Explanatory Notes

METAPHYSICS, as a natural disposition of reason, is actual, but if considered by itself alone (as the analytical solution of the third principal question showed), dialectical and illusory. If we think of taking principles from it, and in using them follow the natural, but on that account not less false, illusion, we can never produce science, but only a vain dialectical art, in which one school may outdo another, but none can ever acquire a just and lasting approbation.

Metaphysics exists as a natural tendency of reason. But considered on its own, it is dialectical and illusory. The analytical solution to the third main question showed this. Suppose we try to take principles from metaphysics and use them according to this natural illusion. The illusion is no less false for being natural. We can never produce a science this way, but only a futile dialectical art. In this art, one school may outdo another, but none can ever earn deserved and lasting approval.

In order that as a science metaphysics may be entitled to claim not mere fallacious plausibility, but in sight and conviction, a Critique of Reason must itself exhibit the whole stock of a priori concepts, their division according to their various sources (Sensibility, Understanding, and Reason), together with a complete table of them, the analysis of all these concepts, with all their consequences, especially by means of the deduction of these concepts, the possibility of synthetical cognition a priori, the principles of its application and finally its bounds, all in a complete system. Critique, therefore, and critique alone, contains in itself the whole well-proved and well-tested plan, and even all the means required to accomplish metaphysics, as a science; by other ways and means it is impossible. The question here therefore is not so much how this performance is possible, as how to set it going, and induce men of clear heads to quit their hitherto perverted and fruitless cultivation for one that will not deceive, and how such a union for the common end may best be directed.

For metaphysics to claim genuine insight and conviction as a science, rather than merely deceptive plausibility, a Critique of Reason must present a complete system. It must set out the entire stock of a priori concepts, divided according to their sources: Sensibility, Understanding, and Reason. It must also provide a complete table of these concepts and analyze them and all their consequences. In particular, it must establish the validity of these concepts through their deduction. It must also show the possibility of synthetic a priori knowledge, the principles governing its application, and finally its limits. Critique alone, therefore, contains the complete, thoroughly proven and tested plan for achieving metaphysics as a science. It also contains all the means required to carry out that plan. No other ways or means can achieve this. The question here, then, is not so much how this achievement is possible as how to set it in motion. The question is how to persuade clear-headed people to abandon their misguided and fruitless work up to now for an approach that will not deceive them. It is also how best to guide their joint efforts toward the common goal.

This much is certain, that whoever has once tasted Critique will be ever after disgusted with all dogmatical twaddle which he formerly put up with, because his reason must have something, and could find nothing better for its support.

One thing is certain: anyone who has once tasted Critique will forever after be disgusted by all the dogmatic nonsense they once tolerated. They tolerated it because their reason needed something to support it, and could find nothing better.

Critique stands in the same relation to the common metaphysics of the schools, as chemistry does to alchemy, or as astronomy to the astrology of the fortune-teller. I pledge myself that nobody who has read through and through, and grasped the principles of, the Critique even in these Prolegomena only, will ever return to that old and sophistical pseudo-science; but will rather with a certain delight look forward to metaphysics which is now indeed in his power, requiring no more preparatory discoveries, and now at last affording permanent satisfaction to reason. For here is an advantage upon which, of all possible sciences, metaphysics alone can with certainty reckon: that it can be brought to such completion and fixity as to be incapable of further change, or of any augmentation by new discoveries; because here reason has the sources of its knowledge in itself, not in objects and their observation (Anschauung), by which latter its stock of knowledge cannot be further increased. When therefore it has exhibited the fundamental laws of its faculty completely and so definitely as to avoid all misunderstanding, there remains nothing for pure reason to cognise a priori, nay, there is even no ground to raise further questions. The sure prospect of knowledge so definite and so compact has a peculiar charm, even though we should set aside all its advantages, of which I shall hereafter speak.

