It took a much longer time before physics entered on the high way of science: for no more than a century and a half has elapsed, since Bacon's ingenious proposal partly initiated that discovery, partly, as others were already on the right track, gave a new impetus to it,—a discovery which, like the former, can only be explained by a rapid intellectual revolution. In what I have to say, I shall confine myself to natural science, so far as it is founded on empirical principles.
When Galilei let balls of a particular weight, which he had determined himself, roll down an inclined plain, or Torricelli made the air carry a weight, which he had previously determined to be equal to that of a definite volume of water; or when, in later times, Stahl2 changed metal into lime, and lime again into metals, by withdrawing and restoring something, a new light flashed on all students of nature. They comprehended that reason has insight into that only, which she herself produces on her own plan, and that she must move forward with the principles of her judgments, according to fixed law, and compel nature to answer her questions, but not let herself be led by nature, as it were in leading strings, because otherwise accidental observations, made on no previously fixed plan, will never converge towards a necessary law, which is the only thing that reason seeks and requires. Reason, holding in one hand its principles, according to which concordant phenomena alone can be admitted as laws of nature, and in the other hand the experiment, which it has devised according to those principles, must approach nature, in order to be taught by it: but not in the character of a pupil, who agrees to everything the master likes, but as an appointed judge, who compels the witnesses to answer the questions which he himself proposes. Therefore even the science of physics entirely owes the beneficial revolution in its character to the happy thought, that we ought to seek in nature (and not import into it by means of fiction) whatever reason must learn from nature, and could not know by itself, and that we must do this in accordance with what reason itself has originally placed into nature. Thus only has the study of nature entered on the secure method of a science, after having for many centuries done nothing but grope in the dark.
Metaphysic, a completely isolated and speculative science of reason, which declines all teaching of experience, and rests on concepts only (not on their application to intuition, as mathematics), in which reason therefore is meant to be her own pupil, has hitherto not been so fortunate as to enter on the secure path of a science, although it is older than all other sciences, and would remain, even if all the rest were swallowed up in the abyss of an all-destroying barbarism. In metaphysic, reason, even if it tries only to understand a priori (as it pretends to do) those laws which are confirmed by the commonest experience, is constantly brought to a standstill, and we are obliged again and again to retrace our steps, because they do not lead us where we want to go; while as to any unanimity among those who are engaged in the same work, there is so little of it in metaphysic, that it has rather become an arena, specially destined, it would seem, for those who wish to exercise themselves in mock fights, and where no combatant has, as yet, succeeded in gaining an inch of ground that he could call permanently his own. It cannot be denied, therefore, that the method of metaphysic has hitherto consisted in groping only, and, what is the worst, in groping among mere concepts.
What then can be the cause that hitherto no secure method of science has been discovered? Shall we say that it is impossible? Then why should nature have visited our reason with restless aspiration to look for it, as if it were its most important concern? Nay more, how little should we be justified in trusting our reason if, with regard to one of the most important objects we wish to know, it not only abandons us, but lures us on by vain hopes, and in the end betrays us! Or, if hitherto we have only failed to meet with the right path, what indications are there to make us hope that, if we renew our researches, we shall be more successful than others before us?
The examples of mathematics and natural science, which by one revolution have become what they now are, seem to me sufficiently remarkable to induce us to consider, what may have been the essential element in that intellectual revolution which has proved so beneficial to them, and to make the experiment, at least, so far as the analogy between them, as sciences of reason, with metaphysic allows it, of imitating them. Hitherto it has been supposed that all our knowledge must conform to the objects: but, under that supposition, all attempts to establish anything about them a priori, by means of concepts, and thus to enlarge our knowledge, have come to nothing. The experiment therefore ought to be made, whether we should not succeed better with the problems of metaphysic, by assuming that the objects must conform to our mode of cognition, for this would better agree with the demanded possibility of an a priori knowledge of them, which is to settle something about objects, before they are given us. We have here the same case as with the first thought of Copernicus, who, not being able to get on in the explanation of the movements of the heavenly bodies, as long as he assumed that all the stars turned round the spectator, tried, whether he could not succeed better, by assuming the spectator to be turning round, and the stars to be at rest. A similar experiment may be tried in metaphysic, so far as the intuition of objects is concerned. If the intuition had to conform to the constitution of objects, I do not see how we could know anything of it a priori; but if the object (as an object of the senses) conforms to the constitution of our faculty of intuition, I can very well conceive such a possibility. As, however, I cannot rest in these intuitions, if they are to become knowledge, but have to refer them, as representations, to something as their object, and must determine that object by them, I have the choice of admitting, either that the concepts, by which I carry out that determination, conform to the