I.2.1. The Deduction of the Pure Concepts of Understanding: Principles of Any Transcendental Deduction

Jurists, when speaking of rights and claims, distinguish in a legal action the question of right (quid juris) from the question of fact (quid facti); and they demand that both be proved. Proof of the former, which has to state the right or the legal claim, they entitle the deduction. Many empirical concepts are employed without question from anyone. Since experience is always available for the proof of their objective reality, we believe ourselves, even without a deduction, to be justified in appropriating to them a meaning, an ascribed significance. But there are also usurpatory concepts, such as fortune, fate, which, though allowed to circulate by almost universal indulgence, are yet from time to time challenged by the question: quid juris. This demand for a deduction involves us in considerable perplexity, no clear legal title, sufficient to justify their employment, being obtainable either from experience or from reason.

Now among the manifold concepts which form the highly complicated web of human knowledge, there are some which are marked out for pure a priori employment, in complete independence of all experience; and their right to be so employed always demands a deduction. For since empirical proofs do not suffice to justify this kind of employment, we are faced by the problem how these concepts can relate to objects which they yet do not obtain from any experience. The explanation of the manner in which concepts can thus relate a priori to objects I entitle their transcendental deduction; and from it I distinguish empirical deduction, which shows the manner in which a concept is acquired through experience and through reflection upon experience, and which therefore concerns, not its legitimacy, but only its de facto mode of origination.

We are already in possession of concepts which are of two quite different kinds, and which yet agree in that they relate to objects in a completely a priori manner, namely, the concepts of space and time as forms of sensibility, and the categories as concepts of understanding. To seek an empirical deduction of either of these types of concept would be labour entirely lost. For their distinguishing feature consists just in this, that they relate to their objects without having borrowed from experience anything that can serve in the representation of these objects. If, therefore, a deduction of such concepts is indispensable, it must in any case be transcendental.

We can, however, with regard to these concepts, as with regard to all knowledge, seek to discover in experience, if not the principle of their possibility, at least the occasioning causes of their production. The impressions of the senses supplying the first stimulus, the whole faculty of knowledge opens out to them, and experience is brought into existence. That experience contains two very dissimilar elements, namely, the matter of knowledge [obtained] from the senses, and a certain form for the ordering of this matter, [obtained] from the inner source of the pure intuition and thought which, on occasion of the sense-impressions, are first brought into action and yield concepts. Such an investigation of the first strivings of our faculty of knowledge, whereby it advances from particular perceptions to universal concepts, is undoubtedly of great service. We are indebted to the celebrated Locke for opening out this new line of enquiry. But a deduction of the pure a priori concepts can never be obtained in this manner; it is not to be looked for in any such direction. For in view of their subsequent employment, which has to be entirely independent of experience, they must be in a position to show a certificate of birth quite other than that of descent from experiences. Since this attempted physiological derivation concerns a quaestio facti, it cannot strictly be called deduction; and I shall therefore entitle it the explanation of the possession of pure knowledge. Plainly the only deduction that can be given of this knowledge is one that is transcendental, not empirical. In respect to pure a priori concepts the latter type of deduction is an utterly useless enterprise which can be engaged in only by those who have failed to grasp the quite peculiar nature of these modes of knowledge.