Critique stands in the same relation to traditional academic metaphysics as chemistry does to alchemy, or astronomy to fortune-tellers’ astrology. I pledge that no one who has thoroughly read and grasped the principles of the Critique, even in these Prolegomena alone, will ever return to that old, deceptive pseudo-science. Instead, that person will look forward with some delight to a metaphysics now within reach. This metaphysics requires no more preparatory discoveries and can at last give reason lasting satisfaction. For metaphysics alone, of all possible sciences, can count with certainty on one advantage. It can be brought to a state of completion and permanence that allows no further change or additions from new discoveries. This is because reason finds the sources of its knowledge within itself, not in objects or their observation (Anschauung). These cannot add to reason’s stock of knowledge. Once reason has fully set out the fundamental laws of its own capacity, clearly enough to prevent all misunderstanding, nothing remains for pure reason to know a priori. Indeed, there is no basis even for asking further questions. The sure prospect of knowledge so definite and complete has a special charm. This is true even if we set aside all its advantages, which I shall discuss later.

All false art, all vain wisdom, lasts its time, but finally destroys itself, and its highest culture is also the epoch of its decay. That this time is come for metaphysics appears from the state into which it has fallen among all learned nations, despite of all the zeal with which other sciences of every kind are prosecuted. The old arrangement of our university studies still preserves its shadow; now and then an Academy of Science tempts men by offering prizes to write essays on it, but it is no longer numbered among thorough sciences; and let any one judge for himself how a man of genius, if he were called a great metaphysician, would receive the compliment, which may be well-meant, but is scarce envied by anybody.

All false art and all empty wisdom last for a time, but finally destroy themselves. They reach their highest development just as their decline begins. The state of metaphysics among all learned nations shows that this time has come for metaphysics. This is so despite the zeal with which other sciences of every kind are pursued. The old structure of university studies still preserves a shadow of metaphysics. Now and then, an Academy of Science offers prizes to tempt people to write essays on it. But metaphysics is no longer counted among the rigorous sciences. Let anyone judge how a person of genius would take being called a great metaphysician. The compliment may be well meant, but hardly anyone would envy it.

Yet, though the period of the downfall of all dogmatical metaphysics has undoubtedly arrived, we are yet far from being able to say that the period of its regeneration is come by means of a thorough and complete Critique of Reason. All transitions from a tendency to its contrary pass through the stage of indifference, and this moment is the most dangerous for an author, but, in my opinion, the most favorable for the science. For, when party spirit has died out by a total dissolution of former connexions, minds are in the best state to listen to several proposals for an organisation according to a new plan.

The age of dogmatic metaphysics has undoubtedly come to an end. Yet we are still far from seeing its rebirth through a thorough and complete critique of reason. Every shift from one tendency to its opposite passes through a stage of indifference. This stage is the most dangerous for an author, but, in my opinion, the most favorable for the science. For when old connections have completely dissolved, party spirit dies out. Minds are then most ready to listen to different proposals for organizing the science according to a new plan.

When I say, that I hope these Prolegomena will excite investigation in the field of critique and afford a new and promising object to sustain the general spirit of philosophy, which seems on its speculative side to want sustenance, I can imagine beforehand, that every one, whom the thorny paths of my Critique have tired and put out of humor, will ask me, upon what I found this hope. My answer is, upon the irresistible law of necessity.

I hope these Prolegomena will encourage inquiry in the field of critique. I also hope they will give philosophy a new and promising subject to sustain its general spirit. Philosophy seems to need such support on its speculative side. But I can already imagine how everyone worn out and frustrated by the thorny paths of my Critique will respond. They will ask what gives me this hope. My answer is: the irresistible law of necessity.

That the human mind will ever give up metaphysical researches is as little to be expected as that we should prefer to give up breathing altogether, to avoid inhaling impure air. There will therefore always be metaphysics in the world; nay, every one, especially every man of reflexion, will have it, and for want of a recognised standard, will shape it for himself after his own pattern. What has hitherto been called metaphysics, cannot satisfy any critical mind, but to forego it entirely is impossible; therefore a Critique of Pure Reason itself must now be attempted or, if one exists, investigated, and brought to the full test, because there is no other means of supplying this pressing want, which is something more than mere thirst for knowledge.

