philosophy he was at first a Wolffian rationalist, but doubts grew. The Dreams of a Spirit-Seer (1766), a satire on the visionary Emanuel Swedenborg, also mocked the “dreams of metaphysics” — speculative systems as baseless as Swedenborg’s spirits. In 1770, finally appointed Professor of Logic and Metaphysics, he wrote an Inaugural Dissertation that already distinguished the sensible from the intelligible world and treated space and time as forms of sensibility.

The silent decade

Then came eleven years of near-silence. Kant was working on the problem that, as he later said, David Hume had made unavoidable. In 1781, at fifty-seven, he published the Critique of Pure Reason. Over the next decade came a torrent: Prolegomena (1783), Groundwork of the Metaphysics of Morals (1785), Metaphysical Foundations of Natural Science (1786), the revised second edition of the first Critique (1787), Critique of Practical Reason (1788), Critique of the Power of Judgment (1790), Religion within the Boundaries of Mere Reason (1793), Toward Perpetual Peace (1795), The Metaphysics of Morals (1797), Anthropology from a Pragmatic Point of View (1798) — as well as essays on Enlightenment, history, race, the French Revolution, and the conflict of the faculties.

Censorship and old age

Under the reactionary King Frederick William II, Kant’s Religion drew a royal reprimand in 1794 and he promised, as “Your Majesty’s most loyal subject”, to refrain from public statements on religion — a promise he considered void after the king’s death. In his last years his memory and mind declined; he was cared for by a former student, Wasianski. He died on 12 February 1804. His last words are reported as “Es ist gut” — “It is good.” On his tomb in Königsberg cathedral are inscribed the words from the end of the second Critique: “the starry heavens above me and the moral law within me.”

The man

The clock-setting legend exaggerates. Kant was sociable, hosted daily dinners with guests from many walks of life (never fewer than the Graces, three, nor more than the Muses, nine), loved conversation about politics and travel, followed the American and French revolutions with sympathy, and was considered an entertaining companion. He lived alone, never married, and was famously meticulous about his health.

The problem Kant inherited

Rationalism and empiricism

The early modern debate — see Rationalism vs. Empiricism — had reached an impasse.

The rationalists (René Descartes, Baruch Spinoza, Gottfried Wilhelm Leibniz, and the systematizer Christian Wolff) held that reason alone could know necessary truths about substance, God, the soul and the world. But their systems contradicted each other, and their metaphysics seemed to have no way to settle its disputes. Kant called metaphysics “a battlefield of endless controversies” (A viii).

The empiricists (John Locke, Berkeley, David Hume) held that all ideas come from experience. But Hume had shown where this leads: the necessity we attribute to causal laws cannot be found in experience and is only a habit of the mind. If Hume was right, then not only metaphysics but Newtonian science — which Kant revered — rested on habit.

Hume’s challenge

I freely admit that it was the remembrance of David Hume which, many years ago, first interrupted my dogmatic slumber and gave my investigations in the field of speculative philosophy a quite new direction. (Prolegomena, Ak. 4:260)

Kant generalized Hume’s problem. It was not only the concept of cause; all the basic concepts of metaphysics — substance, unity, necessity — claim a necessity that experience cannot supply. The question was whether any knowledge of this kind is possible at all.

The master question: synthetic a priori judgments

Two distinctions

Kant’s way of posing the problem rests on two distinctions (see Synthetic a priori):

• A priori / a posteriori — an epistemological distinction. A priori knowledge is independent of experience; its marks are necessity and strict universality. A posteriori knowledge depends on experience and is always contingent.
• Analytic / synthetic — a logical distinction about judgments. In an analytic judgment, the predicate is “contained in” the concept of the subject (“all bodies are extended”); denying it is a contradiction. A synthetic judgment adds something new (“all bodies are heavy”).

Analytic judgments are all a priori and merely clarify our concepts. A posteriori judgments are all synthetic. The question is whether there are synthetic a priori judgments — informative, yet necessary and known independently of experience.

Where they are found

Kant claims there are, and that they are everywhere:

• Mathematics. “7 + 5 = 12” is synthetic: the concept of the sum of seven and five contains only the uniting of the two numbers, not the concept of twelve; you must “go beyond” the concepts with the help of intuition — counting on your fingers — to reach it (B 15). “A straight line is the shortest between two points” is synthetic too: “straight” is a quality, “shortest” a quantity.
• Natural science. “In all changes of the corporeal world the quantity of matter remains unchanged”; “in all communication of motion, action and reaction must always be equal” (B 17–18).
• Metaphysics at least aims at synthetic a priori knowledge: “the world must have a first beginning”, “every event has a cause”.

