[4] This method, accordingly, which we have borrowed from the natural philosopher, consists in seeking for the elements of pure reason in that which admits of confirmation or refutation by experiment. Now the propositions of pure reason, especially when they transcend the limits of possible experience, do not admit of our making any experiment with their objects, as in natural science. Hence, with regard to those conceptions and principles which we assume à priori, our only course will be to view them from two different sides. We must regard one and the same conception, on the one hand, in relation to experience as an object of the senses and of the understanding, on the other hand, in relation to reason, isolated and transcending the limits of experience, as an object of mere thought. Now if we find that, when we regard things from this double point of view, the result is in harmony with the principle of pure reason, but that, when we regard them from a single point of view, reason is involved in self-contradiction, then the experiment will establish the correctness of this distinction.
This attempt succeeds as well as we could desire, and promises to metaphysics, in its first part—that is, where it is occupied with conceptions à priori, of which the corresponding objects may be given in experience—the certain course of science. For by this new method we are enabled perfectly to explain the possibility of à priori cognition, and, what is more, to demonstrate satisfactorily the laws which lie à priori at the foundation of nature, as the sum of the objects of experience—neither of which was possible according to the procedure hitherto followed. But from this deduction of the faculty of à priori cognition in the first part of metaphysics, we derive a surprising result, and one which, to all appearance, militates against the great end of metaphysics, as treated in the second part. For we come to the conclusion that our faculty of cognition is unable to transcend the limits of possible experience; and yet this is precisely the most essential object of this science. The estimate of our rational cognition à priori at which we arrive is that it has only to do with phenomena, and that things in themselves, while possessing a real existence, lie beyond its sphere. Here we are enabled to put the justice of this estimate to the test. For that which of necessity impels us to transcend the limits of experience and of all phenomena is the unconditioned, which reason absolutely requires in things as they are in themselves, in order to complete the series of conditions. Now, if it appears that when, on the one hand, we assume that our cognition conforms to its objects as things in themselves, the unconditioned cannot be thought without contradiction, and that when, on the other hand, we assume that our representation of things as they are given to us, does not conform to these things as they are in themselves, but that these objects, as phenomena, conform to our mode of representation, the contradiction disappears: we shall then be convinced of the truth of that which we began by assuming for the sake of experiment; we may look upon it as established that the unconditioned does not lie in things as we know them, or as they are given to us, but in things as they are in themselves, beyond the range of our cognition.[[5]]
[5] This experiment of pure reason has a great similarity to that of the Chemists, which they term the experiment of reduction, or, more usually, the synthetic process. The analysis of the metaphysician separates pure cognition à priori into two heterogeneous elements, viz., the cognition of things as phenomena, and of things in themselves. Dialectic combines these again into harmony with the necessary rational idea of the unconditioned, and finds that this harmony never results except through the above distinction, which is, therefore, concluded to be just.
But, after we have thus denied the power of speculative reason to make any progress in the sphere of the supersensible, it still remains for our consideration whether data do not exist in practical cognition which may enable us to determine the transcendent conception of the unconditioned, to rise beyond the limits of all possible experience from a practical point of view, and thus to satisfy the great ends of metaphysics. Speculative reason has thus, at least, made room for such an extension of our knowledge: and, if it must leave this space vacant, still it does not rob us of the liberty to fill it up, if we can, by means of practical data—nay, it even challenges us to make the attempt.[[6]]
[6] So the central laws of the movements of the heavenly bodies established the truth of that which Copernicus, first, assumed only as a hypothesis, and, at the same time, brought to light that invisible force (Newtonian attraction) which holds the universe together. The latter would have remained forever undiscovered, if Copernicus had not ventured on the experiment—contrary to the senses but still just—of looking for the observed movements not in the heavenly bodies, but in the spectator. In this Preface I treat the new metaphysical method as a hypothesis with the view of rendering apparent the first attempts at such a change of method, which are always hypothetical. But in the Critique itself it will be demonstrated, not hypothetically, but apodeictically, from the nature of our representations of space and time, and from the elementary conceptions of the understanding.
This attempt to introduce a complete revolution in the procedure of metaphysics, after the example of the geometricians and natural philosophers, constitutes the aim of the Critique of Pure Speculative Reason. It is a treatise on the method to be followed, not a system of the science itself. But, at the same time, it marks out and defines both the external boundaries and the internal structure of this science. For pure speculative reason has this peculiarity, that, in choosing the various objects of thought, it is able to define the limits of its own faculties, and even to give a complete enumeration of the possible modes of proposing problems to itself, and thus to sketch out the entire system of metaphysics. For, on the one hand, in cognition à priori, nothing must be attributed to the objects but what the thinking subject derives from itself; and, on the other hand, reason is, in regard to the principles of cognition, a perfectly distinct, independent unity, in which, as in an organized body, every member exists for the sake of the others, and all for the sake of each, so that no principle can be viewed, with safety, in one relationship, unless it is, at the same time, viewed in relation to the total use of pure reason. Hence, too, metaphysics has this singular advantage—an advantage which falls to the lot of no other science which has to do with objects—that, if once it is conducted into the sure path of science, by means of this criticism, it can then take in the whole sphere of its cognitions, and can thus complete its work, and leave it for the use of posterity, as a capital which can never receive fresh accessions. For metaphysics has to deal only with principles and with the limitations of its own employment as determined by these principles. To this perfection it is, therefore, bound, as the fundamental science, to attain, and to it the maxim may justly be applied:
Nil actum reputans, si quid superesset agendum.
