Immanuel Kant (1787)

Critique of Pure Reason.

Source: Last section of Analytic of Concepts, from Transcendental Logic, from Norman Kemp Smith translation of Second Edition of Kant's Critique of Pure Reason.

Deduction of the Pure Concepts of the Understanding

Section 2
Transcendental Deduction of the Pure Concepts of the Understanding

§15. The Possibility of Combination in General

THE manifold of representations can be given in an intuition which is purely sensible, that is, nothing but receptivity; and the form of this intuition can lie a priori in our faculty of representation, without being anything more than the mode in which the subject is affected. But the combination (conjunctio) of a manifold in general can never come to us through the senses, and cannot, therefore, be already contained in the pure form of sensible intuition. For it is an act of spontaneity of the faculty of representation; and since this faculty, to distinguish it from sensibility, must be entitled understanding, all combination -- be we conscious of it or not, be it a combination of the manifold of intuition, empirical or non-empirical, or of various concepts -- is an act of the understanding. To this act the general title 'synthesis' may be assigned, as indicating that we cannot represent to ourselves anything as combined in the object which we have not ourselves previously combined, and that of all representations combination is the only one which cannot be given through objects. Being an act of the self- activity of the subject, it cannot be executed save by the subject itself. It will easily be observed that this action is originally one and is equipollent for all combination, and that is dissolution, namely, analysis, which appears to be its opposite, yet always presupposes it. For where the understanding has not previously combined, it cannot dissolve, since only as having been combined by the understanding can anything that allows of analysis be given to the faculty of representation.

But the concept of combination includes, besides the concept of the manifold and of its synthesis, also the concept of the unity of the manifold. Combination is representation of the synthetic unity of the manifold. The representation of this unity cannot, therefore, arise out of the combination. On the contrary, it is what, by adding itself to the representation of the manifold, first makes possible the concept of the combination. This unity, which precedes a priori all concepts of combination, is not the category of unity (§10); for all categories are grounded in logical functions of judgment, and in these functions combination, and therefore unity of given concepts, is already thought. Thus the category already presupposes combination. We must therefore look yet higher for this unity (as qualitative, §12), namely in that which itself contains the ground of the unity of diverse concepts in judgment, and therefore of the possibility of the understanding, even as regards its logical employment.

The Original Synthetic Unity of Apperception

It must be possible for the 'I think' to accompany all my representations; for otherwise something would be represented in me which could not be thought at all, and that is equivalent to saying that the representation would be impossible, or at least would be nothing to me.

Whether the representations are in themselves identical, and whether, therefore, one can be analytically thought through the other, is not a question that here arises. The consciousness of the one, when the manifold is under consideration, has always to be distinguished from the consciousness of the other; and it is with the synthesis of this (possible) consciousness that we are here alone concerned.

That representation which can be given prior to all thought is entitled intuition. All the manifold of intuition has, therefore, a necessary relation to the 'I think' in the same subject in which this manifold is found. But this representation is an act of spontaneity, that is, it cannot be regarded as belonging to sensibility. I call it pure apperception, to distinguish it from empirical apperception, or, again, origninal apperception, because it is that self-consiousness which, while generating the representation 'I think' (a representation which must be capable of accompanying all other representations, and which in all consciousness is one and the same), cannot itself be accompanied by any further representation. The unity of this apperception I likewise entitle the transcendental unity of self-consciousness, in order to indicate the possibility of a priori knowledge arising from it. For the manifold representations, which are given in an intuition, would not be one and all my representations, if they did not all belong to one self-consciousness. As my representations (even if I am not conscious of them as such) they must conform to the condition under which alone they can stand together in one universal self-consciousness, because otherwise they would not all without exception belong to me. From this original combination many consequences follow.

This thoroughgoing identity of the apperception of a manifold which is given in intuition contains a synthesis of representations, and is possible only through the consciousness of this synthesis. For the empirical consciousness, which accompanies different representations, is in itself diverse and without relation to the identity of the subject. That relation comes about, not simply through my accompanying each representation with consciousness, but only in so far as I conjoin one representation with another, and am conscious of the synthesis of them. Only in so far, therefore, as I can unite a manifold of given representations in one consciousness, is it possible for me to represent to myself the identity of the consciousness in [i.e. throughout] these representations. In other words, the analytic unity of apperception is possible only under the presupposition of a certain synthetic unity.

