If a metaphysics that could assert itself as science were actual, if one could say: here is metaphysics, you need only to learn it, and it will convince you of its truth irresistibly and immutably, then this question would be unnecessary, and there would remain only that question which would pertain more to a test of our acuteness than to a proof of the existence of the subject matter itself, namely: how it is possible, and how reason should set about attaining it. Now it has not gone so well for human reason in this case. One can point to no single book, as for instance one presents a Euclid, and say: this is metaphysics, here you will find the highest aim of this science, knowledge of a supreme being and a future life, proven from principles of pure reason. For one can indeed show us many propositions that are apodictically certain and have never been disputed; but these are collectively analytic and pertain more to the materials and building blocks of metaphysics than to the expansion of knowledge, which after all ought to be our real aim for it (§2c). But although you present synthetic propositions as well (e.g., the principle of sufficient reason), which you have never proven from bare reason and consequently a priori, as was indeed your obligation, and which are gladly ceded to you all the same: then if you want to use them toward your main goal, you still fall into assertions so illicit and precarious that one metaphysics has always contradicted the other, either in regard to the assertions themselves or their proofs, and thereby metaphysics has itself destroyed its claim to lasting approbation.

It was characteristic of the great modern philosophers to attempt, each in his own way, to rebuild philosophy from the ground up. Kant embraced this goal more fully than any other classical modern philosopher. And his work did in fact change philosophy permanently, though not always as he had intended. He wanted to show that philosophers and natural scientists were not able, and would never be able, to give final answers to questions about the nature of the physical world and of the human mind or soul, and about the existence and attributes of a supreme being. While he did not accomplish precisely that, his work changed philosophy's conception of what can be known, and how it can be known. Kant also wanted to set forth new and permanent doctrines in metaphysics and morals. Though his exact teachings have not gained general acceptance, they continue to inspire new positions in philosophical discussion today.

Kant stands at the center of modern philosophy. His criticism of previous work in metaphysics and the theory of knowledge, propounded in the Critique of Pure Reason and summarized in the Prolegomena, provided a comprehensive response to early modern philosophy and a starting point for subsequent work. He rejected previous philosophical explanations of philosophical cognition itself. His primary target was the rationalist use of reason or “pure intellect” – advanced by Descartes and Leibniz – as a basis for making claims about God and the essences of mind and matter.

Whether or not the cultivation of those cognitions that belong to the occupation of reason treads the sure path of a science can be assessed quickly from the results. If, after repeated preparations and provisions, this cultivation gets bogged down as soon as it arrives at its goal, or if it must often backtrack and take another path to attain this goal; or equally, if it is not possible to unite the various collaborators on the manner in which their common aim should be pursued: then one can always be convinced that such a pursuit has not yet (by far) taken the sure path of science, but is merely groping about; and the discovery of this path, if possible, is already a service to reason, even if much should have to be abandoned as futile that was contained in the goal as previously accepted (without reflection).

That logic has tread this sure path from the most ancient times up to now can be seen from the fact that since Aristotle it has not had to take a single step backward, if the removal of a few superfluous subtleties or the clearer determination of what is presented are not to be reckoned as improvements, which anyway pertain more to the elegance than to the surety of the science. It is further noteworthy about logic that it also has not, up to now, been able to take any step forward, and therefore seems, to all appearance, to be finished and complete.

From all of this now follows the idea of a special science, which can be called the critique of pure reason. For reason is the faculty that supplies the principles of a priori cognition. Hence, pure reason is what contains the principles for unconditionally cognizing something a priori. An organon of pure reason would be a collection of those principles according to which all pure cognition a priori can be acquired and actually brought about. The complete execution of such an organon would yield a system of pure reason. But since this system is greatly desired, and since it is still undecided whether, and in what instances, an expansion of our cognition is in general possible here, we can look upon a science of the mere appraisal of pure reason, its sources and boundaries, as the propaedeutic to the system of pure reason. Such a science should not be called a doctrine, but only a critique of pure reason, and its benefit, with respect to speculation, would actually be only negative, serving not for the expansion but only for the purification of our reason, and for keeping reason free of error, which is already a very great gain. I call all cognition transcendental that is in general concerned not so much with objects as with the mode of our cognition of objects, insofar as this cognition is supposed to be possible a priori.