So the central question of the Critique is: How are synthetic a priori judgments possible? (B 19). If we can answer it for mathematics and natural science, we will know whether, and how far, metaphysics is possible.

The Copernican revolution

The answer is the famous reversal announced in the Preface to the second edition (see The Copernican Revolution):

Hitherto it has been assumed that all our knowledge must conform to objects. But all attempts to extend our knowledge of objects by establishing something in regard to them a priori, by means of concepts, have, on this assumption, ended in failure. We must therefore make trial whether we may not have more success in the tasks of metaphysics, if we suppose that objects must conform to our knowledge. (B xvi)

Copernicus, unable to explain the heavens on the assumption that the stars revolve around the observer, tried making the observer revolve. Kant tries making objects conform to the structure of our cognition. Then a priori knowledge becomes intelligible: we can know in advance the forms that any object must have in order to be an object for us, because those forms come from us.

This is transcendental idealism. “Transcendental” knowledge, in Kant’s sense, is knowledge “occupied not so much with objects as with the mode of our knowledge of objects in so far as this mode of knowledge is to be possible a priori” (B 25). And it implies a radical limit: we can know objects only as they appear to us (phenomena), never as they are in themselves (noumena, things in themselves — see The Thing-in-Itself).

Sensibility: space and time

Two sources of knowledge

Human knowledge has two sources: sensibility, through which objects are given to us in intuitions, and understanding, through which they are thought through concepts. Neither is sufficient alone:

Thoughts without content are empty, intuitions without concepts are blind. (A 51/B 75)

Rationalists had treated sensibility as confused thinking; empiricists treated concepts as faded sensations. Kant insists the two are different in kind and both indispensable.

The Transcendental Aesthetic

The first part of the Critique, the “Transcendental Aesthetic” (from Greek aisthēsis, perception), argues that space and time are the forms of our sensible intuition — not properties of things in themselves, but the way any object must appear to beings like us.

His arguments include:

1. Space is presupposed by experience, not derived from it. To represent objects as outside me and beside one another, I must already have the representation of space.
2. Space is necessary. I can think of space without objects, but I cannot represent the absence of space.
3. Space is an intuition, not a concept. There is only one space; “spaces” are parts of it, not instances of a general concept.
4. Geometry. Geometry gives synthetic a priori truths about space. How could we know them with necessity unless space were a form contributed by us? If space were a property of things in themselves, we could learn about it only by experience — and then without necessity.

Parallel arguments apply to time, the form of inner sense — of all our representations.

The “neglected alternative”

A famous objection, pressed by the nineteenth-century philosopher Adolf Trendelenburg: even granting that space is a form of our intuition, why couldn’t things in themselves also be spatial? Kant seems to have overlooked this possibility. Defenders reply that Kant’s arguments, if sound, show that spatial properties are essentially relational and dependent on perceivers, and so cannot belong to things as they are in themselves.

The problem of geometry

Kant’s view that Euclidean geometry is synthetic a priori truth about space suffered a blow when nineteenth-century mathematicians (Gauss, Lobachevsky, Bolyai, Riemann) developed consistent non-Euclidean geometries, and a greater blow when Einstein’s general relativity (1915) showed that physical space is not Euclidean. Neo-Kantians responded in different ways — some by distinguishing mathematical from physical geometry, others (like Hans Reichenbach) by making the a priori “relativized” and revisable.

Understanding: the categories

The table of categories

The “Transcendental Analytic” turns to the understanding. Kant argues that just as sensibility has its forms, understanding has its pure concepts — the categories. He derives them from the logical forms of judgment (the “metaphysical deduction”), producing twelve, in four groups of three:

Group	Categories
Quantity	Unity, Plurality, Totality
Quality	Reality, Negation, Limitation
Relation	Substance and accident, Cause and effect, Community (reciprocity)
Modality	Possibility, Existence, Necessity

These are not learned from experience; they are the rules by which the understanding unifies the manifold of intuition into experience of objects.

The transcendental deduction

The heart of the Critique — and the part Kant rewrote most extensively for the second edition — is the Transcendental Deduction of the Categories. “Deduction” here is a legal term: a proof of right (quid juris), not a derivation. Kant must show that the categories have a legitimate claim to apply to objects of experience.