[The analytic unity of consciousness belongs to all general concepts, as such. If, for instance, I think red in general, I thereby represent to myself a property which (as a characteristic) can be found in something, or can he combined with other representations; that is, only by means of a presupposed possible synthetic unity can I represent to myself the analytic unity. A representation which is to be thought as common to different representations is regarded as belonging to such as have, in addition to it, also something different. Consequently it must previously be thought in synthetic unity with other (though, it may be, only possible) representations, before I can think in it the analytic unity of consciousness, which makes it a conceptus communis. The synthetic unity of apperception is therefore that highest point, to which we must ascribe all employment of the understanding, even the whole of logic, and conformably therewith, transcendental philosophy. Indeed this faculty of apperception is the understanding itself.]

The thought that the representations given in intuition one and all belong to me, is therefore equivalent to the thought that I unite them in one self-consciousness, or can at least so unite them; and although this thought is not itself the consciousness of the synthesis of the representations, it presupposes the possibility of that synthesis. In other words, only in so far as I can grasp the manifold of the representations in one consciousness, do I call them one and all mine. For otherwise I should have as many-coloured and diverse a self as I have representations of which I am conscious to myself. Synthetic unity of the manifold of intuitions, as generated a - priori, is thus the ground of the identity of apperception itself, which precedes a priori all my determinate thought. Combination does not, however, lie in the objects, and cannot be borrowed from them, and so, through perception, first taken up into the understanding. On the contrary, it is an affair of the understanding alone, which itself is nothing but the faculty of combining a priori, and of bringing the manifold of given representations under the unity of apperception. The principle of apperception is the highest principle in the whole sphere of human knowledge.

This principle of the necessary unity of apperception is itself, indeed, an identical, and therefore analytic, proposition; nevertheless it reveals the necessity of a synthesis of the manifold given in intuition, without which the thoroughgoing identity of self-consciousness cannot be thought. For through the 'I', as simple representation, nothing manifold is given; only in intuition, which is distinct from the 'I', can a manifold be given; and only through combination in one consciousness can it be thought. An understanding in which through self-consciousness all the manifold would eo ipso be given, would be intuitive; our understanding can only think, and for intuition must look to the senses. I am conscious of the self as identical in respect of the manifold of representations that are given to me in an intuition, because I call them one and all my representations, and so apprehend them as constituting one intuition. This amounts to saying, that I am conscious to myself a priori of a necessary synthesis of representations -- to be entitled the original synthetic unity of apperception -- under which all representations that are given to me must stand, but under which they have also first to be brought by means of a synthesis.


The Principle of the Synthetic Unity is the Supreme Principle of all Employment of the Understanding The supreme principle of the possibility of all intuition in its relation to sensibility is, according to the Transcendental Aesthetic, that all the manifold of intuition should be subject to the formal conditions of space and time. The supreme principle of the same possibility, in its relation to understanding, is that all the manifold of intuition should be subject to conditions of the original synthetic unity of apperception.

[Space and time, and all their parts, are intuitions, and are, therefore, with the manifold which they contain, singular representations (see the Transcendental Aesthetic). Consequently they are not mere concepts through which one and the same consciousness is found to be contained in a number of representations. On the contrary, through them many representations are found to be contained in one representation, and in the consciousness of that representation; and they are thus composite. The unity of that consciousness is therefore synthetic and yet is also original. The singularity of such intuitions is found to have important consequences (see §25).]

In so far as the manifold representations of intuition are given to us, they are subject to the former of these two principles; in so far as they must allow of being combined in one consciousness, they are subject to the latter. For without such combination nothing can be thought or known, since the given representations would not have in common the act of the apperception 'I think', and so could not be apprehended together in knowledge.