But, it will be asked, what kind of a treasure is this that we propose to bequeath to posterity? What is the real value of this system of metaphysics, purified by criticism, and thereby reduced to a permanent condition? A cursory view of the present work will lead to the supposition that its use is merely negative, that it only serves to warn us against venturing, with speculative reason, beyond the limits of experience. This is, in fact, its primary use. But this, at once, assumes a positive value, when we observe that the principles with which speculative reason endeavours to transcend its limits lead inevitably, not to the extension, but to the contraction of the use of reason, inasmuch as they threaten to extend the limits of sensibility, which is their proper sphere, over the entire realm of thought and, thus, to supplant the pure (practical) use of reason. So far, then, as this criticism is occupied in confining speculative reason within its proper bounds, it is only negative; but, inasmuch as it thereby, at the same time, removes an obstacle which impedes and even threatens to destroy the use of practical reason, it possesses a positive and very important value. In order to admit this, we have only to be convinced that there is an absolutely necessary use of pure reason—the moral use—in which it inevitably transcends the limits of sensibility, without the aid of speculation, requiring only to be insured against the effects of a speculation which would involve it in contradiction with itself. To deny the positive advantage of the service which this criticism renders us would be as absurd as to maintain that the system of police is productive of no positive benefit, since its main business is to prevent the violence which citizen has to apprehend from citizen, that so each may pursue his vocation in peace and security. That space and time are only forms of sensible intuition, and hence are only conditions of the existence of things as phenomena; that, moreover, we have no conceptions of the understanding, and, consequently, no elements for the cognition of things, except in so far as a corresponding intuition can be given to these conceptions; that, accordingly, we can have no cognition of an object, as a thing in itself, but only as an object of sensible intuition, that is, as phenomenon—all this is proved in the analytical part of the Critique; and from this the limitation of all possible speculative cognition to the mere objects of experience, follows as a necessary result. At the same time, it must be carefully borne in mind that, while we surrender the power of cognizing, we still reserve the power of thinking objects, as things in themselves.[[7]] For, otherwise, we should require to affirm the existence of an appearance, without something that appears—which would be absurd. Now let us suppose, for a moment, that we had not undertaken this criticism and, accordingly, had not drawn the necessary distinction between things as objects of experience and things as they are in themselves. The principle of causality, and, by consequence, the mechanism of nature as determined by causality, would then have absolute validity in relation to all things as efficient causes. I should then be unable to assert, with regard to one and the same being, e.g., the human soul, that its will is free, and yet, at the same time, subject to natural necessity, that is, not free, without falling into a palpable contradiction, for in both propositions I should take the soul in the same signification, as a thing in general, as a thing in itself—as, without previous criticism, I could not but take it. Suppose now, on the other hand, that we have undertaken this criticism, and have learnt that an object may be taken in two senses, first, as a phenomenon, secondly, as a thing in itself; and that, according to the deduction of the conceptions of the understanding, the principle of causality has reference only to things in the first sense. 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. Now, it is true that I cannot, by means of speculative reason, and still less by empirical observation, cognize my soul as a thing in itself and consequently, cannot cognize liberty as the property of a being to which I ascribe effects in the world of sense. For, to do so, I must cognize this being as existing, and yet not in time, which—since I cannot support my conception by any intuition—is impossible. At the same time, while I cannot cognize, I can quite well think freedom, that is to say, my representation of it involves at least no contradiction, if we bear in mind the critical distinction of the two modes of representation (the sensible and the intellectual) and the consequent limitation of the conceptions of the pure understanding and of the principles which flow from them. Suppose now that morality necessarily presupposed liberty, in the strictest sense, as a property of our will; suppose that reason contained certain practical, original principles à priori, which were absolutely impossible without this presupposition; and suppose, at the same time, that speculative reason had proved that liberty was incapable of being thought at all. It would then follow that the moral presupposition must give way to the speculative affirmation, the opposite of which involves an obvious contradiction, and that liberty and, with it, morality must yield to the mechanism of nature; for the negation of morality involves no contradiction, except on the presupposition of liberty. Now morality does not require the speculative cognition of liberty; it is enough that I can think it, that its conception involves no contradiction, that it does not interfere with the mechanism of nature. But even this requirement we could not satisfy, if we had not learnt the twofold sense in which things may be taken; and it is only in this way that the doctrine of morality and the doctrine of nature are confined within their proper limits. For this result, then, we are indebted to a criticism which warns us of our unavoidable ignorance with regard to things in themselves, and establishes the necessary limitation of our theoretical cognition to mere phenomena.
[7] In order to cognize an object, I must be able to prove its possibility, either from its reality as attested by experience, or à priori, by means of reason. But I can think what I please, provided only I do not contradict myself; that is, provided my conception is a possible thought, though I may be unable to answer for the existence of a corresponding object in the sum of possibilities. But something more is required before I can attribute to such a conception objective validity, that is real possibility—the other possibility being merely logical. We are not, however, confined to theoretical sources of cognition for the means of satisfying this additional requirement, but may derive them from practical sources.
The positive value of the critical principles of pure reason in relation to the conception of God and of the simple nature of the soul, admits of a similar exemplification; but on this point I shall not dwell. 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. The dogmatism of metaphysics, that is, the presumption that it is possible to advance in metaphysics without previous criticism, is the true source of the unbelief (always dogmatic) which militates against morality.