It is no more likely that the human mind will give up investigating metaphysics than that we would stop breathing to avoid inhaling polluted air. Metaphysics will therefore always exist in the world. Indeed, everyone, especially every thoughtful person, will have a metaphysics of their own. Without an accepted standard, each person will shape it according to their own ideas. What has so far been called metaphysics cannot satisfy any critical mind, yet abandoning metaphysics entirely is impossible. We must therefore attempt a Critique of Pure Reason itself. Or, if one already exists, we must investigate it and put it fully to the test. There is no other way to meet this pressing need, which is more than a mere thirst for knowledge.

Ever since I have come to know critique, whenever I finish reading a book of metaphysical contents, which, by the preciseness of its notions, by variety, order, and an easy style, was not only entertaining but also helpful, I cannot help asking, "Has this author indeed advanced metaphysics a single step?" The learned men, whose works have been useful to me in other respects and always contributed to the culture of my mental powers, will, I hope, forgive me for saying, that I have never been able to find either their essays or my own less important ones (though self-love may recommend them to me) to have advanced the science of metaphysics in the least, and why?

Ever since I became familiar with critique, I have asked myself a question after finishing a book on metaphysics. Even when its precise ideas, variety, organization, and easy style have made it both entertaining and helpful, I cannot help asking: "Has this author really advanced metaphysics even one step?" I hope the scholars whose works have helped me in other ways and always developed my mental abilities will forgive me for saying this. I have never found that their essays have advanced the science of metaphysics in the least. Nor have my own less important essays, though self-love may lead me to think well of them. And why?

Here is the very obvious reason: metaphysics did not then exist as a science, nor can it be gathered piecemeal, but its germ must be fully preformed in the Critique. But in order to prevent all misconception, we must remember what has been already said, that by the analytical treatment of our concepts the understanding gains indeed a great deal, but the science (of metaphysics) is thereby not in the least advanced, because these dissections of concepts are nothing but the materials from which the intention is to carpenter our science. Let the concepts of substance and of accident be ever so well dissected and determined, all this is very well as a preparation for some future use. But if we cannot prove, that in all which exists the substance endures, and only the accidents vary, our science is not the least advanced by all our analyses.

The reason is obvious: metaphysics did not yet exist as a science, and it cannot be assembled piece by piece. Its seed must already be fully formed in the Critique. To avoid any misunderstanding, we must remember what has already been said. Analyzing our concepts greatly improves our understanding, but it does not advance the science of metaphysics at all. Such analyses provide only the materials from which we intend to build our science. We may analyze and define the concepts of substance and accident as thoroughly as we like. This is all well and good as preparation for future use. But unless we can prove that, in everything that exists, the substance endures and only the accidents change, our analyses do not advance our science at all.

Metaphysics has hitherto never been able to prove a priori either this proposition, or that of sufficient reason, still, less any more complex theorem, such as belongs to psychology or cosmology, or indeed any synthetical proposition. By all its analysing therefore nothing is affected, nothing obtained or forwarded, and the science, after all this bustle and noise, still remains as it was in the days of Aristotle, though far better preparations were made for it than of old, if the clue to synthetical cognitions had only been discovered.

So far, metaphysics has never been able to prove a priori either this proposition or the principle of sufficient reason. Still less has it proved a more complex theorem, such as one in psychology or cosmology, or indeed any synthetic proposition. So all its analysis achieves nothing, gains nothing, and advances nothing. After all this bustle and noise, metaphysics remains as it was in Aristotle’s day. Yet far better preparations had been made for it than in earlier times, if only the clue to synthetic knowledge had been discovered.

If any one thinks himself offended, he is at liberty to refute my charge by producing a single synthetical proposition belonging to metaphysics, which he would prove dogmatically a priori, for until he has actually performed this feat, I shall not grant that he has truly advanced the science; even should this proposition be sufficiently confirmed by common experience. No demand can be more moderate or more equitable, and in the (inevitably certain) event of its non-performance, no assertion more just, than that hitherto metaphysics has never existed as a science.

If anyone feels offended, they are free to refute my charge. They need only produce one synthetic proposition belonging to metaphysics and prove it dogmatically a priori. Until they accomplish this feat, I will not grant that they have truly advanced the science, even if common experience sufficiently confirms the proposition. No demand could be more modest or fair. And if this demand is not met, as is inevitably certain, no claim could be more just than this: metaphysics has never yet existed as a science.