Understanding is, to use general terms, the faculty of knowledge. This knowledge consists in the determinate relation of given representations to an object; and an object is that in the concept of which the manifold of a given intuition is united. Now all unification of representations demands unity of consciousness in the synthesis of them. Consequently it is the unity of consciousness that alone constitutes the relation of representations to an object, and therefore their objective validity and the fact that they are modes of knowledge; and upon it therefore rests the very possibility of the understanding.

The first pure knowledge of understanding, then, upon which all the rest of its employment is based, and which also at the same time is completely independent of all conditions of sensible intuition, is the principle of the original synthetic unity of apperception. Thus the mere form of outer sensible intuition, space, is not yet [by itself] knowledge; it supplies only the manifold of a priori intuition for a possible knowledge. To know anything in space (for instance, a line), I must draw it, and thus synthetically bring into being a determinate combination of the given manifold, so that the unity of this act is at the same time the unity of consciousness (as in the concept of a line); and it is through this unity of consciousness that an object (a determinate space) is first known. The synthetic unity of consciousness is, therefore, an objective condition of all knowledge. It is not merely a condition that I myself require in knowing an object, but is a condition under which every intuition must stand in order to become an object for me. For otherwise, in the absence of this synthesis, the manifold would not be united in one consciousness.

Although this proposition makes synthetic unity a condition of all thought, it is, as already stated, itself analytic. For it says no more than that all my representations in any given intuition must be subject to that condition under which alone I can ascribe them to the identical self as my representations, and so can comprehend them as synthetically combined in one apperception through the general expression, 'I think'.

This principle is not, however, to be taken as applying to every possible understanding, but only to that understanding through whose pure apperception, in the representation 'I am', nothing manifold is given. An understanding which through its self-consciousness could supply to itself the manifold of intuition -- an understanding, that is to say, through whose representation the objects of the representation should at the same time exist -- would not require, for the unity of consciousness, a special act of synthesis of the manifold. For the human understanding, however, which thinks only, and does not intuit, that act is necessary. It is indeed the first principle of the human understanding, and is so indispensable to it that we cannot form the least conception of any other possible understanding, either of such as is itself intuitive or of any that may possess an underlying mode of sensible intuition which is different in kind from that in space and time.

The Objective Unity of Self-Consciousness

The transcendental unity of apperception is that unity through which all the manifold given in an intuition is united in a concept of the object. It is therefore entitled objective, and must be distinguished from the subjective unity of consciousness, which is a determination of inner sense -- through which the manifold of intuition for such [objective] combination is empirically given. Whether I can become empirically conscious of the manifold as simultaneous or as successive depends on circumstances or empirical conditions. Therefore the empirical unity of consciousness, through association of representations, itself concerns an appearance, and is wholly contingent. But the pure form of intuition in time, merely as intuition in general, which contains a given manifold, is subject to the original unity of consciousness, simply through the necessary relation of the manifold of the intuition to the one 'I think', and so through the pure synthesis of understanding which is the a priori underlying ground of the empirical synthesis. Only the original unity is objectively valid; the empirical unity of apperception, upon which we are not here dwelling, and which besides is merely derived from the former under given conditions in concreto, has only subjective validity. To one man, for instance, a certain word suggests one thing, to another some other thing; the unity of consciousness in that which is empirical is not, as regards what is given, necessarily and universally valid.

The Logical Form of all Judgments consists in the Objective Unity of the Apperception of the Concepts which they contain

I have never been able to accept the interpretation which logicians give of judgment in general. It is, they declare, the representation of a relation between two concepts. I do not here dispute with them as to what is defective in this interpretation -- that in any case it applies only to categorical, not to hypothetical and disjunctive judgments (the two latter containing a relation not of concepts but of judgments), an oversight from which many troublesome consequences have followed. I need only point out that the definition does not determine in what the asserted relation consists.

[The lengthy doctrine of the four syllogistic figures concerns categorical syllogisms only; and although it is indeed nothing more than an artificial method of securing, through the surreptitious introduction of immediate inferences (consequentiae immediatae) among the premisses of a pure syllogism, the appearance that there are more kinds of inference than that of the first figure, this would hardly have met with such remarkable acceptance, had not its authors succeeded in bringing categorical judgments into such exclusive respect, as being those to which all others must allow of being reduced -- teaching which, as indicated in §9, is none the less erroneous.]