But although it may be admitted that the only kind of deduction of pure a priori knowledge which is possible is on transcendental lines, it is not at once obvious that a deduction is indispensably necessary. We have already, by means of a transcendental deduction, traced the concepts of space and time to their sources, and have explained and determined their a priori objective validity. Geometry, however, proceeds with security in knowledge that is completely a priori, and has no need to beseech philosophy for any certificate of the pure and legitimate descent of its fundamental concept of space. But the concept is employed in this science only in its reference to the outer sensible world—of the intuition of which space is the pure form—where all geometrical knowledge, grounded as it is in a priori intuition, possesses immediate evidence. The objects, so far as their form is concerned, are given, through the very knowledge of them, a priori in intuition. In the case of the pure concepts of understanding, it is quite otherwise; it is with them that the unavoidable demand for a transcendental deduction, not only of themselves, but also of the concept of space, first originates. For since they speak of objects through predicates not of intuition and sensibility but of pure a priori thought, they relate to objects universally, that is, apart from all conditions of sensibility. Also, not being grounded in experience, they cannot, in a priori intuition, exhibit any object such as might, prior to all experience, serve as ground for their synthesis. For these reasons, they arouse suspicion not merely in regard to the objective validity and the limits of their own employment, but owing to their tendency to employ the concept of space beyond the conditions of sensible intuition, that concept also they render ambiguous; and this, indeed, is why we have already found a transcendental deduction of it necessary. The reader must therefore be convinced of the unavoidable necessity of such a transcendental deduction before he has taken a single step in the field of pure reason. Otherwise he proceeds blindly, and after manifold wanderings must come back to the same ignorance from which he started. At the same time, if he is not to lament over obscurity in matters which are by their very nature deeply veiled, or to be too easily discouraged in the removal of obstacles, he must have a clear foreknowledge of the inevitable difficulty of the undertaking. For we must either completely surrender all claims to make judgments of pure reason in the most highly esteemed of all fields, that which transcends the limits of all possible experience, or else bring this critical enquiry to completion.

We have already been able with but little difficulty to explain how the concepts of space and time, although a priori modes of knowledge, must necessarily relate to objects, and how independently of all experience they make possible a synthetic knowledge of objects. For since only by means of such pure forms of sensibility can an object appear to us, and so be an object of empirical intuition, space and time are pure intuitions which contain a priori the condition of the possibility of objects as appearances, and the synthesis which takes place in them has objective validity.

The categories of understanding, on the other hand, do not represent the conditions under which objects are given in intuition. Objects may, therefore, appear to us without their being under the necessity of being related to the functions of understanding; and understanding need not, therefore, contain their a priori conditions. Thus a difficulty such as we did not meet with in the field of sensibility is here presented, namely, how subjective conditions of thought can have objective validity, that is, can furnish conditions of the possibility of all knowledge of objects. For appearances can certainly be given in intuition independently of functions of the understanding. Let us take, for instance, the concept of cause, which signifies a special kind of synthesis, whereby upon something, A, there is posited something quite different, B, according to a rule. It is not manifest a priori why appearances should contain anything of this kind (experiences cannot be cited in its proof, for what has to be established is the objective validity of a concept that is a priori)] and it is therefore a priori doubtful whether such a concept be not perhaps altogether empty, and have no object anywhere among appearances. That objects of sensible intuition must conform to the formal conditions of sensibility which lie a priori in the mind is evident, because otherwise they would not be objects for us. But that they must likewise conform to the conditions which the understanding requires for the synthetic unity of thought, is a conclusion the grounds of which are by no means so obvious. Appearances might very well be so constituted that the understanding should not find them to be in accordance with the conditions of its unity. Everything might be in such confusion that, for instance, in the series of appearances nothing presented itself which might yield a rule of synthesis and so answer to the concept of cause and effect. This concept would then be altogether empty, null, and meaningless. But since intuition stands in no need whatsoever of the functions of thought, appearances would none the less present objects to our intuition.

If we thought to escape these toilsome enquiries by saying that experience continually presents examples of such regularity among appearances and so affords abundant opportunity of abstracting the concept of cause, and at the same time of verifying the objective validity of such a concept, we should be overlooking the fact that the concept of cause can never arise in this manner. It must either be grounded completely a priori in the understanding, or must be entirely given up as a mere phantom of the brain. For this concept makes strict demand that something, A, should be such that something else, B, follows from it necessarily and in accordance with an absolutely universal rule. Appearances do indeed present cases from which a rule can be obtained according to which something usually happens, but they never prove the sequence to be necessary. To the synthesis of cause and effect there belongs a dignity which cannot be empirically expressed, namely, that the effect not only succeeds upon the cause, but that it is posited through it and arises out of it. This strict universality of the rule is never a characteristic of empirical rules; they can acquire through induction only comparative universality, that is, extensive applicability. If we were to treat pure concepts of understanding as merely empirical products, we should be making a complete change in [the manner of] their employment.