Since all the paths that have been taken before now have not attained this end, and will never be able to attain it without a preceding critique of pure reason, the demand that the attempt at such a critique which is now before the public be subjected to an exact and careful examination does not seem unreasonable – unless one holds it to be still more advisable rather to relinquish completely all claims to metaphysics, in which case, if one only remains true to one's intention, there is nothing to object to in this. If the course of events is taken as it actually runs and not as it should run, then there are two kinds of judgments: a judgment that precedes the investigation, and in our case this is one in which the reader, from his own metaphysics, passes judgment on the Critique of Pure Reason (which is supposed first of all to investigate the possibility of that metaphysics); and then a different judgment that comes after the investigation, in which the reader is able to set aside for a while the consequences of the critical investigation, which may strongly repudiate the metaphysics he otherwise accepts, and first tests the grounds from which these consequences may have been derived. If what ordinary metaphysics presents were undeniably certain (like geometry, for instance), the first way of judging would be valid; for if the consequences of certain principles conflict with undeniable truths, then these principles are false and are to be rejected without any further investigation.

Nature is the existence of things, insofar as that existence is determined according to universal laws. If nature meant the existence of things in themselves, we would never be able to cognize it, either a priori or a posteriori. Not a priori, for how are we to know what is suited to things in themselves, since this can never come about through the analysis of our concepts (analytical propositions), since I do not want to know what may be contained in my concept of a thing (for that belongs to its logical being), but what would be added to this concept in the actuality of a thing and through which the thing itself would be determined in its existence apart from my concept. My understanding, and the conditions under which alone it can connect the determinations of things in their existence, prescribes no rule to the things themselves; these do not conform to my understanding, but my understanding would have to conform to them; they would therefore have to be given to me in advance so that these determinations could be taken from them, but then they would not be cognized a priori.

Such cognition of the nature of things in themselves would also be impossible a posteriori. For if experience is supposed to teach me laws to which the existence of things is subject, then these laws, insofar as they relate to things in themselves, would have to apply to them necessarily even apart from my experience.

In whatever manner and through whatever means a cognition may relate to objects, intuition is that by which it relates to objects immediately, and that toward which, as a means, all thought aims. But intuition takes place only insofar as the object is given to us; this in turn, however, is made possible (for us human beings at least) only through the object's affecting the mind in a specific manner. The capacity (receptivity) to obtain representations through the way in which we are affected by objects is called sensibility. By means of sensibility, therefore, objects are given to us, and it alone provides us with intuitions; but through the understanding objects are thought, and from it there arise concepts. All thinking, however, whether it do so directly (directe), or indirectly (indirecte), by means of certain characters, must ultimately relate to intuitions, and hence, for us, to sensibility, for no object can be given to us in any other way.

The effect of an object upon the capacity for representation, insofar as we are affected by that object, is sensation. The intuition that is related to the object through sensation is called empirical. The indeterminate object of an empirical intuition is called appearance.

Within appearance, that which corresponds to sensation I call the matter of appearance, but that which makes it that the multiplicity of appearance can be ordered in specific relations I call the form of appearance.

By the analytic of concepts I do not mean the analysis of concepts, nor the ordinary procedure in philosophical investigation, of analyzing and clarifying (as regards their content) concepts that present themselves, but the as yet little attempted analysis of the faculty of understanding itself, so as to investigate the possibility of a priori concepts by seeking that possibility in the understanding alone (as the birthplace of those concepts), and by analyzing the pure use of those concepts (in general); for this is the proper business of transcendental philosophy; the rest is the logical treatment of concepts in philosophy in general. We will therefore pursue the pure concepts as far as their first seeds and predispositions in the human understanding, in which they lie ready until those concepts are, on the occasion of experience, finally developed, and are, by the very same understanding, exhibited in their purity, free of the empirical conditions attaching to them.

First section

The discipline of pure reason in its dogmatic use

Mathematics provides the brightest example of pure reason augmenting itself successfully by itself, without any help from experience. Examples are contagious, especially to one and the same faculty, which naturally flatters itself that it will have the very same luck in other cases as has come its way in the one case. Hence, pure reason hopes to be able to extend itself just as successfully and well-foundedly in its transcendental use as it has managed to do in its mathematical use, especially if it uses the same method in the former case as has been of such manifest benefit in the latter. It is therefore very important for us to know: whether the method for achieving apodictic certainty that one calls mathematical in the latter science is the same as the method by which one seeks to achieve the same kind of certainty in philosophy, and which would in that field have to be called dogmatic.

Philosophical cognition is cognition through reason from concepts; mathematical cognition is cognition through reason from the construction of concepts. To construct a concept means, however: to exhibit a priori the intuition corresponding to it. For the construction of a concept, then, a nonempirical intuition is required, which therefore, as intuition, is a single object, but which, as the construction of a concept (a universal representation), must nonetheless express (in the representation) universal validity for all possible intuitions belonging under that same concept.

Metaphysics, as a natural predisposition of reason, is actual, but it is also of itself (as the analytical solution to the third main question proved) dialectical and deceitful. The desire to derive principles from it, and to follow the natural but nonetheless false illusion in their use, can therefore never bring forth science, but only vain dialectical art, in which one school outdoes another but none can ever gain legitimate and lasting approbation.

