Immanuel Kant (22 April 1724 – 12 February 1804) was an 18th-century German philosopher from the Prussian city of Königsberg. Kant was the last influential philosopher of modern Europe in the classic sequence of the theory of knowledge during the Enlightenment beginning with thinkers John Locke, George Berkeley, and David Hume. Kant created a new perspective in philosophy which had widespread influences on philosophy continuing through to the 21st century. He published important works on epistemology, as well as works relevant to religion, law, ####and history. One of his most prominent works is the Critique of Pure Reason, an investigation into the limitations and structure of reason itself. It encompasses an attack on traditional metaphysics and epistemology, and highlights Kant's own contribution to these areas. The other main works of his maturity are the Critique of Practical Reason, which concentrates on ethics, and the Critique of Judgment, which investigates aesthetics and teleology.
Kant suggested that metaphysics can be reformed through epistemology. He suggested that by understanding the sources and limits of human knowledge we can ask fruitful metaphysical questions. He asked if an object can be known to have certain properties prior to the experience of that object. He concluded that all objects about which the mind can think must conform to its manner of thought. Therefore if the mind can think only in terms of causality – which he concluded that it does – then we can know prior to experiencing them that all objects we experience must either be a cause or an effect. However, it follows from this that it is possible that there are objects of such nature which the mind cannot think, and so the principle of causality, for instance, cannot be applied outside of experience: hence we cannot know, for example, whether the world always existed or if it had a cause. And so the grand questions of speculative metaphysics cannot be answered by the human mind, but the sciences are firmly grounded in laws of the mind. Research on the structure and development of the brain in animals supports Kant's theory, at least for the perception of space; that is, spatial representation of the environment includes an innate component that predates any actual perception of the environment itself.
Kant believed himself to be creating a compromise between the empiricists and the rationalists. The empiricists believed that knowledge is acquired through experience alone, but the rationalists maintained that such knowledge is open to Cartesian doubt and that reason alone provides us with knowledge. Kant argues, however, that using reason without applying it to experience will only lead to illusions, while experience will be purely subjective without first being subsumed under pure reason.
Kant’s thought was very influential in Germany during his lifetime, moving philosophy beyond the debate between the rationalists and empiricists. The philosophers Fichte, Schelling, Hegel and Schopenhauer each saw themselves as correcting and expanding the Kantian system, thus bringing about various forms of German idealism. Kant continues to be a major influence on philosophy, influencing both analytic and continental philosophy.
In Kant's essay "Answering the Question: What is Enlightenment?", Kant defined the Enlightenment as an age shaped by the Latin motto Sapere aude ("Dare to Know"). Kant maintained that one ought to think autonomously, free of the dictates of external authority. His work reconciled many of the differences between the rationalist and empiricist traditions of the 18th century. He had a decisive impact on the Romantic and German Idealist philosophies of the 19th century. His work has also been a starting point for many 20th century philosophers.
The sense of an enlightened approach and the critical method required that "If one cannot prove that a thing is, he may try to prove that it is not. And if he succeeds in doing neither (as often occurs), he may still ask whether it is in his interest to accept one or the other of the alternatives hypothetically, from the theoretical or the practical point of view. Hence the question no longer is as to whether perpetual peace is a real thing or not a real thing, or as to whether we may not be deceiving ourselves when we adopt the former alternative, but we must act on the supposition of its being real." The presupposition of God, soul, and freedom was then a practical concern, for "Morality, by itself, constitutes a system, but happiness does not, unless it is distributed in exact proportion to morality. This, however, is possible in an intelligible world only under a wise author and ruler. Reason compels us to admit such a ruler, together with life in such a world, which we must consider as future life, or else all moral laws are to be considered as idle dreams… ."