object, being then again in the same perplexity on account of the manner how I can know anything about it a priori; or that the objects, or what is the same, the experience in which alone they are known (as given objects), must conform to those concepts. In the latter case, the solution becomes more easy, because experience, as a kind of knowledge, requires under standing, and I must therefore, even before objects are given to me, presuppose the rules of the understanding as existing within me a priori, these rules being expressed in concepts a priori, to which all objects of experience must necessarily conform, and with which they must agree. With regard to objects, so far as they are conceived by reason only, and conceived as necessary, and which can never be given in experience, at least in that form in which they are conceived by reason, we shall find that the attempts at conceiving them (for they must admit of being conceived) will furnish afterwards an excellent test of our new method of thought, according to which we do not know of things anything a priori except what we ourselves put into them.3
This experiment succeeds as well as we could desire, and promises to metaphysic, in its first part, which deals with concepts a priori, of which the corresponding objects may be given in experience, the secure method of a science. For by thus changing our point of view, the possibility of knowledge a priori can well be explained, and, what is still more, the laws which a priori lie at the foundation of nature, as the sum total of the objects of experience, may be supplied with satisfactory proofs, neither of which was possible with the procedure hitherto adopted. But there arises from this deduction of our faculty of knowing a priori, as given in the first part of metaphysic, a somewhat startling result, apparently most detrimental to the objects of metaphysic that have to be treated in the second part, namely, the impossibility of going with it beyond the frontier of possible experience, which is precisely the most essential purpose of metaphysical science. But here we have exactly the experiment which, by disproving the opposite, establishes the truth of our first estimate of the knowledge of reason a priori, namely, that it can refer to phenomena only, but must leave the thing by itself as unknown to us, though as existing by itself. For that which impels us by necessity to go beyond the limits of experience and of all phenomena, is the unconditioned, which reason postulates in all things by themselves, by nec
essity and by right, for everything conditioned, so that the series of conditions should thus become complete. If then we find that, under the supposition of our experience conforming to the objects as things by themselves, it is impossible to conceive the unconditioned without contradiction, while, under the supposition of our representation of things, as they are given to us, not conforming to them as things by themselves, but, on the contrary, of the objects conforming to our mode of representation, that contradiction vanishes, and that therefore the unconditioned must not be looked for in things, so far as we know them (so far as they are given to us), but only so far as we do not know them (as things by themselves), we clearly perceive that, what we at first assumed tentatively only, is fully confirmed.4 But, after all progress in the field of the supersensuous has thus been denied to speculative reason, it is still open to us to see, whether in the practical knowledge of reason data may not be found which enable us to determine that transcendent concept of the unconditioned which is demanded by reason, in order thus, according to the wish of metaphysic, to get beyond the limits of all possible experience, by means of our knowledge a priori, which is possible to us for practical purposes only. In this case, speculative reason has at least gained for us room for such an extension of knowledge, though it had to leave it empty, so that we are not only at liberty, but are really called upon to fill it up, if we are able, by practical data of reason.5
The very object of the critique of pure speculative reason consists in this attempt at changing the old procedure of metaphysic, and imparting to it the secure method of a science, after having completely revolutionised it, following the example of geometry and physical science. That critique is a treatise on the method (Traité de la méthode), not a system of the science itself; but it marks out nevertheless the whole plan of that science, both with regard to its limits, and to its internal organisation. For pure speculative reason has this peculiar advantage that it is able, nay, bound to measure its own powers, according to the different ways in which it chooses its own objects, and to completely enumerate the different ways of choosing problems; thus tracing a complete outline of a system of metaphysic.
This is due to the fact that, with regard to the first point, nothing can be attributed to objects in knowledge a priori, except what the thinking subject takes from within itself; while, with regard to the second point, reason, so far as its principles of cognition are concerned, forms a separate and independent unity, in which, as in an organic body, every member exists for the sake of all others, and all others exist for the sake of the one, so that no principle can be safely applied in one relation, unless it has been carefully examined in all its relations, to the whole employment of pure reason. Hence, too, metaphysic has this singular advantage, an advantage which cannot be shared by any other science, in which reason has to deal with objects (for Logic deals only with the form of thought in general) that, if it has once attained, by means of this critique, to the secure method of a science, it can completely comprehend the whole field of knowledge pertaining to it, and thus finish its work and leave it to posterity, as a capital that can never be added to, because it has only to deal with principles and the limits of their employment, which are fixed by those principles themselves. And this completeness becomes indeed an obligation, if it is to be a fundamental science, of which we must be able to say, 'nil actum reputans, si quid superesset agendum.'