The argument, in very simplified form, turns on the unity of consciousness:

It must be possible for the “I think” to accompany all my representations. (B 131)

For any representations to be mine — to belong to a single consciousness — they must be capable of being combined in one self-consciousness. Kant calls this the transcendental unity of apperception. But combination is an act of the understanding, governed by rules. And the most basic rules of combination are the categories. So the very unity of self-consciousness requires that experience be organized by the categories; and since objects can be objects for us only in such unified experience, the categories necessarily apply to all objects of possible experience.

Argument (simplified).

1. All my representations must be capable of belonging to one self-consciousness (“I think”).
2. This unity requires that the manifold of intuition be combined according to rules.
3. The fundamental rules of combination are the categories.
4. Objects of experience are just what is combined in such unified representation.
5. Therefore the categories necessarily apply to all objects of possible experience — but only to them.

The deduction is notoriously obscure, and interpretations differ widely. Its last clause matters as much as its conclusion: the categories have objective validity only for objects of possible experience. Applied beyond experience, they are empty.

The schematism

How can pure concepts, which are entirely intellectual, apply to sensible intuitions, which are entirely different? Kant’s answer is the schematism: each category has a “schema”, a rule for its application in time, produced by the imagination. The schema of substance is permanence in time; the schema of cause is regular succession according to a rule. Kant calls schematism “an art concealed in the depths of the human soul” (A 141/B 180) — a phrase readers have found both profound and evasive.

The principles: Kant’s answer to Hume

The “Analytic of Principles” spells out the synthetic a priori principles that follow. The most important for Hume’s problem are the Analogies of Experience:

1. First Analogy: In all change of appearances, substance persists, and its quantum in nature is neither increased nor diminished.
2. Second Analogy: All alterations take place in conformity with the law of the connection of cause and effect.
3. Third Analogy: All substances, insofar as they can be perceived to coexist in space, are in thoroughgoing reciprocity.

The Second Analogy is Kant’s direct reply to Hume. Kant notes that our perceptions are always successive — when I look over a house, I see the roof, then the walls, then the foundation; when I watch a ship sail downstream, I see it upstream, then downstream. How do I distinguish the objective succession of an event (the ship moving) from the merely subjective succession of my perceptions of an unchanging object (the house)? Only by thinking the event as following according to a rule from what preceded it. So the very concept of an objective event already presupposes causal ordering. Causation is not a habit acquired from repeated experience; it is a condition of experiencing events at all.

Whether this refutes Hume or merely answers a different question is debated. Hume asked how we know particular causal laws (that bread nourishes); Kant shows at most that every event has some cause.

The Refutation of Idealism

In the second edition, Kant added a short argument against Descartes’ “problematic idealism” — the view that the existence of external things is doubtful. He argues that I am conscious of my own existence as determined in time; but all time-determination presupposes something permanent in perception; this permanent cannot be a representation in me; so it must be a thing outside me. “The mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me” (B 275). The inner experience Descartes thought certain presupposes the outer experience he doubted.

The limits of reason: the Dialectic

Reason’s natural illusion

The second half of the Critique, the “Transcendental Dialectic”, is about what happens when reason goes beyond experience. Reason is the faculty that seeks the unconditioned — the complete series of conditions for anything conditioned. This drive is natural and inevitable, and it produces transcendental illusion: the conviction that we can know the unconditioned objects it posits. Like the optical illusion by which the moon looks larger at the horizon, the illusion persists even after we see through it.

Reason generates three ideas: the soul (the unconditioned subject), the world as a whole (the unconditioned series of appearances), and God (the unconditioned condition of everything). Each gives rise to a family of fallacies.

The paralogisms: the soul

Rational psychology claims to prove from the “I think” that the soul is a substance, simple, a single identical person through time, and distinct from the body. Kant argues that each proof is a paralogism — a fallacy of ambiguity. The “I” of the “I think” is a purely formal unity of consciousness; it does not give me an intuition of myself as a substance. From the logical simplicity of the thinking subject nothing follows about the simplicity of a substance.

The antinomies: the world

In the antinomies reason contradicts itself — producing equally compelling proofs of opposite conclusions about the world as a whole:

	Thesis	Antithesis
1	The world has a beginning in time and is limited in space.	The world has no beginning and no limits; it is infinite.
2	Every composite substance consists of simple parts.	No composite thing consists of simple parts.
3	There is causality through freedom, as well as by laws of nature.	There is no freedom; everything happens by the laws of nature.
4	There exists an absolutely necessary being, as part or cause of the world.	No absolutely necessary being exists.

