Kant reacted to the Enlightenment, to the Age of Reason, and to Newtonian mechanics (which he understood better than any other philosopher), by accepting determinism as a fact in the physical world, which he called the phenomenal world. He then put limits on what we can know by pure speculative Reason, in order to make room for belief in a timeless noumenal world that includes God, freedom, and immortality.

Kant's noumenal world is a variation on Plato's concept of Soul, Descartes' mental world, and the Scholastic idea of a world in which all times are present to the eye of God. His idea of free will is a most esoteric form of compatibilism. Our decisions are made in our souls outside of time and only appear determined to our senses, which are governed by our built-in a priori categories of understanding, like space and time.

"We then see how it does not involve any contradiction to assert, on the one hand, that the will, in the phenomenal sphere − in visible action − is necessarily obedient to the law of nature, and, in so far, not free; and, on the other hand, that, as belonging to a thing in itself, it is not subject to that law, and, accordingly, is free." (Preface to Second Edition, Critique of Practical Reason)

"I cannot even make the assumption− as the practical interests of morality require− of God, freedom, and immortality, if I do not deprive speculative reason of its pretensions to transcendent insight. For to arrive at these, it must make use of principles which, in fact, extend only to the objects of possible experience, and which cannot be applied to objects beyond this sphere without converting them into phenomena, and thus rendering the practical extension of pure reason impossible. I must therefore, abolish knowledge, to make room for belief." (Preface to Second Edition, Critique of Practical Reason)

As early as 1784, in his Idea for a Universal History with a Cosmopolitan Intent, Kant saw lack of free will in the statistical regularities of social data on births, deaths, and marriages.

No matter what conception may form of the freedom of the will in metaphysics, the phenomenal appearances of the will, i.e., human actions, are determined by general laws of nature like any other event of nature. History is concerned with telling about these events. History allows one to hope that when history considers in the large the play of the freedom of human will, it will be possible to discover the regular progressions thereof. Thus (it is to be hoped) that what appears to be complicated and accidental in individuals, may yet be understood as a steady, progressive, though slow, evolution of the original endowments of the entire species. Thus marriages, the consequent births and the deaths, since the free will seems to have such a great influence on them, do not seem to be subject to any law according to which one could calculate their number beforehand. Yet the annual (statistical) tables about them in the major countries show that they occur according to stable natural laws. It is like the erratic weather the occurrence of which cannot be determined in particular instances, although it never fails in maintaining the growth of plants, the flow of streams, and other of nature's arrangements at a uniform, uninterrupted pace. Individual human beings, each pursuing his own ends according to his inclination and often one against another (and even one entire people against another) rarely unintentionally promote, as if it were their guide, an end of nature which is unknown to them. They thus work to promote that which they would care little for if they knew about it.

Since men in their endeavors do not act like animals merely according to instinct, nor like rational citizens according to an agreed plan, no planned history seems to be possible (as in the case of bees and beavers). It is hard to suppress a certain disgust when contemplating men's action upon the world stage. For one finds, in spite of apparent wisdom in detail that everything, taken as a whole, is interwoven with stupidity, childish vanity, often with childish viciousness and destructiveness. In the end, one does not know what kind of conception one should have of our species which is so conceited about its superior qualities. Since the philosopher must assume that men have a flexible purpose of their own, it is left to him to attempt to discover an end of nature in this senseless march of human events. A history of creatures who proceed without a plan would be possible in keeping with such an end; the history would proceed according to such an end of nature.