But if I investigate more precisely the relation of the given modes of knowledge in any judgment, and distinguish it, as belonging to the understanding, from the relation according to laws of the reproductive imagination, which has only subjective validity, I find that a judgment is nothing but the manner in which given modes of knowledge are brought to the objective unity of apperception. This is what is intended by the copula 'is'. It is employed to distinguish the objective unity of given representations from the subjective.

It indicates their relation to original apperception, and its necessary unity. It holds good even if the judgment is itself empirical, and therefore contingent, as, for example, in the judgment, 'Bodies are heavy'. I do not here assert that these representations necessarily belong to one another in the empirical intuition, but that they belong to one another in virtue of the necessary unity of apperception in the synthesis of intuitions, that is, according to principles of the objective determination of all representations, in so far as knowledge can be acquired by means of these representations -- principles which are all derived from the fundamental principle of the transcendental unity of apperception. Only in this way does there arise from this relation a judgment, that is, a relation which is objectively valid, and so can be adequately distinguished from a relation of the same representations that would have only subjective validity -- as when they are connected according to laws of association. In the latter case, all that I could say would be, 'If I support a body, I feel an impression of weight'; I could not say, 'It, the body, is heavy'. Thus to say 'The body is heavy' is not merely to state that the two representations have always been conjoined in my perception, however often that perception be repeated; what we are asserting is that they are combined in the object, no matter what the state of the subject may be.


All Sensible Intuitions are subject to the Categories, as Conditions under which alone their Manifold can come together in one Consciousness

The manifold given in a sensible intuition is necessarily subject to the original synthetic unity of apperception, because in no other way is the unity of intuition possible (§17). But that act of understanding by which the manifold of given representations (be they intuitions or concepts) is brought under one apperception, is the logical function of judgment (cf. §19). All the manifold, therefore, so far as it is given in a single empirical intuition, is determined in respect of one of the logical functions of judgment, and is thereby brought into one consciousness. Now the categories are just these functions of judgment, in so far as they are employed in determination of the manifold of a given intuition (cf. §13). Consequently, the manifold in a given intuition is necessarily subject to the categories.


A manifold, contained in an intuition which I call mine, is represented, by means of the synthesis of the understanding, as belonging to the necessary unity of self-consciousness; and this is effected by means of the category.

[The proof of this rests on the represented unity of intuition, by which an object is given. This unity of intuition always includes in itself a synthesis of the manifold given for an intuition, and so already contains the relation of this manifold to the unity of apperception.]

This [requirement of a] category therefore shows that the empirical consciousness of a given manifold in a single intuition is subject to a pure self- consciousness a priori, just as is empirical intuition to a pure sensible intuition, which likewise takes place a priori. Thus in the above proposition a beginning is made of a deduction of the pure concepts of understanding; and in this deduction, since the categories have their source in the understanding alone, independently of sensibility, I must abstract from the mode in which the manifold for an empirical intuition is given, and must direct attention solely to the unity which, in terms of the category, and by means of the understanding, enters into the intuition. In what follows (cf. §26) it will be shown, from the mode in which the empirical intuition is given in sensibility, that its unity is no other than that which the category (according to §20) prescribes to the manifold of a given intuition in general. Only thus, by demonstration of the a priori validity of the categories in respect of all objects of our senses, will the purpose of the deduction be fully attained.

But in the above proof there is one feature from which I could not abstract, the feature, namely, that the manifold to be intuited must be given prior to the synthesis of understanding, and independently of it. How this takes place, remains here undetermined. For were I to think an understanding which is itself intuitive (as, for example, a divine understanding which should not represent to itself given objects, but through whose representation the objects should themselves be given or produced), the categories would have no meaning whatsoever in respect of such a mode of knowledge. They are merely rules for an understanding whose whole power consists in thought, consists, that is, in the act whereby it brings the synthesis of a manifold, given to it from elsewhere in intuition, to the unity of apperception -- a faculty, therefore, which by itself knows nothing whatsoever, but merely combines and arranges the material of knowledge, that is, the intuition, which must be given to it by the object. This peculiarity of our understanding, that it can produce a priori unity of apperception solely by means of the categories, and only by such and so many, is as little capable of further explanation as why we have just these and no other functions of judgment, or why space and time are the only forms of our possible intuition.