Kant called the antinomies the discovery that first woke him from his dogmatic slumber (in a 1798 letter to Christian Garve) — alongside Hume. They are the strongest argument for transcendental idealism: if appearances were things in themselves, one side of each antinomy would have to be true, and the contradictions would be irresolvable. Recognizing that the world-whole is not given as a thing in itself dissolves them. In the first two, both sides are false (the world is neither finite nor infinite as a whole in itself, since it is not a thing in itself). In the last two, both may be true: natural causality holds for phenomena, while freedom may hold for the noumenal self — making room for morality.

The ideal: God

Kant demolishes the three traditional proofs of God’s existence:

• The ontological argument (Anselm, Descartes): God is the most perfect being; existence is a perfection; so God exists. Kant replies: “Being is obviously not a real predicate” (A 598/B 626). To say a thing exists is not to add a property to its concept but to posit the thing with all its properties. “A hundred real thalers do not contain the least coin more than a hundred possible thalers” — though they make a difference to my financial position.
• The cosmological argument (from a contingent world to a necessary being): Kant argues that it secretly relies on the ontological argument to identify the necessary being with God.
• The physico-theological argument (from design): Kant treats it with respect — it is “the oldest, the clearest, and the most accordant with the common reason of mankind” — but it proves at most an architect of the world, not a creator, and only by relying on the cosmological argument.

What reason is for

Yet the ideas of reason are not useless. They have a regulative use: they guide inquiry, urging us to seek ever more systematic unity in nature — as if everything had a single intelligent cause — without being knowledge of objects. And the limit of knowledge has a positive meaning:

I had to deny knowledge in order to make room for faith. (B xxx)

Because speculative reason cannot prove that the soul is mortal, that the will is unfree, or that God does not exist, these questions are left open for practical reason.

Morality: autonomy and the categorical imperative

The good will

The Groundwork of the Metaphysics of Morals (1785) begins with a famous sentence:

It is impossible to think of anything at all in the world, or indeed even beyond it, that could be considered good without limitation except a good will. (Ak. 4:393)

Intelligence, courage, wealth, even happiness can be bad if the will that uses them is bad. A good will is good not because of what it achieves but in itself; “like a jewel, it would still shine by itself, as something that has its full worth in itself”, even if it accomplished nothing.

Duty

What makes a will good? Acting from duty, not merely in accordance with it. Kant’s examples: a shopkeeper who charges inexperienced customers fair prices because honesty is good for business acts in accordance with duty but not from duty; his action has no moral worth. A man who, sunk in grief and without any inclination, preserves his life only because it is his duty, acts from duty. Even beneficence from a warm temperament, admirable as it is, lacks the specifically moral worth of beneficence done from duty by someone whose sympathy has dried up.

Critics, beginning with Schiller’s epigram — “Gladly I serve my friends, but alas I do it with pleasure, / And so I am often troubled that I am not virtuous” — have charged that this makes the cold-hearted more moral than the warm. Defenders reply that Kant’s point is about what grounds moral worth, not that feelings are bad; later works give the cultivation of sympathetic feelings a moral role.

Hypothetical and categorical imperatives

An imperative commands. Hypothetical imperatives command conditionally: if you want X, do Y. Rules of skill and counsels of prudence are hypothetical. Morality, however, must command categorically — unconditionally, regardless of what you want. If morality depended on our desires, it would lack the necessity and universality we recognize in it.

The formulations

Kant gives the categorical imperative (see The Categorical Imperative) several formulations, which he claims are equivalent:

1. The Formula of Universal Law: “Act only according to that maxim through which you can at the same time will that it become a universal law” (4:421). Variant — the Formula of the Law of Nature: act as if your maxim were to become by your will a universal law of nature.
2. The Formula of Humanity: “So act that you use humanity, whether in your own person or in the person of any other, always at the same time as an end, never merely as a means” (4:429).
3. The Formula of Autonomy: the idea of “the will of every rational being as a will giving universal law” (4:431).
4. The Formula of the Kingdom of Ends: act as a legislating member of a “kingdom of ends” — a systematic union of rational beings under common laws (4:433).

Four examples

Kant tests the universal law formula with four cases:

1. Suicide from despair. Can I will “from self-love I make it my principle to shorten my life when its continuance threatens more evil than it promises satisfaction” as a law of nature? Kant argues that a nature whose law it was to destroy life by the very feeling meant to promote it would contradict itself.
2. The false promise. Can I borrow money with a promise I don’t intend to keep? Universalized, the maxim destroys itself: if everyone made false promises when convenient, no one would believe a promise, and promising — and so my scheme — would be impossible. This is a contradiction in conception.
3. Neglecting one’s talents. A world in which everyone let their talents rust is conceivable (the South Sea Islanders, Kant unflatteringly suggests), but a rational being cannot will it, since it necessarily wills that its capacities be developed. This is a contradiction in the will.
4. Refusing to help others. A world without mutual aid is conceivable, but no one could will it, since everyone may one day need help.

