Hume criticized the Theory of Ideas of his fellow British empiricists John Locke and George Berkeley. If knowledge is limited to perceptions of sense data, we cannot "know" anything about external objects, even our own bodies. Hume said that we could have a natural belief in the external world and causal laws.

"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, B XXX)

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, B XXVIII)

If Kant's Critique of Pure Reason can be seen as a reaction to David Hume's skeptical attitude toward knowledge that depends on sense data, the parallel between Hume and Kant is even stronger in Kant's Critique of Practical Reason.

Hume and Kant both sought a reconciling of freedom and necessity or causality.

Where Hume said we could not reason to knowledge of causality, for example, but could have a natural belief in causality because of our moral sentiments and feelings, so Kant claims that his Practical Reason establishes freedom in a noumenal realm whose grounding principle is morality. Freedom is the condition for the moral law.

"Freedom, however, is the only one of all the ideas of the speculative reason of which we know the possibility a priori (without, however, understanding it), because it is the condition of the moral law which we know." (Critique of Practical Reason, p.329)

In an early letter to a friend, Kant described the workings of his mind as involving chance, and in terms that sound remarkably like our Cogito model, - "The mind must...lie open to any chance suggestion which may present itself." He described his method...

"In mental labour of so delicate a character nothing is more harmful than preoccupation with extraneous matters. The mind, though not constantly on the stretch, must still, alike in its idle and in its favourable moments, lie uninterruptedly open to any chance suggestion which may present itself. Relaxations and diversions must maintain its powers in freedom and mobility, so that it may be enabled to view the object afresh from every side, and so to enlarge its point of view from a microscopic to a universal outlook that it adopts in turn every conceivable standpoint, verifying the observations of each by means of all the others."¹

1. Letter to Marcus Herz, February 21, 1772, Werke, x, p.127
(cited by Norman Kemp Smith, Commentary to Kant's Critique of Pure Reason, p. xxii)

Kant's command of Newtonian physics was greater than Hume's admiration of it as a model for his human nature. He asked how we could call a man free while he is subject to physical necessity. He thought that ideas could be determined (bestimmt) by psychological forces just as as the motion of matter is determined by mechanical forces.

how can a man be called quite free at the same moment, and with respect to the same action in which he is subject to an inevitable physical necessity?

Comparative freedom (cf., the compatibilism of Hobbes and Hume) as being determined by internal causes, without external constraints
(freedom of action)

Some try to evade this by saying that the causes that determine his causality are of such a kind as to agree with a comparative notion of freedom. According to this, that is sometimes called a free effect, the determining physical cause of which lies within the acting thing itself, e. g., that which a projectile performs when it is in free motion, in which case we use the word freedom, because while it is in flight it is not urged by anything external; or as we call the motion of a clock a free motion, because it moves its hands itself, which therefore do not require to be pushed by external force; so although the actions of man are necessarily determined by causes which precede in time, we yet call them free, because these causes are ideas produced by our own faculties, whereby desires are evoked on occasion of circumstances, and hence actions are wrought according to our own pleasure.

Locke had warned that the words we use could be a source of confusion. Kant accuses philosophers of juggling words

This is a wretched subterfuge ("miserable substitute" is a better translation of ein elender Behelf, but the English phrase is now famous in philosophy) with which some persons still let themselves be put off, and so think they have solved, with a petty word-jugglery (again, "a little quibbling" is better for einer kleinen Wortklauberei), that difficult problem, at the solution of which centuries have laboured In vain, and which can therefore scarcely be found so completely on the surface.

An internal causal chain is just as deterministic as an external chain

In fact, in the question about the freedom which must be the foundation of all moral laws and the consequent responsibility, it does not matter whether the principles which necessarily determine causality by a physical law reside within the subject or without him, or in the former case whether these principles are instinctive or are conceived by reason, if, as is admitted by these men themselves, these determining ideas have the ground of their existence in time and in the antecedent state, and this again in an antecedent. etc.

Psychological freedom is not free, just because it is not mechanicaly caused, if there is an internal determining principle, a causal chain of mental events. Transcendental freedom is independent of the empirical.

Then it matters not that these are internal; it matters not that they have a psychological and not a mechanical causality, that is, produce actions by means of ideas and not by bodily movements; they are still determining principles of the causality of a being whose existence is determinable in time, and therefore under the necessitation of conditions of past time, which therefore, when the subject has to act, are no longer in his power. This may imply psychological freedom (if we choose to apply this term to a merely internal chain of ideas in the mind), but it involves physical necessity and, therefore, leaves no room for transcendental freedom, which must be conceived as independence on everything empirical, and, consequently, on nature generally, whether it is an object of the internal sense considered in time only, or of the exterlnal in time and space. Without this freedom (in the latter and true sense), which alone is practical a priori, no moral law and no moral imputation are possible. Just for this reason the necessity of events in time, according to the physical law of causality, may be called the mechanism of nature, although we do not mean by this that things which are subject to it must be really material machines. We look here only to the necessity of the connection of events in a time-series as it is developed according to the physical law,

Kant says ideas can be determined by psychological forces, just as matter is determined by mechanical forces, without some absolute transcendental free element

whether the subject in which this development takes place is called automaton materiale when the mechanical being is moved by matter, or with Leibnitz spirituale when it is impelled by ideas; and if the freedom of our will were no other than the latter (say the psychological and comparative, not also transcendental, that is, absolute), then it would at bottom be nothing better than the freedom of a turnspit, which, when once it is wound up, accomplishes its motions of itself.

(Critique of Practical Reason, "Examination of the Analytic of Pure Practical Reason", Great Books, v. 42, p.332)

"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."