The Category has no other Application in Knowledge than to Objects of Experience

To think an object and to know an object are thus by no means the same thing. Knowledge involves two factors: first, the concept, through which an object in general is thought (the category); and secondly, the intuition, through which it is given. For if no intuition could be given corresponding to the concept, the concept would still indeed be a thought, so far as its form is concerned, but would be without any object, and no knowledge of anything would be possible by means of it. So far as I could know, there would be nothing, and could be nothing, to which my thought could be applied. Now, as the Aesthetic has shown, the only intuition possible to us is sensible; consequently, the thought of an object in general, by means of a pure concept of understanding, can become knowledge for us only in so far as the concept is related to objects of the senses. Sensible intuition is either pure intuition (space and time) or empirical intuition of that which is immediately represented, through sensation, as actual in space and time. Through the determination of pure intuition we can acquire a priori knowledge of objects, as in mathematics, but only in regard to their form, as appearances; whether there can be things which must be intuited in this form, is still left undecided. Mathematical concepts are not, therefore, by themselves knowledge, except on the supposition that there are things which allow of being presented to us only in accordance with the form of that pure sensible intuition. Now things in space and time are given only in so far as they are perceptions (that is, representations accompanied by sensation) -- therefore only through empirical representation. Consequently, the pure concepts of understanding, even when they are applied to a priori intuitions, as in mathematics, yield knowledge only in so far as these intuitions -- and therefore indirectly by their means the pure concepts also -- can be applied to empirical intuitions. Even, therefore, with the aid of [pure] intuition, the categories do not afford us any knowledge of things; they do so only through their possible application to empirical intuition. In other words, they serve only for the possibility of empirical knowledge; and such knowledge is what we entitle experience. Our conclusion is therefore this: the categories, as yielding knowledge of things, have no kind of application, save only in regard to things which may be objects of possible experience.


The above proposition is of the greatest importance; for it determines the limits of the employment of the pure concepts of understanding in regard to objects, just as the Transcendental Aesthetic determined the limits of the employment of the pure form of our sensible intuition. Space and time, as conditions under which alone objects can possibly be given to us, are valid no further than for objects of the senses, and therefore only for experience. Beyond these limits they represent nothing; for they are only in the senses, and beyond them have no reality. The pure concepts of understanding are free from this limitation, and extend to objects of intuition in general, be the intuition like or unlike ours, if only it be sensible and not intellectual. But this extension of concepts beyond our sensible intuition is of no advantage to us. For as concepts of objects they are then empty, and do not even enable us to judge of their objects whether or not they are possible. They are mere forms of thought, without objective reality, since we have no intuition at hand to which the synthetic unity of apperception, which constitutes the whole content of these forms, could be applied, and in being so applied determine an object. Only our sensible and empirical intuition can give to them body and meaning.

If we suppose an object of a non-sensible intuition to be given, we can indeed represent it through all the predicates which are implied in the presupposition that it has none of the characteristics proper to sensible intuition; that it is not extended or in space, that its duration is not a time, that no change (succession of determinations in time) is to be met with in it, etc. But there is no proper knowledge if I thus merely indicate what the intuition of an object is not, without being able to say what it is that is contained in the intuition. For I have not then shown that the object which I am thinking through my pure concept is even so much as possible, not being in a position to give any intuition corresponding to the concept, and being able only to say that our intuition is not applicable to it. But what has chiefly to be noted is this, that to such a something [in general] not a single one of all the categories could

be applied. We could not, for instance, apply to it the concept of substance, meaning something which can exist as subject and never as mere predicate. For save in so far as empirical intuition provides the instance to which to apply it, I do not know whether there can be anything that corresponds to such a form of thought. But of this more hereafter.