The first two generate “perfect” (strict) duties; the last two “imperfect” duties that allow latitude in how they are fulfilled.

Dignity

The Formula of Humanity grounds Kant’s most influential idea:

In the kingdom of ends everything has either a price or a dignity. What has a price can be replaced by something else as its equivalent; what on the other hand is raised above all price and therefore admits of no equivalent has a dignity. (4:434)

Rational beings — persons — have dignity, not price. They may never be treated merely as means: deceived, coerced, used. This idea lies behind the opening of the Universal Declaration of Human Rights (1948) and many modern constitutions — Article 1 of the German Basic Law begins, “Human dignity shall be inviolable.”

Autonomy and freedom

Morality is autonomy: the will gives the moral law to itself. Every other ethical theory, Kant argues, is heteronomy — grounding the law in something outside the will (happiness, God’s command, moral sense) — and so cannot yield an unconditional obligation.

But autonomy presupposes freedom. In the theoretical Critique, freedom could only be shown to be thinkable. In the Critique of Practical Reason (1788), Kant argues that the moral law is a “fact of reason”: we are immediately conscious of standing under it. And since “ought implies can”, consciousness of the moral law reveals our freedom. Freedom is the ratio essendi (ground of being) of the moral law; the moral law is the ratio cognoscendi (ground of knowledge) of freedom.

The postulates

The second Critique also argues that morality commands us to pursue the highest good — virtue crowned with proportionate happiness. Since this is not achievable in the natural world, practical reason must “postulate” the immortality of the soul (for endless progress toward holiness) and the existence of God (to guarantee the proportion of happiness to virtue). These are not knowledge, but rational faith.

The murderer at the door

The most notorious test case for Kant’s ethics came in a 1797 exchange with the French writer Benjamin Constant. Constant objected that the duty to tell the truth, taken unconditionally, would make it a crime to lie to a murderer who asks whether your friend is hiding in your house. Kant’s essay “On a Supposed Right to Lie from Philanthropy” bit the bullet: truthfulness is an unconditional duty, and if you lie, you are responsible for any consequences. Most Kantians since have tried to show that Kant’s principles, properly applied, don’t require this conclusion — for instance, by considering the maxim “lie to murderers to protect the innocent”, or by distinguishing ethical duties from rights.

Objections

• Empty formalism — Georg Wilhelm Friedrich Hegel: the universal-law test can justify any content if the maxim is described suitably.
• Rigorism — no exceptions to perfect duties, even to save lives.
• Coldness — moral worth seems to require acting against inclination.
• Moral luck and particular attachments — Bernard Williams’s “one thought too many”: a man who saves his drowning wife rather than a stranger should not need the thought “and in situations like this it is permissible to save one’s wife”.
• The scope of dignity — does it cover infants, the severely cognitively disabled, animals? Kant held that we have duties only regarding animals (cruelty to them damages our humanity), not to them.

The third Critique: beauty, the sublime, and purpose

The judgment of taste

The Critique of the Power of Judgment (1790) aims to bridge the gulf between nature (governed by causal law) and freedom (governed by the moral law). Its first part is the founding text of modern aesthetics.

A judgment of taste (“this rose is beautiful”) has four “moments”:

1. Quality: disinterested. The pleasure in the beautiful is independent of any desire for the object’s existence — unlike the agreeable (food) or the good (useful or moral).
2. Quantity: universal without a concept. When I call something beautiful, I speak “with a universal voice” — I demand that everyone agree — yet my judgment is not based on any concept or rule.
3. Relation: purposiveness without a purpose. The beautiful seems designed, as if for our faculties, though we can name no purpose it serves.
4. Modality: necessary. The beautiful is thought of as necessarily pleasing, on the basis of a “common sense” (sensus communis) shared by all.

The pleasure arises from the free play of the imagination and the understanding — the harmonious engagement of our faculties without any determinate concept closing the game.

The sublime

The sublime is different. Faced with the boundless ocean, towering mountains, a raging storm — the mathematically vast or the dynamically overwhelming — imagination fails; we feel our physical smallness. But this failure reveals a faculty superior to nature: reason, with its idea of infinity and its moral vocation. The feeling of the sublime is thus “a pleasure that is possible only by means of a displeasure”. Kant’s analysis shaped Romanticism, and Schiller, Coleridge and Wordsworth drew on it.

