Kant reacted to the Enlightenment, to the Age of Reason, and to Newtonian mechanics (which he probably understood better than any other philosopher), by accepting determinism as a fact in the physical world, which he calls the phenomenal world. Kant's goal was to rescue the physical sciences from the devastating and unanswerable skepticism of David Hume, especially Hume's assertion that no number of "constant conjunctions" of cause and effect could logically prove causality. Kant called this assertion the "crux metaphysicorum." If Hume is right, he said, metaphysics is impossible. Perhaps even knowledge is impossible? Kant's Critique of Pure Reason was to prove that Hume was wrong. Hume criticized the Theory of Ideas of his fellow British empiricists John Locke and George Berkeley. If, as they claim, knowledge is limited to perceptions of sense data, we cannot "know" anything about external objects, even our own bodies. But Hume said that we do have a natural belief in the external world and causal laws. Hume's idea of the mind having a "feeling" (not a reason) that leads to natural beliefs became Kant's "second Copernican revolution" that the mind projects "concepts of the understanding" and "forms of perception" on the external world.Kant put limits on what we can know by pure speculative Reason, in order to make room for Belief in a timeless noumenal (or mental) world of "analytic," a priori truths that include God, freedom, and immortality.
Kant's main change in the second edition of the Critique of Pure Reason was an attempted refutation of this British idealism (B 274). He thought he had a proof of the existence of the external world. Kant thought it a scandal in philosophy that we must accept the existence of material things outside ourselves merely as a belief, with no proof.
The only thing which might be called an addition, though in the method of proof only, is the new refutation of psychological idealism, and the strict (and as I believe the only possible) proof of the objective reality of outer intuition. However innocent idealism may be considered with respect to the essential purposes of metaphysics (without being so in reality), it remains a scandal to philosophy, and to human reason in general, that we should have to accept the existence of things outside us (from which after all we derive the whole material for our knowledge, even for that of our inner sense) merely on trust, and have no satisfactory proof with which to counter any opponent who chooses to doubt it.Martin Heidegger commented on Kant's scandal:
The 'scandal of philosophy' is not that this proof has yet to be given, but that such proofs are expected and attempted again and again.
The Dialectical Antinomy of CausalityIn Kant's 1781 Critique of Pure Reason, Transcendental Doctrine of Elements, Second Part: Transcendental Logic, Second Division: Transcendental Dialectic, Book II: Of the Dialectical Procedure of Pure Reason, Chapter II: The Antinomy of Pure Reason, Section II: The Antithetic of Pure Reason, Third Conflict of the Transcendental Ideas (his third Antinomy), Kant writes
THIRD CONFLICT OF THE TRANSCENDENTAL IDEAS. THESIS. Causality according to the laws of nature, is not the only causality operating to originate the phenomena of the world. A causality of freedom is also necessary to account fully for these phenomena. PROOF. Let it be supposed, that there is no other kind of causality than that according to the laws of nature. Consequently, everything that happens presupposes a previous condition, which it follows with absolute certainty, in conformity with a rule. But this previous condition must itself be something that has happened (that has arisen in time, as it did not exist before), for, if it has always been in existence, its consequence or effect would not thus originate for the first time, but would likewise have always existed. The causality, therefore, of a cause, whereby something happens, is itself a thing that has happened. Now this again presupposes, in conformity with the law of nature, a previous condition and its causality, and this another anterior to the former, and so on. If, then, everything happens solely in accordance with the laws of nature, there cannot be any real first beginning of things, but only a subaltern or comparative beginning. There cannot, therefore, be a completeness of series on the side of the causes which originate the one from the other. But the law of nature is that nothing can happen without a sufficient à priori determined cause. The proposition therefore—if all causality is possible only in accordance with the laws of nature—is, when stated in this unlimited and general manner, self-contradictory. It follows that this cannot be the only kind of causality. From what has been said, it follows that a causality must be admitted, by means of which something happens, without its cause being determined according to necessary laws by some other cause preceding. That is to say, there must exist an absolute spontaneity of cause, which of itself originates a series of phenomena which proceeds according to natural laws—consequently transcendental freedom, without which even in the course of nature the succession of phenomena on the side of causes is never complete. ANTITHESIS. There is no such thing as freedom, but everything in the world happens solely according to the laws of nature. PROOF. Granted, that there does exist freedom in the transcendental sense, as a peculiar kind of causality, operating to produce events in the world—a faculty, that is to say, of originating a state, and consequently a series of consequences from that state. In this case, not only the series originated by this spontaneity, but the determination of this spontaneity itself to the production of the series, that is to say, the causality itself must have an absolute commencement, such that nothing can precede to determine this action according to unvarying laws. But every beginning of action presupposes in the acting cause a state of inaction; and a dynamically primal beginning of action presupposes a state, which has no connection—as regards causality—with the preceding state of the cause—which does not, that is, in any wise result from it. Transcendental freedom is therefore opposed to the natural law of cause and effect, and such a conjunction of successive states in effective causes is destructive of the possibility of unity in experience and for that reason not to be found in experience—is consequently a mere fiction of thought. We have, therefore, nothing but nature to which we must look for connection and order in cosmical events. Freedom—independence of the laws of nature—is certainly a deliverance from restraint, but it is also a relinquishing of the guidance of law and rule. For it cannot be alleged that, instead of the laws of nature, laws of freedom may be introduced into the causality of the course of nature. For, if freedom were determined according to laws, it would be no longer freedom, but merely nature. Nature, therefore, and transcendental freedom are distinguishable as conformity to law and lawlessness. The former imposes upon understanding the difficulty of seeking the origin of events ever higher and higher in the series of causes, inasmuch as causality is always conditioned thereby; while it compensates this labour by the guarantee of a unity complete and in conformity with law. The latter, on the contrary, holds