The Application of the Categories to Objects of the Senses in General

The pure concepts of understanding relate, through the mere understanding, to objects of intuition in general, whether that intuition be our own or any other, provided only it be sensible. The concepts are, however, for this very reason, mere forms of thought, through which alone no determinate object is known. The synthesis or combination of the manifold in them relates only to the unity of apperception, and is thereby the ground of the possibility of a priori knowledge, so far as such knowledge rests on the understanding. This synthesis, therefore, is at once transcendental and also purely intellectual. But since there lies in us a certain form of a priori sensible intuition, which depends on the receptivity of the faculty of representation (sensibility), the understanding, as spontaneity, is able to determine inner sense through the manifold of given representations, in accordance with the synthetic unity of apperception, and so to think synthetic unity of the apperception of the manifold of a priori sensible intuition -- that being the condition under which all objects of our human intuition must necessarily stand. In this way the categories, in themselves mere forms of thought, obtain objective reality, that is, application to objects which can be given us in intuition. These objects, however, are only appearances, for it is solely of appearances that we can have a priori intuition.

This synthesis of the manifold of sensible intuition, which is possible and necessary a priori, may be entitled figurative synthesis (synthesis speciosa), to distinguish it from the synthesis which is thought in the mere category in respect of the manifold of an intuition in general, and which is entitled combination through the understanding (synthesis intellectualis). Both are transcendental, not merely as taking place a priori, but also as conditioning the possibility of other a priori knowledge.

But the figurative synthesis, if it be directed merely to the original synthetic unity of apperception, that is, to the transcendental unity which is thought in the categories, must, in order to be distinguished from the merely intellectual combination, be called the transcendental synthesis of imagination. Imagination is the faculty of representing in intuition an object that is not itself present. Now since all our intuition is sensible, the imagination, owing to the subjective condition under which alone it can give to the concepts of understanding a corresponding intuition, belongs to sensibility. But inasmuch as its synthesis is an expression of spontaneity, which is determinative and not, like sense, determinable merely, and which is therefore able to determine sense a priori in respect of its form in accordance with the unity of apperception, imagination is to that extent a faculty which determines the sensibility a priori; and its synthesis of intuitions, conforming as it does to the categories, must be the transcendental synthesis of imagination.

It is an operation of the understanding on sensibility, and the first application of the understanding to objects of possible intuition, and at the same time the basis for the exercise of the other functions of that faculty. As figurative, it is distinguished from the merely intellectual synthesis, which is produced by the understanding alone, without the aid of imagination. Now, in so far as imagination is spontaneity, I sometimes call it also the productive imagination, and distinguish it from the reproductive, the synthesis of which is subject entirely to empirical laws, those of association, namely, and which, therefore, contributes nothing to the explanation of the possibility of a priori cognition, and for this reason belongs not to transcendental philosophy, but to psychology.

* * *

This is a suitable place for explaining the paradox which must have struck every one in our exposition of the internal sense, namely- how this sense represents us to our own consciousness, only as we appear to ourselves, not as we are in ourselves, because, to wit, we intuit ourselves only as we are inwardly affected, and this would seem to be contradictory, since we thus stand in a passive relation to ourselves. It is to avoid this contradiction that in systems of psychology, inner sense, which we have carefully distinguished from the faculty of apperception, is commonly regarded as being identical with it.

That which determines the internal sense is the understanding, and its original power of conjoining the manifold of intuition, that is, of bringing this under an apperception (upon which rests the possibility of the understanding itself). Now, as the human understanding is not in itself a faculty of intuition, and is unable to exercise such a power, in order to conjoin, as it were, the manifold of its own intuition, the synthesis of understanding is, considered per se, nothing but the unity of action, of which, as such, it is self-conscious, even apart from sensibility, by which, moreover, it is able to determine our internal sense in respect of the manifold which may be presented to it according to the form of sensuous intuition. Thus, under the name of a transcendental synthesis of imagination, the understanding exercises an activity upon the passive subject, whose faculty it is; and so we are right in saying that the internal sense is affected thereby. Apperception and its synthetical unity are by no means one and the same with inner sense. The former, as the source of all combination, applies to the manifold of intuition in general, prior to all sensible intuition, to objects in general. Inner sense, on the other hand, contains the mere form of intuition, but without combination of the manifold in t, and therefore so far contains no determinate intuition, which is possible only through the consciousness of the determination of the manifold by the transcendental act of imagination (synthetic influence of the understanding upon inner sense), which I have entitled figurative synthesis.