Genius and art

Fine art is “the art of genius”. Genius is “the innate mental predisposition through which nature gives the rule to art” — it produces original, exemplary works that cannot be produced by following rules.

Teleology

The second part of the third Critique considers organisms. In a living thing, each part exists for the sake of the others and of the whole — it is both cause and effect of itself. Mechanical explanation alone cannot make organisms intelligible: “there will never be a Newton who could make comprehensible even the generation of a blade of grass according to natural laws that no intention has ordered” (5:400). (Darwin, seventy years later, is often said to have been that Newton.) Kant concludes that we must judge organisms as if purposive — a regulative, not constitutive, principle.

History, politics and peace

What is Enlightenment?

In his 1784 essay “An Answer to the Question: What Is Enlightenment?”, Kant gave the age its definition:

Enlightenment is man’s emergence from his self-incurred immaturity. Immaturity is the inability to use one’s own understanding without the guidance of another… Sapere aude! Have courage to use your own understanding! (8:35)

He distinguishes the public use of reason — the scholar addressing the reading world, which must always be free — from the private use — the officer or clergyman performing an office, which may be restricted. “Argue as much as you will, and about whatever you will, but obey!” — a slogan he attributes, with praise and some irony, to Frederick the Great.

History

In “Idea for a Universal History with a Cosmopolitan Purpose” (1784), Kant suggests that history can be read as nature’s plan to develop all human capacities, through unsocial sociability — our tendency both to come together and to compete. Without vanity, ambition and greed, “all talents would remain hidden for ever in their germ” in an Arcadian shepherd’s life. “Out of the crooked timber of humanity no straight thing was ever made” (8:23).

Perpetual peace

Toward Perpetual Peace (1795) — a sketch in the form of a peace treaty — proposes three “definitive articles”: every state should have a republican constitution (with separation of powers and representation); international right should be based on a federation of free states; and cosmopolitan right should be limited to universal hospitality — the right of a stranger not to be treated with hostility on arrival. Kant argues that republics are less likely to go to war, since the citizens who would bear its costs must consent. The essay is the ancestor of the League of Nations, the United Nations and “democratic peace theory”.

Kant’s shadows

Kant also wrote, especially in his lectures on anthropology and physical geography and in essays of the 1770s–80s, about human “races”, ranking them in a hierarchy with white Europeans at the top, and made demeaning remarks about Africans, Indigenous Americans and others. He also held views about women’s capacities that are disparaging by any modern standard. Scholars debate how deep these views go and whether Kant revised them in the 1790s (Perpetual Peace condemns European colonialism severely). The tension between his universal moral principles and his prejudices has become a central topic in Kant studies (see work by Charles Mills, Pauline Kleingeld, Robert Bernasconi).

Religion, right and virtue: the later Kant

Radical evil

Religion within the Boundaries of Mere Reason (1793) opens with one of Kant’s most surprising doctrines. Human beings, he argues, have a propensity to evil that is “radical” — rooted in the very ground of our maxims, though freely adopted and imputable to us. It does not consist in sensuous desires (which are innocent) nor in a diabolical will to evil for its own sake (which humans do not have), but in a reversal of the order of incentives: we all recognize the moral law, but we make obeying it conditional on self-love, rather than subordinating self-love to it. Its stages are frailty (“I will the good, but cannot do it”), impurity (doing right, but from mixed motives), and depravity (making self-love the supreme condition).

Kant’s critics on the Enlightenment side — Goethe accused him of slobbering on his philosopher’s mantle with the stain of radical evil to lure Christians — thought he had smuggled original sin back in. But the doctrine is a remarkable piece of moral psychology: it explains how rational beings who acknowledge the law nonetheless routinely make exceptions for themselves, and it anticipates the analysis of self-deception and “bad faith” in Jean-Paul Sartre.

The rest of the Religion reinterprets Christianity as a symbolic expression of moral truths: Christ as the personified ideal of moral perfection; grace as the hope that our moral conversion will be completed; the Church as an “ethical commonwealth” — a people of God under moral laws — of which historical churches are imperfect vehicles. Rituals, creeds and “statutory” observances undertaken to win divine favour are “counterfeit service” (Afterdienst).

The Doctrine of Right

The Metaphysics of Morals (1797) has two parts. The first, the Doctrine of Right (Rechtslehre), is Kant’s political and legal philosophy. Its universal principle: “Any action is right if it can coexist with everyone’s freedom in accordance with a universal law” (6:230). Right concerns only external freedom and may be enforced by coercion — coercion that hinders a hindrance to freedom is consistent with freedom.