But there are two things which, in case the challenge be accepted, I must deprecate: first, trifling about probability and conjecture, which are suited as little to metaphysics, as to geometry; and secondly, a decision by means of the magic wand of common sense, which does not convince every one, but which accommodates itself to personal peculiarities.

But if the challenge is accepted, I must object to two things. First, I object to playing with probability and conjecture, which are no more suited to metaphysics than to geometry. Second, I object to settling the matter with the magic wand of common sense. Common sense does not convince everyone, but adapts itself to each person's peculiarities.

For as to the former, nothing can be more absurd, than in metaphysics, a philosophy from pure reason to think of grounding our judgments upon probability and conjecture. Everything that is to be cognised a priori, is thereby announced as apodeictically certain, and must therefore be proved in this way. We might as well think of grounding geometry or arithmetic upon conjectures. As to the doctrine of chances in the latter, it does not contain probable, but perfectly certain, judgments concerning the degree of the probability of certain cases, under given uniform conditions, which, in the sum of all possible cases, infallibly happen according to the rule, though it is not sufficiently determined in respect to every single chance. Conjectures (by means of induction and of analogy) can be suffered in an empirical science of nature only, yet even there the possibility at least of what we assume must be quite certain.

As for the former, nothing could be more absurd than basing our judgments on probability and conjecture in metaphysics. Metaphysics is a philosophy based on pure reason. Everything to be known a priori is, by that very claim, necessarily certain. It must therefore be proved with that same certainty. We might as well think of basing geometry or arithmetic on conjectures. As for probability theory in arithmetic, it does not contain probable judgments. Rather, it contains perfectly certain judgments about how probable certain outcomes are under given uniform conditions. Across all possible cases, those outcomes unfailingly occur according to the rule, although the rule does not sufficiently determine each individual outcome. Conjectures, reached through induction and analogy, can be allowed only in an empirical science of nature. Yet even there, what we assume must at least be certainly possible.

The appeal to common sense is even more absurd, when concept and principles are announced as valid, not in so far as they hold with regard to experience, but even beyond the conditions of experience. For what is common sense? It is normal good sense, so far it judges right. But what is normal good sense? It is the faculty of the knowledge and use of rules in concreto, as distinguished from the speculative understanding, which is a faculty of knowing rules in abstracto. Common sense can hardly understand the rule, "that every event is determined by means of its cause," and can never comprehend it thus generally. It therefore demands an example from experience, and when it hears that this rule means nothing but what it always thought when a pane was broken or a kitchen-utensil missing, it then understands the principle and grants it. Common sense therefore is only of use so far as it can see its rules (though they actually are a priori) confirmed by experience; consequently to comprehend them a priori, or independently of experience, belongs to the speculative understanding, and lies quite beyond the horizon of common sense. But the province of metaphysics is entirely confined to the latter kind of knowledge, and it is certainly a bad index of common sense to appeal to it as a witness, for it cannot here form any opinion whatever, and men look down upon it with contempt until they are in difficulties, and can find in their speculation neither in nor out.

Appealing to common sense is even more absurd when concepts and principles are claimed to hold beyond the conditions of experience, not merely within experience. For what is common sense? It is ordinary good sense, insofar as it judges correctly. But what is ordinary good sense? It is the ability to know and use rules in concrete cases. This differs from speculative understanding, which is the ability to know rules in the abstract. Common sense can hardly understand the rule "that every event is determined by its cause." It can never grasp the rule in such general terms. It therefore demands an example from experience. When told that the rule means nothing more than what it always thought when a windowpane was broken or a kitchen utensil went missing, common sense understands and accepts the principle. Common sense is therefore useful only insofar as it can see its rules confirmed by experience, although those rules are actually a priori. So understanding rules a priori, or independently of experience, belongs to speculative understanding. Such understanding lies entirely beyond the reach of common sense. But metaphysics deals entirely with this latter kind of knowledge. It is certainly a poor sign of common sense to appeal to common sense as a witness here, for it cannot form any opinion on the matter. People look down on common sense with contempt until they run into difficulties and can find no way forward or back in their speculation.