out to the understanding the promise of a point of rest in the chain of causes, by conducting it to an unconditioned causality, which professes to have the power of spontaneous origination, but which, in its own utter blindness, deprives it of the guidance of rules, by which alone a completely connected experience is possible. OBSERVATIONS ON THE THIRD ANTINOMY. ON THE THESIS. The transcendental idea of freedom is far from constituting the entire content of the psychological conception so termed, which is for the most part empirical. It merely presents us with the conception of spontaneity of action, as the proper ground for imputing freedom to the cause of a certain class of objects. It is, however, the true stumbling-stone to philosophy, which meets with unconquerable difficulties in the way of its admitting this kind of unconditioned causality. That element in the question of the freedom of the will, which has for so long a time placed speculative reason in such perplexity, is properly only transcendental, and concerns the question, whether there must be held to exist a faculty of spontaneous origination of a series of successive things or states. How such a faculty is possible is not a necessary inquiry; for in the case of natural causality itself, we are obliged to content ourselves with the à priori knowledge that such a causality must be presupposed, although we are quite incapable of comprehending how the being of one thing is possible through the being of another, but must for this information look entirely to experience. Now we have demonstrated this necessity of a free first beginning of a series of phenomena, only in so far as it is required for the comprehension of an origin of the world, all following states being regarded as a succession according to laws of nature alone. But, as there has thus been proved the existence of a faculty which can of itself originate a series in time—although we are unable to explain how it can exist—we feel ourselves authorized to admit, even in the midst of the natural course of events, a beginning, as regards causality, of different successions of phenomena, and at the same time to attribute to all substances a faculty of free action. But we ought in this case not to allow ourselves to fall into a common misunderstanding, and to suppose that, because a successive series in the world can only have a comparatively first beginning—another state or condition of things always preceding—an absolutely first beginning of a series in the course of nature is impossible. For we are not speaking here of an absolutely first beginning in relation to time, but as regards causality alone. When, for example, I, completely of my own free will, and independently of the necessarily determinative influence of natural causes, rise from my chair, there commences with this event, including its material consequences in infinitum, an absolutely new series; although, in relation to time, this event is merely the continuation of a preceding series. For this resolution and act of mine do not form part of the succession of effects in nature, and are not mere continuations of it; on the contrary, the determining causes of nature cease to operate in reference to this event, which certainly succeeds the acts of nature, but does not proceed from them. For these reasons, the action of a free agent must be termed, in regard to causality, if not in relation to time, an absolutely primal beginning of a series of phenomena. The justification of this need of reason to rest upon a free act as the first beginning of the series of natural causes is evident from the fact, that all philosophers of antiquity (with the exception of the Epicurean school) felt themselves obliged, when constructing a theory of the motions of the universe, to accept a prime mover, that is, a freely acting cause, which spontaneously and prior to all other causes evolved this series of states. They always felt the need of going beyond mere nature, for the purpose of making a first beginning comprehensible. ON THE ANTITHESIS. The assertor of the all-sufficiency of nature in regard to causality (transcendental Physiocracy), in opposition to the doctrine of freedom, would defend his view of the question somewhat in the following manner. He would say, in answer to the sophistical arguments of the opposite party: If you do not accept a mathematical first, in relation to time, you have no need to seek a dynamical first, in regard to causality. Who compelled you to imagine an absolutely primal condition of the world, and therewith an absolute beginning of the gradually progressing successions of phenomena—and, as some foundation for this fancy of yours, to set bounds to unlimited nature? Inasmuch as the substances in the world have always existed—at least the unity of experience renders such a supposition quite necessary—there is no difficulty in believing also, that the changes in the conditions of these substances have always existed; and, consequently, that a first beginning, mathematical or dynamical, is by no means required. The possibility of such an infinite derivation, without any initial member from which all the others result, is certainly quite incomprehensible. But, if you are rash enough to deny the enigmatical secrets of nature for this reason, you will find yourselves obliged to deny also the existence of many fundamental properties of natural objects (such as fundamental forces), which you can just as little comprehend; and even the possibility of so simple a conception as that of change must present to you insuperable difficulties. For if experience did not teach you that it was real, you never could conceive à priori the possibility of this ceaseless sequence of being and non-being. But if the existence of a transcendental faculty of freedom is granted—a faculty of originating changes in the world—this faculty must at least exist out of and apart from the world; although it is certainly a bold assumption, that, over and above the complete content of all possible intuitions, there still exists an object which cannot be presented in any possible perception. But, to attribute to substances in the world itself such a faculty, is quite inadmissible; for, in this case; the connection of phenomena reciprocally determining and determined according to general laws, which is termed nature, and along with it the criteria of empirical truth, which enable us to distinguish experience from mere visionary dreaming, would almost entirely disappear. In proximity with such a lawless faculty of freedom, a system of nature is hardly cogitable; for the laws of the latter would be continually subject to the intrusive influences of the former, and the course of phenomena, which would otherwise proceed regularly and uniformly, would become thereby confused and disconnected.
Putting Limits on ReasonNeither Hume's Idea of "natural belief" nor Kant's "concepts of the understanding" are the apodictic and necessary truths sought by metaphysicians. They are both abstract theories about the world, whose information content is validated by experiments.
"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."Kant's noumenal world outside of space and time 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 forms of sensible perception, like space and time, and built-in categories or concepts of intelligible understanding.
"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)