• Innate right. There is only one: freedom, “independence from being constrained by another’s choice”, belonging to every human being by virtue of humanity.
• Property. We can rightfully have external things as our own; but private property claims are only provisional in the state of nature and become conclusive only in a civil condition. Hence we have a duty to leave the state of nature and enter a state.
• The state. It is grounded in the original contract — not a historical event but “an idea of reason” which obliges legislators to frame laws “as if they could have arisen from the united will of a whole people” (8:297).
• Punishment. Kant is a strict retributivist: punishment may never be imposed merely as a means to promote some other good, but only because the criminal has committed a crime; and the measure is equality (ius talionis). His most notorious example: even if a civil society were to dissolve itself, the last murderer in prison would first have to be executed, “so that each has done to him what his deeds deserve” (6:333).
• No right of revolution. Kant denies any right to rebel against even an unjust sovereign — while admiring the French Revolution. He resolves the tension by distinguishing the illegitimate act of revolution from the spectators’ disinterested enthusiasm for it (see below).

The Doctrine of Virtue

The second part, the Doctrine of Virtue (Tugendlehre), sets out ethical duties that cannot be externally enforced. There are two ends that are also duties: one’s own perfection and the happiness of others. Duties to oneself include not committing suicide, not lying (even inwardly), not being servile (“one who makes himself a worm cannot complain afterwards if people step on him”, 6:437), and developing one’s talents. Duties to others include beneficence, gratitude and sympathy — and respect, which forbids arrogance, defamation and ridicule. The book ends with a rich account of friendship as the union of love and respect.

The Conflict of the Faculties and the Revolution

In The Conflict of the Faculties (1798), Kant asks whether the human race is constantly progressing toward the better. Experience cannot prove it; but a “historical sign” can indicate it. That sign is the reaction of spectators to the French Revolution — not the revolution’s deeds, which were full of “misery and atrocities”, but the “sympathy bordering on enthusiasm” it aroused in disinterested observers across Europe, at their own risk. Such enthusiasm could only have a moral cause: a predisposition in humanity toward the idea of right. “Such a phenomenon in human history is not to be forgotten” (7:88).

The Opus postumum

In his last years Kant worked on a manuscript, unfinished and published only in the twentieth century as the Opus postumum, attempting a “transition” from the metaphysical foundations of natural science to physics — including speculations about an all-pervading ether, and new reflections on God and the self as “self-positing”. Some readers see in it a turn toward the Idealism of Fichte; most see the struggles of an old man with a failing memory working on a problem he could not solve.

The Kant crisis

Kant’s philosophy could be devastating to those who took it to heart. The most famous case is the dramatist Heinrich von Kleist, who in March 1801 wrote to his fiancée Wilhelmine von Zenge that reading Kant had destroyed his life’s aim:

If all men had green glasses instead of eyes, they would have to judge that the objects which they see through them are green — and they would never be able to decide whether their eye shows them things as they are, or whether it adds something to them which belongs not to them but to the eye. So it is with the understanding. We cannot decide whether what we call truth is truly truth, or whether it only seems so to us.

“My one, my highest goal has sunk, and I have no other.” Kleist’s “Kant crisis” sent him wandering, and stands as the first great literary record of the vertigo transcendental idealism can produce. (Whether he read Kant or a popularization is uncertain.) Kleist went on to write some of the most unsettling stories and plays in German literature, filled with the gap between appearance and reality.

Kant today

Kant remains the reference point for several live debates:

• Human dignity in medicine and technology. Informed consent, the prohibition on using patients as mere means, and debates on organ markets, surrogacy and genetic enhancement are often framed in Kantian terms.
• Artificial intelligence. If the mark of a person is rational autonomy, could a machine be a person? Conversely, does manipulative design (“dark patterns”) violate the duty not to treat users merely as means?
• Animals. Kant’s indirect-duty view is widely rejected; Christine Korsgaard’s Fellow Creatures (2018) argues that Kantian principles properly understood require direct duties to animals.
• Cosmopolitanism. International law, refugee rights (Kant’s “right of hospitality”) and global justice draw on Perpetual Peace.
• Constructivism. Rawls’s “Kantian constructivism” — principles of justice as those free and equal persons would choose — remains one of the dominant frameworks in political philosophy.
• Cognitive science. The question of how much of our experience is structured by innate forms — space, objects, causation — is Kant’s question in experimental form.

