3.As to metaphysics, even if we look upon it merely as an attempted science, yet, from the nature of human reason, an indispensable one, we find that it must contain synthetical propositions a priori.It is not merely the duty of metaphysics to dissect, and thereby analytically to illustrate the conceptions which we form a priori of things; but we seek to widen the range of our a priori knowledge.
For this purpose, we must avail ourselves of such principles as add something to the original conception- something not identical with, nor contained in it, and by means of synthetical judgements a priori, leave far behind us the limits of experience; for example, in the proposition, "the world must have a beginning," and such like.Thus metaphysics, according to the proper aim of the science, consists merely of synthetical propositions a priori.
VI.The Universal Problem of Pure Reason.
It is extremely advantageous to be able to bring a number of investigations under the formula of a single problem.For in this manner, we not only facilitate our own labour, inasmuch as we define it clearly to ourselves, but also render it more easy for others to decide whether we have done justice to our undertaking.The proper problem of pure reason, then, is contained in the question: "How are synthetical judgements a priori possible?"That metaphysical science has hitherto remained in so vacillating a state of uncertainty and contradiction, is only to be attributed to the fact that this great problem, and perhaps even the difference between analytical and synthetical judgements, did not sooner suggest itself to philosophers.Upon the solution of this problem, or upon sufficient proof of the impossibility of synthetical knowledge a priori, depends the existence or downfall of the science of metaphysics.Among philosophers, David Hume came the nearest of all to this problem; yet it never acquired in his mind sufficient precision, nor did he regard the question in its universality.On the contrary, he stopped short at the synthetical proposition of the connection of an effect with its cause (principium causalitatis), insisting that such proposition a priori was impossible.According to his conclusions, then, all that we term metaphysical science is a mere delusion, arising from the fancied insight of reason into that which is in truth borrowed from experience, and to which habit has given the appearance of necessity.Against this assertion, destructive to all pure philosophy, he would have been guarded, had he had our problem before his eyes in its universality.For he would then have perceived that, according to his own argument, there likewise could not be any pure mathematical science, which assuredly cannot exist without synthetical propositions a priori- an absurdity from which his good understanding must have saved him.
In the solution of the above problem is at the same time comprehended the possibility of the use of pure reason in the foundation and construction of all sciences which contain theoretical knowledge a priori of objects, that is to say, the answer to the following questions:
How is pure mathematical science possible?
How is pure natural science possible?
Respecting these sciences, as they do certainly exist, it may with propriety be asked, how they are possible?- for that they must be possible is shown by the fact of their really existing.* But as to metaphysics, the miserable progress it has hitherto made, and the fact that of no one system yet brought forward, far as regards its true aim, can it be said that this science really exists, leaves any one at liberty to doubt with reason the very possibility of its existence.
*As to the existence of pure natural science, or physics, perhaps many may still express doubts.But we have only to look at the different propositions which are commonly treated of at the commencement of proper (empirical) physical science- those, for example, relating to the permanence of the same quantity of matter, the vis inertiae, the equality of action and reaction, etc.- to be soon convinced that they form a science of pure physics (physica pura, or rationalis), which well deserves to be separately exposed as a special science, in its whole extent, whether that be great or confined.
Yet, in a certain sense, this kind of knowledge must unquestionably be looked upon as given; in other words, metaphysics must be considered as really existing, if not as a science, nevertheless as a natural disposition of the human mind (metaphysica naturalis).For human reason, without any instigations imputable to the mere vanity of great knowledge, unceasingly progresses, urged on by its own feeling of need, towards such questions as cannot be answered by any empirical application of reason, or principles derived therefrom; and so there has ever really existed in every man some system of metaphysics.It will always exist, so soon as reason awakes to the exercise of its power of speculation.And now the question arises: "How is metaphysics, as a natural disposition, possible?" In other words, how, from the nature of universal human reason, do those questions arise which pure reason proposes to itself, and which it is impelled by its own feeling of need to answer as well as it can?