In order that metaphysics might, as science, be able to lay claim, not merely to deceitful persuasion, but to insight and conviction, a critique of reason itself must set forth the entire stock of a priori concepts, their division according to the different sources (sensibility, understanding, and reason), further, a complete table of those concepts, and the analysis of all of them together with everything that can be derived from that analysis; and then, especially, such a critique must set forth the possibility of synthetic cognition a priori through a deduction of these concepts, it must set forth the principles of their use, and finally the boundaries of that use; and all of this in a complete system. Therefore a critique, and that alone, contains within itself the whole well-tested and verified plan, and even all the means for carrying it out, by which metaphysics as science can be achieved; by any other ways or means it is impossible.

Introduction: The idea of a transcendental logic

Our cognition arises from two fundamental sources in the mind, the first of which is the receiving of representations (receptivity of impressions), the second, the capacity to cognize an object by means of these representations (spontaneity of concepts); through the first an object is given to us, through the second this object is, in relation to this representation (as a mere determination of the mind), thought. Intuition and concepts therefore constitute the elements of all our cognition, in such a way that neither concepts without intuition in some way corresponding to them, nor intuition without concepts, can yield a cognition. Both are either pure or empirical. Empirical, if sensation (which presupposes the actual presence of the object) is contained in it; but pure if no sensation is intermixed with the representation. The sensation can be called the matter of sensory cognition. Hence, pure intuition contains only the form under which something is intuited, and a pure concept contains only the form of the thought of an object in general. Only pure intuitions or concepts alone are possible a priori; empirical intuitions or concepts are possible only a posteriori.

If the receptivity of our mind, its capacity for receiving representations insofar as it is affected in some way, is to be called sensibility; then, by comparison, the capacity for generating representations by itself, or the spontaneity of cognition, is the understanding.

Here now is a great and proven body of cognition, which is already of admirable extent and promises unlimited expansion in the future, which carries with it thoroughly apodictic certainty (i.e., absolute necessity), hence rests on no grounds of experience, and so is a pure product of reason, but beyond this is thoroughly synthetic. “How is it possible then for human reason to achieve such cognition wholly a priori?” Does not this capacity, since it is not, and cannot be, based on experience, presuppose some a priori basis for cognition, which lies deeply hidden, but which might reveal itself through these its effects, if their first beginnings were only diligently tracked down?

We find, however, that all mathematical cognition has this distinguishing feature, that it must present its concept beforehand in intuition and indeed a priori, consequently in an intuition that is not empirical but pure, without which means it cannot take a single step; therefore its judgments are always intuitive, in the place of which philosophy can content itself with discursive judgments from mere concepts, and can indeed exemplify its apodictic teachings through intuition but can never derive them from it. This observation with respect to the nature of mathematics already guides us toward the first and highest condition of its possibility; namely, it must be grounded in some pure intuition or other, in which it can present, or, as one calls it, construct all of its concepts in concreto yet a priori.

On the sources of metaphysics

If one wishes to present a body of cognition as science, then one must first be able to determine precisely the differentia it has in common with no other science, and which is therefore its distinguishing feature; otherwise the boundaries of all the sciences run together, and none of them can be dealt with thoroughly according to its own nature.

Whether this distinguishing feature consists in a difference of the object or the source of cognition, or even of the type of cognition, or several if not all of these things together, the idea of the possible science and its territory depends first of all upon it.

First, concerning the sources of metaphysical cognition, it already lies in the concept of metaphysics that they cannot be empirical. The principles of such cognition (which include not only its fundamental propositions or basic principles, but also its fundamental concepts) must therefore never be taken from experience; for the cognition is supposed to be not physical but metaphysical, i.e., lying beyond experience. Therefore it will be based upon neither outer experience, which constitutes the source of physics proper, nor inner, which provides the foundation of empirical psychology. It is therefore cognition a priori, or from pure understanding and pure reason.

These prolegomena are not for the use of apprentices, but of future teachers, and indeed are not to help them to organize the presentation of an already existing science, but to discover this science itself for the first time.

There are scholars for whom the history of philosophy (ancient as well as modern) is itself their philosophy; the present prolegomena have not been written for them. They must wait until those who endeavor to draw from the wellsprings of reason itself have finished their business, and then it will be their turn to bring news of these events to the world. Failing that, in their opinion nothing can be said that has not already been said before; and in fact this opinion can stand for all time as an infallible prediction, for since the human understanding has wandered over countless subjects in various ways through many centuries, it can hardly fail that for anything new something old should be found that has some similarity with it.

My intention is to convince all of those who find it worthwhile to occupy themselves with metaphysics that it is unavoidably necessary to suspend their work for the present, to consider all that has happened until now as if it had not happened, and before all else to pose the question: “whether such a thing as metaphysics is even possible at all.”