The reception

The immediate storm

The Critique was misunderstood at first (the 1782 Garve–Feder review treated it as Berkeleyan idealism). Within a decade it dominated German philosophy. K. L. Reinhold’s Letters on the Kantian Philosophy popularized it; Salomon Maimon, Gottlob Ernst Schulze (“Aenesidemus”) and Friedrich Heinrich Jacobi raised the objections that would drive the next generation. Jacobi’s quip about the thing-in-itself — without it he could not enter the system, with it he could not stay — became proverbial.

German Idealism

Fichte, Schelling and Georg Wilhelm Friedrich Hegel — see German Idealism — took Kant’s insight that the mind shapes experience and pushed it further: abolish the thing-in-itself, derive the forms of experience from a single principle, and let reason know the absolute. Hegel’s charge of “empty formalism” against Kant’s ethics and his historicization of the categories mark the decisive turn.

Schopenhauer

Arthur Schopenhauer considered himself Kant’s true heir, keeping the phenomena/noumena distinction but identifying the noumenon with will — see The Will.

Nietzsche’s mockery

Friedrich Nietzsche (Beyond Good and Evil §11) mocked Kant’s answer to his own question: “How are synthetic judgments a priori possible?” Kant answered, “By means of a faculty” — which is as much an explanation as Molière’s doctor who says opium puts you to sleep because of its virtus dormitiva. The real question, Nietzsche said, is why belief in such judgments is necessary for creatures like us — a question of life, not of logic. In Twilight of the Idols he called the Königsberg Chinaman a “catastrophic spider” whose “true world” was the last disguise of Christian Platonism.

Neo-Kantianism and beyond

“Back to Kant!” became a slogan in the 1860s. The Marburg school (Hermann Cohen, Paul Natorp, Ernst Cassirer) read Kant as a philosopher of mathematical science; the Southwest school (Windelband, Rickert) as a philosopher of values. Husserl’s phenomenology called itself “transcendental”. Martin Heidegger‘s Kant and the Problem of Metaphysics (1929) read the first Critique as a work of fundamental ontology, centred on the imagination; his 1929 debate with Cassirer at Davos was a turning point in European philosophy.

In the English-speaking world, P. F. Strawson’s The Bounds of Sense (1966) reconstructed Kant’s arguments as “transcendental arguments” without transcendental idealism. John Rawls’s A Theory of Justice (1971) revived Kantian constructivism in political philosophy; Christine Korsgaard, Onora O’Neill, Allen Wood and Barbara Herman made Kantian ethics a living research programme. Wilfrid Sellars, John McDowell and Robert Brandom built analytic philosophies on Kantian foundations.

Science

Hermann von Helmholtz connected Kant’s forms of intuition to the physiology of perception; ethologist Konrad Lorenz argued that Kant’s a priori is the a posteriori of evolution — the categories are innate because they were selected for. Cognitive scientists studying “core knowledge” of objects and causation in infants are, in a sense, testing Kant.

Kant on the map

• Answered: David Hume (causation), Gottfried Wilhelm Leibniz and Wolff (dogmatic metaphysics), René Descartes (problematic idealism, the ontological argument).
• Answered by: Georg Wilhelm Friedrich Hegel (dialectic, Spirit), Arthur Schopenhauer (Will), Friedrich Nietzsche (genealogy, perspectivism), Søren Kierkegaard (the ethical and the religious), Martin Heidegger (finitude and Being).
• Echoes in India: the knower that cannot be known as an object recalls Yājñavalkya (“you cannot see the seer of seeing”); the veil between appearance and reality recalls Māyā — though Kant thinks the veil can never be lifted, whereas the Upanishads promise liberating knowledge.

Key terms

A priori / a posteriori

Known independently of experience (necessary, universal) / known through experience.

Analytic / synthetic

Predicate contained in the subject concept / predicate adds something new.

Transcendental

Concerning the a priori conditions of the possibility of experience.

Transcendental idealism

The doctrine that space, time and the categories are forms of our cognition, so that we know only appearances.

Intuition (Anschauung)

Immediate, singular representation of an object — given through sensibility.

Using Kant in conversation

• When someone says “it’s all subjective”: Kant would answer that the forms of experience are universal — shared by every rational mind — so subjectivity is not arbitrariness.
• When someone says science simply reads the world off: the world we read is partly written by us.
• On people as tools: treat humanity always as an end, never merely as a means. Dignity, not price.
• On lies and exceptions: “what if everyone did that?” is the popular form of the universal law test.