Kant is most famous for his ideas on transcendental idealism that we bring innate forms and concepts to the raw experience of the world, which otherwise would be completely unknowable. Kant's philosophy of nature and human nature was both immediately controversial and very durable in its influence. Kant provided both a summation of many of the currents of his own time, and a challenge for philosophy in the future to connect rational with empirical and moral philosophy.

Life of Immannuel Kant
Kant was born, lived and died in Königsberg (at the time a town in Prussia; today it is the town of Kaliningrad in Russia). He spent much of his youth as a solid, albeit unspectacular, student living more off playing pool than his writings. He lived a very regulated life: the walk he took at three-thirty every afternoon was so punctual that local housewives would set their clocks by him. He never married and he owned only one piece of art in his household, advocating the absence of passion in favor of logic so that he may better serve. He never left Prussia, and rarely stepped outside his own home town. However, despite his reputation of being a solitary man, he was considered a very sociable person: he would regularly have guests over for dinner, insisting that sociable company was good for his constitution, as was laughter. Kant was a respected and competent university professor for most of his life, although he was in his late fifties before he did anything that would bring him historical repute.

He entered the local university in 1740, and studied the philosophy of Leibniz and Christian Wolff under Martin Knutsen, a follower of Wolff. He also studied the then new mathematics of Sir Isaac Newton. In 1746 he wrote a paper on measurement, reflecting Leibniz's influence. He, at the same time, absorbed pietism as a basic part of his make up. Different scholars hold different views on the importance of each of these aspects, for Paul Guyer, and many others, it is rationalism which is the most important element - in this view Kant is seen as a philosopher, like many others, trying to replace Wolffian rationalism with an empiricism drawn from Hume and others.

In 1755 he became a private lecturer at the University, and while there published "Inquiry into the Distinctness of the Principles of Natural Theology and Morals", where he examined the problem of having a logical system of philosophy that connected with the world of natural philosophy, a concern typical of The Enlightenment period, indeed, Kant left one of the most influential definitions of Aufklärung, or enlightenment, in philosophy. In 1763 he wrote The Only Possible Ground of Proof for a Demonstration of God's Existence, which questioned the Anslemic ontological argument for God: essentially, that the idea of the greatest of all possible ideas proves that the idea exists. René Descartes had used this argument in his philosophy, as had others after him.

Having questioned both the principle of contradiction - that the seeming opposite of a false idea must be true - and the ontological proof of God - Kant had attacked the fundamental tools of axiomic rational philosophy, but, as yet, he had nothing to replace them with.

He was of the rather curious conviction that a person did not have a firm direction in life until their thirty-ninth year; when this came and passed and he was just a minor metaphysician in a Prussian University a brief mid-life crisis ensued; perhaps it can be credited with some of his later direction. In 1770, he became a full professor, and began reading the works of David Hume. Hume was fiercely empirical, scorned all metaphysics, and systematically debunked great quantities of it. His most famous thesis is that nothing in our experience can justify our assuming that there are "causal powers" inherent in things—that, for example, when one billiard ball strikes another, how can we assume the second one "must" move. Of course, things have always happened this way, and through "custom and habit" we tend to assume they will continue to do so, even though we have no rational grounds for the assumption. He simultaneously found Hume's argument irrefutable and his conclusions unacceptable.

"It was this that roused me from my slumber", he would later write. For the next 10 years he worked on the architecture of his own philosophy, beginning with what he called "the scandal of reality", that there was no philosophical proof of the outside world. During this period he published nothing, and then, in 1781, he released the massive Critique of Pure Reason, one of the most widely argued over, widely cited - and widely influential works in Western Philosophy. He followed this with Groundwork of the Metaphysics of Morals, and then in 1785 Critique of Practical Reason and in 1790, Critique of Judgement. The effect was immediate in the German speaking world, with readership including Ludwig van Beethoven and Johann Wolfgang von Goethe. But the attention was far from universally approving: on the contrary, almost every aspect of the works were attacked and criticized fiercely, particularly his ideas on categories, the place of free will and determinism and particularly on the knowledge of the outside world. His early critics included Johann Schaumann, Friedrich Hienrich Jacobi and Hermann Pistorius. Pistorius' criticisms were particularly influential and are still cited in contra-Kantian arguments.

The Critique of Practical Reason dealt with morality, or action, in the same way that the first Critique dealt with knowledge, and the Critique of Judgement dealt with the various uses of our mental powers that neither confer factual knowledge nor determine us to action, such as aesthetic judgment, for example of the beautiful and sublime, and teleological judgment , that is construing things as having "purposes".

As Kant understood them, aesthetic and teleological judgment connected our moral and empirical judgments to one another, unifying his system.

Two shorter works, the Prolegomena to any Future Metaphysics and the Groundwork to the Metaphysics of Morals treated the same matter as the first and second critiques respectively, in a more cursory form—assuming the answer and working backward, so to speak. They serve as his introductions to the critical system. The epistemological material of the first Critique was put into application in the Metaphysical Foundations of Natural Science; the ethical dictums of the second were put into practice in Metaphysics of Morals.

Aside from this Kant wrote a number of semi-popular essays on history, politics, and the application of philosophy to life. When he died he was working on a projected "fourth critique", having come to the conviction that his system was incomplete; this incomplete manuscript has been published as Opus Postumum. Kant died in 1804.

See also Famous Immanuel Kant Quotations