But it will be asked, what kind of treasure is it which we mean to bequeath to posterity in this metaphysic of ours, after it has been purified by criticism, and thereby brought to a permanent condition? After a superficial view of this work, it may seem that its advantage is negative only, warning us against venturing with speculative reason beyond the limits of experience. Such is no doubt its primary use: but it becomes positive, when we perceive that the principles with which speculative reason ventures beyond its limits, lead inevitably, not to an extension, but, if carefully considered, to a narrowing of the employment of reason, because, by indefinitely extending the limits of sensibility, to which they properly belong, they threaten entirely to supplant the pure (practical) employment of reason. Hence our critique, by limiting sensibility to its proper sphere, is no doubt negative; but by thus removing an impediment, which threatened to narrow, or even entirely to destroy its practical employment, it is in reality of positive, and of very important use, if only we are convinced that there is an absolutely necessary practical use of pure reason (the moral use), in which reason must inevitably go beyond the limits of sensibility, and though not requiring for this purpose the assistance of speculative reason, must at all events be assured against its opposition, lest it be brought in conflict with itself. To deny that this service, which is rendered by criticism, is a positive advantage, would be the same as to deny that the police confers upon us any positive advantage, its principal occupation being to prevent violence, which citizens have to apprehend from citizens, so that each may pursue his vocation in peace and security. We had established in the analytical part of our critique the following points:—First, that space and time are only forms of sensuous intuition, therefore conditions of the existence of things, as phenomena only; Secondly, that we have no concepts of the understanding, and therefore nothing whereby we can arrive at the knowledge of things, except in so far as an intuition corresponding to these concepts can be given, and consequently that we cannot have knowledge of any object, as a thing by itself, but only in so far as it is an object of sensuous intuition, that is, a phenomenon. This proves no doubt that all speculative knowledge of reason is limited to objects of experience; but it should be carefully borne in mind, that this leaves it perfectly open to us, to think the same objects as things by themselves, though we cannot know them.6 For otherwise we should arrive at the absurd conclusion, that there is phenomenal appearance without something that appears. Let us suppose that the necessary distinction, established in our critique, between things as objects of experience and the same things by themselves, had not been made. In that case, the principle of causality, and with it the mechanism of nature, as determined by it, would apply to all things in general, as efficient causes. I should then not be able to say of one and the same being, for instance the human soul, that its will is free, and, at the same time, subject to the necessity of nature, that is, not free, without involving myself in a palpable contradiction: and this because I had taken the soul, in both propositions, in one and the same sense, namely, as a thing in general (as something by itself), as, without previous criticism, I could not but take it. If, however, our criticism was true, in teaching us to take an object in two senses, namely, either as a phenomenon, or as a thing by itself, and if the deduction of our concepts of the understanding was correct, and the principle of causality applies to things only, if taken in the first sense, namely, so far as they are objects of experience, but not to things, if taken in their second sense, we can, without any contradiction, think the same will when phenomenal (in visible actions) as necessarily conforming to the law of nature, and so far, not free, and yet, on the other hand, when belonging to a thing by itself, as not subject to that law of nature, and therefore free. Now it is quite true that I may not know my soul, as a thing by itself, by means of speculative reason (still less through empirical observation), and consequently may not know freedom either, as the quality of a being to which I attribute effects in the world of sense, because, in order to do this, I should have to know such a being as determined in its existence, and yet as not determined in time (which, as I cannot provide my concept with any intuition, is impossible). This, however, does not prevent me from thinking freedom; that is, my representation of it contains at least no contradiction within itself, if only our critical distinction of the two modes of representation (the sensible and the intelligible), and the consequent limitation of the concepts of the pure understanding, and of the principles based on them, has been properly carried out. If, then, morality necessarily presupposed freedom (in the strictest sense) as a property of our wil
l, producing, as a priori data of it, practical principles, belonging originally to our reason, which, without freedom, would be absolutely impossible, while speculative reason had proved that such a freedom cannot even be thought, the former supposition, namely, the moral one, would necessarily have to yield to another, the opposite of which involves a palpable contradiction, so that freedom, and with it morality (for its opposite contains no contradiction, unless freedom is presupposed), would have to make room for the mechanism of nature. Now, however, as morality requires nothing but that freedom should only not contradict itself, and that, though unable to understand, we should at least be able to think it, there being no reason why freedom should interfere with the natural mechanism of the same act (if only taken in a different sense) the doctrine of morality may well hold its place, and the doctrine of nature may hold its place too, which would have been impossible, if our critique had not previously taught us our inevitable ignorance with regard to things by themselves, and limited everything, which we are able to know theoretically, to mere phenomena. The same discussion as to the positive advantage to be derived from the critical principles of pure reason might be repeated with regard to the concept of God, and of the simple nature of our soul; but, for the sake of brevity, I shall pass this by. I am not allowed therefore even to assume, for the sake of the necessary practical employment of my reason, God, freedom, and immortality, if I cannot deprive speculative reason of its pretensions to transcendent insights, because reason, in order to arrive at these, must use principles which are intended originally for objects of possible experience only, and which, if in spite of this, they are applied to what cannot be an object of experience, really changes this into a phenomenon, thus rendering all practical extension of pure reason impossible. I had therefore to remove knowledge, in order to make room for belief. For the dogmatism of metaphysic, that is, the presumption that it is possible to achieve anything in metaphysic without a previous criticism of pure reason, is the source of all that unbelief, which is always very dogmatical, and wars against all morality.
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