There are only two possible ways in which synthetic representations and their objects can establish connection, obtain necessary relation to one another, and, as it were, meet one another. Either the object alone must make the representation possible, or the representation alone must make the object possible. In the former case, this relation is only empirical, and the representation is never possible a priori. This is true of appearances, as regards that [element] in them which belongs to sensation. In the latter case, representation in itself does not produce its object in so far as existence is concerned, for we are not here speaking of its causality by means of the will. None the less the representation is a priori determinant of the object, if it be the case that only through the representation is it possible to know anything as an object. Now there are two conditions under which alone the knowledge of an object is possible, first, intuition, through which it is given, though only as appearance; secondly, concept, through which an object is thought corresponding to this intuition. It is evident from the above that the first condition, namely, that under which alone objects can be intuited, does actually lie a priori in the mind as the formal ground of the objects. All appearances necessarily agree with this formal condition of sensibility, since only through it can they appear, that is, be empirically intuited and given. The question now arises whether a priori concepts do not also serve as antecedent conditions under which alone anything can be, if not intuited, yet thought as object in general. In that case all empirical knowledge of objects would necessarily conform to such concepts, because only as thus presupposing them is anything possible as object of experience. Now all experience does indeed contain, in addition to the intuition of the senses through which something is given, a concept of an object as being thereby given, that is to say, as appearing. Concepts of objects in general thus underlie all empirical knowledge as its a priori conditions. The objective validity of the categories as a priori concepts rests, therefore, on the fact that, so far as the form of thought is concerned, through them alone does experience become possible. They relate of necessity and a priori to objects of experience, for the reason that only by means of them can any object whatsoever of experience be thought.

The transcendental deduction of all a priori concepts has thus a principle according to which the whole enquiry must be directed, namely, that they must be recognised as a priori conditions of the possibility of experience, alike of the intuition which is to be met with in it and of the thought. Concepts which yield the objective ground of the possibility of experience are for this very reason necessary. But the unfolding of the experience wherein they are encountered is not their deduction; it is only their illustration. For on any such exposition they would be merely accidental. Save through their original relation to possible experience, in which all objects of knowledge are found, their relation to any one object would be quite incomprehensible.

The illustrious Locke, failing to take account of these considerations, and meeting with pure concepts of the understanding in experience, deduced them also from experience, and yet proceeded so inconsequently that he attempted with their aid to obtain knowledge which far transcends all limits of experience. David Hume recognised that, in order to be able to do this, it was necessary that these concepts should have an a priori origin. But since he could not explain how it can be possible that the understanding must think concepts, which are not in themselves connected in the understanding, as being necessarily connected in the object, and since it never occurred to him that the understanding might itself, perhaps, through these concepts, be the author of the experience in which its objects are found, he was constrained to derive them from experience, namely, from a subjective necessity (that is, from custom), which arises from repeated association in experience, and which comes mistakenly to be regarded as objective. But from these premisses he argued quite consistently. It is impossible, he declared, with these concepts and the principles to which they give rise, to pass beyond the limits of experience. Now this empirical derivation, in which both philosophers agree, cannot be reconciled with the scientific a priori knowledge which we do actually possess, namely, pure mathematics and general science of nature; and this fact therefore suffices to disprove such derivation.

While the former of these two illustrious men opened a wide door to enthusiasm—for if reason once be allowed such rights, it will no longer allow itself to be kept within bounds by vaguely defined recommendations of moderation—the other gave himself over entirely to scepticism, having, as he believed, discovered that what had hitherto been regarded as reason was but an all-prevalent illusion infecting our faculty of knowledge. We now propose to make trial whether it be not possible to find for human reason safe conduct between these two rocks, assigning to her determinate limits, and yet keeping open for her the whole field of her appropriate activities.

But first I shall introduce a word of explanation in regard to the categories. They are concepts of an object in general, by means of which the intuition of an object is regarded as determined in respect of one of the logical functions of judgment. Thus the function of the categorical judgment is that of the relation of subject to predicate; for example, ‘All bodies are divisible’. But as regards the merely logical employment of the understanding, it remains undetermined to which of the two concepts the function of the subject, and to which the function of predicate, is to be assigned. For we can also say, ‘Something divisible is a body’. But when the concept of body is brought under the category of substance, it is thereby determined that its empirical intuition in experience must always be considered as subject and never as mere predicate. Similarly with all the other categories.