This was a radio broadcast in honour of the 150th anniversary of Kant's death. It was published in the BBC Listener in 1954 and a number of very interesting footnotes have been added in this version.
After the introduction there are six sections. Popper started with an account of the large crowd that came to Kant's funeral, to the amazement of his friends who expected a quiet ceremony. Popper speculated that this had little to do with Kant's reputation as a great philosopher, instead he suggested that the people came to show their gratitude for a man who taught the rights of man, equality before the law, world citizenship, peace on earth and, "perhaps most important", of emancipation through learning.
The footnote to this reads: "I say 'most important' because Kant's well-deserved rise from near poverty to fame and comparatively easy circumstances helped to create on the Continent the idea of emancipation through self-education (in this form hardly known in England where the 'self-made man' was the uncultured upstart). The significance of this idea is connected with the fact that on the Continent, the educated had been for a long time the middle classes, while in England they were the upper classes."
1. Kant and the Enlightenment
It seems that the good ideas that are associated with Kant's name reached the Continent from England, in a book by Voltaire (Letters About the English, 1732) which drew many comparisons between England and the mainland which favoured English representative government, tolerance and empirical science over the tyranny and bigotry that prevailed in the rest of Europe.
"Voltaire's book was burnt, but its publication marks the beginning of a philosophical movement -- a movement whose peculiar mood of intellectual aggressiveness was little understood in England, where there was no occasion for it."
Kant is one of Popper's favorite philosophers and he associated him with all the positive ideas of the Englightenment, and especially with the struggle for intellectual and spiritual freedom. He quoted Kant: "Enlightenment is the emancipation of man from a state of self-imposed tutelage... of incapacity to use his own intelligence without external guidance... Dare to use your intelligence! This is the battle-cry of the Enlightenment."
However there is a different usage of the term 'Enlightenment' to mean shallow and pretentious intellectualism. This has given rise to considerable literature about the different aspects of the 'Enlightenment Project' which is no doubt complicated by the existence of two radically different meanings of the term. For some people the project derives from the radicalism of the French revolution and it is represented by all those utopian revolutionary movements that have causesd the death of millions. For others it is the simple goodness and gentle meliorism of Kant which is the line taken by Popper, although that is not pursued in this essay.
2. Kant's Newtonian Cosmology
Kant is best known (if not at first hand) as a philosopher but his major inspiration came from Newton's theory and he made a major contribution to cosmology and cosmogony with the Kant-Laplace hypothesis on the origin of the solar system. In the way that one problem leads to another, in the case of Kant his work on the structure of the universe led him to the problem of human knowledge.
3. The Critique and the Cosmological Problem
The central problem of the Critique of Pure Reason turned up when Kant was wrestling with the problem of the limit of the universe in time. Either it had a beginning in time or it did not. Kant found that he could produce apparently valid proofs for each option. That was the first of the "antimonies" of pure reason.
4. Space and Time
"What lesson did Kant draw from these bewildering antimonies? He concluded that our ideas of space and time are inapplicable to the universe as a whole. We can, of course, apply the ideas of space and time to ordinary physical things and physical events. But space and time themselves are neither things nor events... They are a kind of framework for things and events."
He gave his theory the unfortunate name 'Transcendental Idealism' which gave the impression that he denied the reality of physical objects. This opened the way for others to follow, using his difficult style of writing, to promulgate wildly speculative metaphysical views which Kant found repulsive. Popper quoted from the Critique:
"There is no need for a critique of reason in its empirical use; for its principles are continuously submitted to tests, being tested by the touchstone of experience. Similarly there is no need for it within the field of mathematic... But in a field in which reason is constrained neither by sensation nor by pure intuition to follow a visible track -- namely, in the field of its transcendent use -- there is much need to discipline reason, so that its tendency to overstep the narrow limits of possible experience may be subdued."
5. Kant's Copernican Revolution
Kant's solution to the problem of the limits of sense experience, and his explanation for the truth of Newton's theory (which clearly exceeded the bounds of sensory experience) was to postulate that "our intellect does not draw its laws from nature, but imposes its laws upon nature". This later became known as the "lamp" theory of the mind, as opposed to the "mirror" theory. This is Copernican in the sense that it puts the observer in the centre of the process of generating knowledge, in the way that Copernicus turned the tables to break the deadlock of cosmology by shifting from a stationary observer and revolving heavens, to envisage that we, the observers, revolve while the heavens stand still.
6. The Doctrine of Autonomy
"Kant makes man the lawgiver of morality just as he makes him the lawgiver of nature. And in so doing he gives back to man his central place both in his moral and in his physical universe. Kant humanised ethics, as he had humanised science... His doctrine of autonomy [states] that we cannot accept the command of an authority, however exalted, as the ultimate basis of ethics... it is our responsibility to judge whether this command is moral or immoral."
He also formulated the doctrine of people as ends in themselves, never to be used merely as the means to the ends of others. He argued that the just state will establish equality in those limitations of the freedom of the people which cannot be avoided "if the freedom of each is to coexist with the freedom of all". He envisaged a league of nations that would establish peace on earth.
"I have tried to sketch in broad outline Kant's philosophy of man and his world, and its two main inspirations -- Newtonian cosmology, and the ethics of freedom; the two inspirations to which Kant referred when he spoke of the starry heavens above and the moral law within us."