Kant's influence on Western thought has been profound. Over and above his influence on specific thinkers, Kant changed the framework within which philosophical inquiry has been carried out. He accomplished a paradigm shift: very little philosophy is now carried out in the style of pre-Kantian philosophy. This shift consists in several closely related innovations that have become axiomatic, in philosophy itself and in the social sciences and humanities generally:
- Kant's "Copernican revolution", that placed the role of the human subject or knower at the center of inquiry into our knowledge, such that it is impossible to philosophize about things as they are independently of us or of how they are for us;
- His invention of critical philosophy, that is of the notion of being able to discover and systematically explore possible inherent limits to our ability to know through philosophical reasoning
- His creation of the concept of "conditions of possibility", as in his notion of "the conditions of possible experience" – that is that things, knowledge, and forms of consciousness rest on prior conditions that make them possible, so that, to understand or to know them, we must first understand these conditions
- His theory that objective experience is actively constituted or constructed by the functioning of the human mind
- His notion of moral autonomy as central to humanity
- His assertion of the principle that human beings should be treated as ends rather than as means
Some or all of these Kantian ideas can be seen in schools of thought as different from one another as German Idealism, Marxism, positivism, phenomenology, existentialism, critical theory, linguistic philosophy, structuralism, post-structuralism, and deconstructionism.
During his own life, there was much critical attention paid to his thought. He had an influence on Reinhold, Fichte, Schelling, Hegel, and Novalis during the 1780s and 1790s. The school of thinking known as German Idealism developed from his writings. The German Idealists Fichte and Schelling, for example, tried to bring traditional "metaphysically" laden notions like "the Absolute", "God", and "Being" into the scope of Kant's critical thought. In so doing, the German Idealists tried to reverse Kant's view that we cannot know what we cannot observe.
Hegel was one of Kant's first major critics. In response to what he saw as Kant's abstract and formal account, Hegel brought about an ethic focused on the "ethical life" of the community. But Hegel's notion of "ethical life" is meant to subsume, rather than replace, Kantian ethics. And Hegel can be seen as trying to defend Kant's idea of freedom as going beyond finite "desires", by means of reason. Thus, in contrast to later critics like Nietzsche or Russell, Hegel shares some of Kant's most basic concerns.
Kant's thinking on religion was used in Britain to challenge the decline in religious faith in the nineteenth century. British Catholic writers, notably G. K. Chesterton and Hilaire Belloc, followed this approach. Ronald Englefield debated this movement, and Kant's use of language. See Englefield's article, reprinted in Englefield. Criticisms of Kant were common in the realist views of the new positivism at that time.
Arthur Schopenhauer was strongly influenced by Kant's transcendental idealism. He, like G. E. Schulze, Jacobi, and Fichte before him, was critical of Kant's theory of the thing in itself. Things in themselves, they argued, are neither the cause of what we observe nor are they completely beyond our access. Ever since the first Critique of Pure Reason philosophers have been critical of Kant's theory of the thing in itself. Many have argued, if such a thing exists beyond experience then one cannot posit that it affects us causally, since that would entail stretching the category 'causality' beyond the realm of experience. For a review of this problem and the relevant literature see The Thing in Itself and the Problem of Affection in the revised edition of Henry Allison's Kant's Transcendental Idealism. For Schopenhauer things in themselves do not exist outside the non-rational will. The world, as Schopenhauer would have it, is the striving and largely unconscious will. Michael Kelly, in the preface to his 1910 book Kant's Ethics and Schopenhauer's Criticism, stated: "Of Kant it may be said that what is good and true in his philosophy would have been buried with him, were it not for Schopenhauer...."
With the success and wide influence of Hegel's writings, Kant's influence began to wane, though there was in Germany a movement that hailed a return to Kant in the 1860s, beginning with the publication of Kant und die Epigonen in 1865 by Otto Liebmann. His motto was "Back to Kant", and a re-examination of his ideas began (See Neo-Kantianism). During the turn of the 20th century there was an important revival of Kant's theoretical philosophy, known as the Marburg School, represented in the work of Hermann Cohen, Paul Natorp, Ernst Cassirer, and anti-Neo-Kantian Nicolai Hartmann.
Kant's notion of "Critique" has been quite influential. The Early German Romantics, especially Friedrich Schlegel in his "Athenaeum Fragments", used Kant's self-reflexive conception of criticism in their Romantic theory of poetry. Also in Aesthetics, Clement Greenberg, in his classic essay "Modernist Painting", uses Kantian criticism, what Greenberg refers to as "immanent criticism", to justify the aims of Abstract painting, a movement Greenberg saw as aware of the key limitiaton—flatness—that makes up the medium of painting. French philosopher Michel Foucault was also greatly influenced by Kant's notion of "Critique" and wrote several pieces on Kant for a re-thinking of the Enlightenment as a form of "critical thought". He went so far as to classify his own philosophy as a "critical history of modernity, rooted in Kant".
Kant believed that mathematical truths were forms of synthetic a priori knowledge, which means they are necessary and universal, yet known through intuition. Kant's often brief remarks about mathematics influenced the mathematical school known as intuitionism, a movement in philosophy of mathematics opposed to Hilbert's formalism, and the logicism of Frege and Bertrand Russell.
Influence on modern thinkers
With his Perpetual Peace, Kant is considered to have foreshadowed many of the ideas that have come to form the democratic peace theory, one of the main controversies in political science.
Prominent recent Kantians include the British philosopher P. F. Strawson, the American philosophers Wilfrid Sellars and Christine Korsgaard. Due to the influence of Strawson and Sellars, among others, there has been a renewed interest in Kant's view of the mind. Central to many debates in philosophy of psychology and cognitive science is Kant's conception of the unity of consciousness.
Jürgen Habermas and John Rawls are two significant political and moral philosophers whose work is strongly influenced by Kant's moral philosophy. They have each argued against relativism, supporting the Kantian view that universality is essential to any viable moral philosophy.
Kant's influence also has extended to the social, behavioral, and physical sciences, as in the sociology of Max Weber, the psychology of Jean Piaget, and the linguistics of Noam Chomsky. Kant's work on mathematics and synthetic a priori knowledge is also cited by theoretical physicist Albert Einstein as an early influence on his intellectual development. Because of the thoroughness of the Kantian paradigm shift, his influence extends to thinkers who neither specifically refer to his work nor use his terminology.