Immanuel Kant later said that David Hume had awakened him from his dogmatic slumbers. The problem was not merely that previous philosophy had failed to justify its claims. It was that the new science itself — whose success was undeniable — rested on assumptions (causation, induction, the uniformity of nature, the existence of an external world) that neither rationalism nor empiricism had been able to secure on its own terms.
Kant set himself a task of extraordinary ambition: to explain how synthetic a priori judgments are possible — judgments that are both informative about the world and known independently of experience — and thereby to place both science and morality on a new and more secure foundation. The result was a revolution in philosophy as profound as the one Copernicus had initiated in astronomy.
The Problem Inherited
The rationalists had claimed too much. Descartes, Spinoza, and Leibniz had believed that clear and distinct ideas, or the geometric method applied to first principles, could yield certain knowledge of the ultimate nature of reality. Hume had shown that this confidence was misplaced: we never observe necessary connections, only constant conjunctions; the self is a bundle of perceptions; causation is a habit of the mind.
The empiricists had claimed too little. If all knowledge comes from experience, then the universal and necessary principles that seem to underwrite Newtonian science — every event has a cause, the future will resemble the past, there exists an external world — cannot be justified. They are not analytic truths, and they cannot be derived from experience without circularity.
Kant accepted the empiricist premise that all knowledge begins with experience, but rejected the conclusion that all knowledge arises from experience. Some elements of knowledge, he argued, are contributed by the mind itself.
The Copernican Turn
The key insight is that the mind is not a passive recipient of experience but an active contributor that structures experience through its own forms and categories. Space and time are not features of things in themselves but forms of human intuition — the necessary conditions under which any object can appear to us at all. The categories of the understanding — substance, causality, unity, plurality, and the rest — are not derived from experience but are the rules by which the mind synthesises the manifold of intuition into coherent experience.
This is the “Copernican revolution” in philosophy. Just as Copernicus had explained the apparent motion of the heavens by making the observer move, Kant explained the apparent necessity and universality of certain principles by making the knowing subject the source of the structure that experience exhibits. Objects must conform to our knowledge, rather than our knowledge conforming to objects.
The result is that we can have genuine knowledge of the world as it appears to us — the world of phenomena — but not of things as they are in themselves, the noumena. Metaphysics, understood as the attempt to know the ultimate nature of reality beyond all possible experience, is impossible. But the foundations of Newtonian science are secured, because the principles that science presupposes (causality, substance, the uniformity of nature) are precisely the conditions that make experience possible in the first place. The mind does not invent the world; it supplies the forms without which there would be no world for us to know.
The Moral Law
Kant did not stop with the critique of theoretical reason. In the Critique of Practical Reason and the Groundwork of the Metaphysics of Morals, he developed a parallel argument for the foundations of morality. The moral law, he argued, is not derived from experience, from divine command, or from the pursuit of happiness. It is a fact of reason, known a priori, that presents itself to every rational being as an unconditional command: act only according to that maxim whereby you can at the same time will that it should become a universal law.
This categorical imperative is the expression of the autonomy of the rational will. To act morally is to act from duty, out of respect for the law that reason gives to itself. The moral agent is not determined by inclination or by external authority; he is determined by the law he legislates for himself as a member of the kingdom of ends. The moral law is universal because it is the law of reason itself; it is unconditional because it does not depend on any desire or interest.
The price of this autonomy is the restriction of theoretical knowledge. We cannot know that we are free, that the soul is immortal, or that God exists, as objects of theoretical cognition. But we can and must postulate these as conditions of the moral life. The moral law commands us to pursue the highest good — happiness in proportion to virtue — and this command would be meaningless if the conditions for its fulfilment were impossible. Reason, in its practical employment, thus points beyond the limits that theoretical reason has established.
The Limits and the Legacy
Kant’s philosophy is a philosophy of limits. It sets strict boundaries to what can be known, what can be said, and what can be hoped. But within those limits it offers a powerful vindication of both the scientific enterprise and the moral life. Science is possible because the mind structures experience according to principles that are universal and necessary for any finite rational being. Morality is possible because reason is practical — because it can determine the will independently of inclination.
The subsequent history of philosophy is in large measure a series of responses to Kant. The German Idealists — Fichte, Schelling, Hegel — would attempt to overcome the distinction between phenomena and noumena and to complete the system that Kant had left unfinished. The empiricists and positivists of the nineteenth and twentieth centuries would accept the limits on metaphysics while rejecting the synthetic a priori as a disguised form of rationalism. Phenomenology and existentialism would explore the structures of lived experience that Kant had treated as formal conditions. Analytic philosophy would inherit the question of how language and logic relate to the world that Kant had posed in a new key.