Immanuel Kant

Key ideas

1. The Copernican turn. Kant proposed that objects must conform to our cognition, not our cognition to objects: the mind brings forms of its own to experience. That, he argued, is how we can know anything about objects a priori.
2. Synthetic a priori knowledge. Analytic judgements only spell out a concept; synthetic ones add to it. Kant held that arithmetic, geometry and the basic principles of natural science are synthetic and known a priori, and the Critique of Pure Reason asks how such knowledge is possible.
3. Transcendental idealism. Space and time are forms of human sensibility, not features of things in themselves. We know the world as it appears, structured by space, time and the categories of the understanding, never as it is independently of our way of knowing it.
4. The limits of reason. Reason that tries to go beyond possible experience, to God, the soul or the world as a whole, falls into illusion. Limiting knowledge, Kant said, left room for faith, and he grounded belief in God, freedom and immortality in morality rather than in theory.
5. The categorical imperative. Morality is a command of reason that binds unconditionally. Its first formula says to act only on a maxim you can will as a universal law; the second to treat humanity, in yourself and in others, never merely as a means; the third to act as a member of a kingdom of ends.
6. The good will and autonomy. Only a good will is good without qualification, and a will is moral when it gives itself the law. What gives an action moral worth is that it is done from duty, not its consequences.
7. Judgement. A judgement of beauty is disinterested yet claims everyone’s agreement. In politics Kant defended freedom of public criticism, republican constitutions and an international federation of states as the route to perpetual peace.

Immanuel Kant (1724–1804) was a German philosopher of the Enlightenment who argued that the human mind actively shapes experience, so that we can know the world only as it appears to us, and that morality is a law that reason gives itself. His Critique of Pure Reason and his ethics of duty, built on the categorical imperative, redirected modern philosophy.

He was born in Königsberg, in East Prussia, into a Lutheran family of Pietist background, and studied at the university there, where Martin Knutzen introduced him to Newton and to the rationalism of Christian Wolff. After six years as a private tutor he returned in 1754 and lectured without a salary until, in 1770, he became professor of logic and metaphysics. The Critique of Pure Reason (1781; revised 1787) followed a decade in which he published little. He retired from teaching in 1796, never married, rarely left Königsberg, and died there on 12 February 1804.

What did Kant make of Hume?

In the Prolegomena (1783) Kant wrote that Hume’s suggestion “first interrupted my dogmatic slumber” (Paul Carus’s translation). Hume had asked what entitles us to the idea of cause and effect, and Kant found the problem wider: metaphysics consists of such connecting concepts. He concluded that they are not drawn from experience, as Hume supposed, but spring from the understanding itself, which put him between rationalism and empiricism.

How can we know anything beforehand?

The Critique begins with a reversal that Kant likened to Copernicus: instead of assuming that “our cognition must conform to the objects”, suppose that “the objects must conform to our cognition” (Meiklejohn’s translation). The mind contributes the forms of space and time and the concepts, or categories, through which we think objects. Without the senses, he says, no object would be given to us; without the understanding none would be thought. “Thoughts without content are void; intuitions without conceptions, blind.”

Kant’s example is arithmetic. The concept of seven and five, he argues, does not contain twelve; we reach 7 + 5 = 12 by adding units in intuition, so the proposition is synthetic yet known a priori, without needing experience. This is the position called transcendental idealism: we know phenomena, things as they appear, and not things in themselves. Reason that tries to reach beyond experience, to God or the whole world, falls into illusion, and Kant says he had to “abolish knowledge, to make room for belief”.

What is the categorical imperative?

The Groundwork (1785) opens: “Nothing can possibly be conceived in the world, or even out of it, which can be called good, without qualification, except a good will” (Abbott’s translation). What gives an action moral worth, Kant argues, is that it is done from duty, not what it brings about, which is why his view is called deontological. Hypothetical imperatives say what to do if you want something; the categorical imperative binds unconditionally.

Its first formula is “Act only on that maxim whereby thou canst at the same time will that it should become a universal law.” The second is “So act as to treat humanity, whether in thine own person or in that of any other, in every case as an end withal, never as means only.” The third asks you to act as a member of a kingdom of ends. Behind all three stands autonomy, the will giving itself the law.

Beauty, religion and politics

In the Critique of Judgement (1790) Kant argues that a judgement of beauty is disinterested, free of desire for the object, yet claims universal agreement; the sublime is the experience of vastness that overwhelms the imagination. In Religion within the Bounds of Bare Reason (1793) he grounds true religion in morality, though how to read his own religious views is disputed.

His 1784 essay on enlightenment gave it the motto Sapere aude, dare to know. Toward Perpetual Peace (1795) argues for republican constitutions and an international federation of states. Scholars criticise his remarks on race and on women as reflecting prejudices of his time that sit badly with his universal principles.

What came after him?

The German idealists Fichte, Schelling and Hegel started from Kant, Schopenhauer engaged with his metaphysics, and Neo-Kantianism became a major movement. In the twentieth century John Rawls and Jürgen Habermas built their political and moral theories on Kantian ground.

How should you start reading Kant?

Start with the Prolegomena, which he wrote as an accessible introduction, then the Groundwork, whose first two sections give the good will and the imperative. For the Critique of Pure Reason, read the Preface to the second edition and the Introduction, which the library has in its Copernican passage. The Critique of Judgement, §§1–9, treats beauty. The Gutenberg translations are over a century old; see the guide to reading philosophy for how to approach such texts.