Immanuel Kant

Immanuel Kant was a German philosopher of the late Enlightenment who spent his whole life in and around the Baltic port of Königsberg. In a series of books written in his late fifties and sixties he tried to settle what human beings can know, how they ought to act and what they may hope for, and his answers changed the course of Western philosophy. Almost every later school, from German idealism to modern analytic philosophy, defines itself partly by how it responds to him.

Kant was born on 22 April 1724 in Königsberg, then a city of the Kingdom of Prussia and today Kaliningrad in Russia. His father was a harness maker, and the family belonged to Pietism, a strict and inward-looking Lutheran movement that stressed moral seriousness and personal devotion. A local pastor noticed the boy's ability and placed him at the Collegium Fridericianum, a Pietist school where he learned Latin thoroughly but later recalled the religious drill with distaste.

In 1740 he entered the University of Königsberg. There a young teacher, Martin Knutzen, introduced him to the physics of Isaac Newton alongside the rationalist philosophy of Gottfried Wilhelm Leibniz and his follower Christian Wolff, which dominated German universities. After his father's death in 1746 Kant left without a post and worked for several years as a private tutor to families in the Prussian countryside.

Kant returned to the university in 1755 as a Privatdozent, an unsalaried lecturer paid directly by students. For fifteen years he lectured on a remarkable range of subjects, including logic, metaphysics, ethics, physics, mathematics, geography and anthropology, often for twenty hours a week. His early writings were mostly scientific. The Universal Natural History and Theory of the Heavens of 1755 proposed that the solar system formed from a rotating cloud of gas, an idea later developed by Laplace and known as the nebular hypothesis.

He was a sociable man who dined with friends, played cards and was admired as a witty lecturer. The image of a rigid bachelor whose daily walk was so punctual that neighbours set their clocks by it comes from his old age and has been exaggerated. In 1770 he was finally appointed professor of logic and metaphysics, and he never travelled more than a short distance from his home city.

Kant later said that reading David Hume woke him from his "dogmatic slumber". Hume had argued that our belief that every event has a cause cannot be proved either by logic or by experience; it is only a habit of the mind. If that was right, science itself seemed to rest on custom. Kant spent the 1770s, his so-called silent decade, working on a reply, and in 1781 published the Critique of Pure Reason, followed by a shorter guide, the Prolegomena, in 1783, and a heavily revised second edition in 1787.

His central move was what he compared to the revolution of Copernicus. Instead of assuming that our knowledge must conform to objects, he suggested that objects, as we experience them, must conform to the structure of our minds. Space and time are not things we discover out there; they are the forms in which anything at all appears to us. In the same way, concepts such as cause and substance are not learned from experience but are rules the mind uses to organise it. Because every possible experience has this shape, we can know some things in advance with complete certainty, for example that every event will have a cause, and this is why mathematics and the basic laws of physics can be universal.

The price was a limit. We know things only as they appear to us, which Kant called phenomena, never as they are in themselves. When reason tries to go beyond any possible experience and prove that God exists, that the soul is immortal or that the universe had a beginning, it tangles itself in contradictions. Kant did not deny God or freedom; he argued that they could be neither proved nor disproved by theoretical reason, which, as he put it, made room for faith. This position is called transcendental idealism.

Kant's ethics appeared in the Groundwork of the Metaphysics of Morals in 1785 and the Critique of Practical Reason in 1788. He held that the moral worth of an action lies not in its results but in the principle behind it. Doing the right thing because it pays, or because we happen to feel kindly, is not yet morality; acting out of respect for duty is.

The supreme principle of morality is what he called the categorical imperative, a command that holds for every rational being regardless of their wishes. One version tells us to act only on a rule we could will to be a universal law. A person tempted to make a false promise to get out of trouble should ask whether a world in which everyone did so could work; it could not, because promises would lose all meaning. Another version tells us always to treat humanity, in ourselves and in others, as an end and never merely as a means. This idea of the equal dignity of every person became one of the foundations of modern thinking about human rights.

For Kant, the fact that we feel bound by moral law shows that we must think of ourselves as free, able to act on reasons rather than only being pushed by causes. Morality and freedom imply each other.

The Critique of Judgment of 1790 dealt with beauty, the sublime and purpose in nature. Kant argued that a judgement of beauty is based on feeling rather than on concepts, yet claims everyone else ought to agree, a puzzle that shaped later aesthetics. His Religion within the Bounds of Mere Reason of 1793 treated Christianity as a vehicle for moral teaching and brought a royal reprimand from the government of Frederick William II, which had abandoned the tolerant line of Frederick the Great. Kant promised not to write further on religion during the king's lifetime.

In the 1784 essay "What is Enlightenment?" he defined it as humanity's emergence from self-imposed immaturity and gave it the motto "dare to know". He welcomed the ideals of the French Revolution while rejecting any right of rebellion, and in Toward Perpetual Peace in 1795 he proposed a federation of free republics under international law, an idea often cited as a forerunner of the League of Nations and the United Nations. His work also contains views on race and on women that many scholars now regard as indefensible, and how central they were to his thought is debated.

Kant's health and memory declined in the late 1790s, and he gave up lecturing in 1796. He died in Königsberg on 12 February 1804 and was buried at the cathedral.

He had absorbed the rationalism of Leibniz, the empiricism of Hume and the moral passion of Jean-Jacques Rousseau, whose portrait was reportedly the only picture in his study, and he tried to unite their strengths within one system. His successors Fichte, Schelling and Hegel tried to overcome his division between appearances and things in themselves, while Arthur Schopenhauer built on it directly, which makes Kant the starting point of nineteenth-century philosophy in Germany. He stands at the summit of the Enlightenment, and his ideas about knowledge, autonomy and human dignity still shape debates in philosophy, ethics and law.