It is a common subterfuge of those false friends of common sense (who occasionally prize it highly, but usually despise it) to say, that there must surely be at all events some propositions which are immediately certain, and of which there is no occasion to give any proof, or even any account at all, because we otherwise could never stop inquiring into the grounds of our judgments. But if we except the principle of contradiction, which is not sufficient to show the truth of synthetical judgments, they can never adduce, in proof of this privilege, anything else indubitable, which they can immediately ascribe to common sense, except mathematical propositions, such as twice two make four, between two points there is but one straight line, etc. But these judgments are radically different from those of metaphysics. For in mathematics I myself can by thinking construct whatever I represent to myself as possible by a concept: I add to the first two the other two, one by one, and myself make the number four, or I draw in thought from one point to another all manner of lines, equal as well as unequal; yet I can draw one only, which is like itself in all its parts. But I cannot, by all my power of thinking, extract from the concept of a thing the concept of something else, whose existence is necessarily connected with the former, but I must call in experience. And though my understanding furnishes me a priori (yet only in reference to possible experience) with the concept of such a connexion (i.e., causation), I cannot exhibit it, like the concepts of mathematics, by (Anschauung) visualising them, a priori, and so show its possibility a priori. This concept, together with the principles of its application, always requires, if it shall hold a priori—as is requisite in metaphysics—a justification and deduction of its possibility, because we cannot otherwise know how far it holds good, and whether it can be used in experience only or beyond it also.

The false friends of common sense have a common trick. They occasionally value common sense highly, but usually despise it. They say that surely some propositions must be immediately certain. These propositions need no proof or even any explanation, they say, because otherwise we could never stop investigating the grounds of our judgments. But apart from the principle of contradiction, they can offer only mathematical propositions as indisputable examples of this privilege. The principle of contradiction is not enough to establish the truth of synthetic judgments. The mathematical propositions they can directly attribute to common sense include, for example, that two times two equals four and that only one straight line connects two points. But mathematical judgments are fundamentally different from those of metaphysics. In mathematics, I can construct in thought whatever a concept represents to me as possible. I add the other two to the first two, one by one, and make the number four myself. Or I draw in thought all kinds of lines from one point to another, both uniform and uneven. Yet I can draw only one line that is the same throughout. But no amount of thinking can draw from the concept of one thing the concept of another thing whose existence is necessarily connected with it. I must turn to experience for that. My understanding does supply me a priori with the concept of such a connection, namely causation, although only in relation to possible experience. But unlike mathematical concepts, I cannot display causation by visualizing it (Anschauung) a priori and so show that it is possible a priori. If this concept and the principles for applying it are to hold a priori, as metaphysics requires, their possibility must always be justified and established by a deduction. Otherwise, we cannot know how far the concept holds or whether it can be used only in experience or beyond experience as well.

Therefore in metaphysics, as a speculative science of pure reason, we can never appeal to common sense, but may do so only when we are forced to surrender it, and to renounce all purely speculative cognition, which must always be knowledge, and consequently when we forego metaphysics itself and its instruction, for the sake of adopting a rational faith which alone may be possible for us, and sufficient to our wants, perhaps even more salutary than knowledge itself. For in this case the attitude of the question is quite altered. Metaphysics must be science, not only as a whole, but in all its parts, otherwise it is nothing; because, as a speculation of pure reason, it finds a hold only on general opinions. Beyond its field, however, probability and common sense may be used with advantage and justly, but on quite special principles, of which the importance always depends on the reference to practical life.

Therefore, in metaphysics, understood as a speculative science of pure reason, we can never appeal to common sense. We can do so only when we are forced to give up metaphysics and all purely speculative cognition. Such cognition must always be knowledge. We must also give up metaphysics itself and what it teaches. We would do this to adopt a rational faith, which alone may be possible for us and sufficient for our needs. Such faith may perhaps be even more beneficial than knowledge itself. For then the question has changed entirely. Metaphysics must be a science, not only as a whole but in all its parts. Otherwise it is nothing. For as speculation based on pure reason, it can rest only on general opinions. Beyond its field, however, probability and common sense may rightly be used to good effect. But their use depends on quite specific principles, whose importance always depends on their bearing on practical life.

This is what I hold myself justified in requiring for the possibility of metaphysics as a science.

This is what I believe I am justified in requiring if metaphysics is to be possible as a science.