This we can indeed always perceive in ourselves. We cannot cogitate a geometrical line without drawing it in thought, nor a circle without describing it, nor represent the three dimensions of space without drawing three lines from the same point perpendicular to one another. We cannot even cogitate time, unless, in drawing a straight line (which is to serve as the external figurative representation of time), we fix our attention on the act of the synthesis of the manifold, whereby we determine successively the internal sense, and thus attend also to the succession of this determination. Motion as an act of the subject (not as a determination of an object),* consequently the synthesis of the manifold in space, if we make abstraction of space and attend merely to the act by which we determine the inner sense according to its form, is that which produces the conception of succession. The understanding, therefore, does by no means find in the internal sense any such synthesis of the manifold, but produces it, in that it affects this sense.

At the same time, how "I who think" is distinct from the "I" which intuits itself (other modes of intuition being cogitable as at least possible), and yet one and the same with this latter as the same subject; how, therefore, I am able to say: "I, as an intelligence and thinking subject, cognise myself as an object thought, so far as I am, moreover, given to myself in intuition- only, like other phenomena, not as I am in myself, and as considered by the understanding, but merely as I appear"- is a question that has in it neither more nor less difficulty than the question- "How can I be an object to myself?" or this- "How I can be an object of my own intuition and internal perceptions?" But that such must be the fact, if we admit that space is merely a pure form of the phenomena of external sense, can be clearly proved by the consideration that we cannot represent time, which is not an object of external intuition, in any other way than under the image of a line, which we draw in thought, a mode of representation without which we could not cognise the unity of its dimension, and also that we are necessitated to take our determination of periods of time, or of points of time, for all our internal perceptions from the changes which we perceive in outward things. It follows that we must arrange the determinations of the internal sense, as phenomena in time, exactly in the same manner as we arrange those of the external senses in space. And consequently, if we grant, respecting this latter, that by means of them we know objects only in so far as we are affected externally, we must also confess, with regard to the internal sense, that by means of it we intuit ourselves only as we are internally affected by ourselves; in other words, as regards internal intuition, we cognise our own subject only as phenomenon, and not as it is in itself.

§ 25

On the other hand, in the transcendental synthesis of the manifold content of representations, consequently in the synthetical unity of apperception, I am conscious of myself, not as I appear to myself, nor as I am in myself, but only that "I am." This representation is a thought, not an intuition. Now, as in order to cognise ourselves, in addition to the act of thinking, which subjects the manifold of every possible intuition to the unity of apperception, there is necessary a determinate mode of intuition, whereby this manifold is given; although my own existence is certainly not mere phenomenon (much less mere illusion), the determination of my existence* Can only take place conformably to the form of the internal sense, according to the particular mode in which the manifold which I conjoin is given in internal intuition, and I have therefore no knowledge of myself as I am, but merely as I appear to myself. The consciousness of self is thus very far from a knowledge of self, in which I do not use the categories, whereby I cogitate an object, by means of the conjunction of the manifold in one apperception. In the same way as I require, for the sake of the cognition of an object distinct from myself, not only the thought of an object in general (in the category), but also an intuition by which to determine that general conception, in the same way do I require, in order to the cognition of myself, not only the consciousness of myself or the thought that I think myself, but in addition an intuition of the manifold in myself, by which to determine this thought. It is true that I exist as an intelligence which is conscious only of its faculty of conjunction or synthesis, but subjected in relation to the manifold which this intelligence has to conjoin to a limitative conjunction called the internal sense. My intelligence (that is, I) can render that conjunction or synthesis perceptible only according to the relations of time, which are quite beyond the proper sphere of the conceptions of the understanding and consequently cognise itself in respect to an intuition (which cannot possibly be intellectual, nor given by the understanding), only as it appears to itself, and not as it would cognise itself, if its intuition were intellectual.

Transcendental Deduction of the Universally Possible Employment in experience of the Pure Concepts of the Understanding

In the metaphysical deduction the a priori origin of the categories has been proved through their complete agreement with the general logical functions of thought; in the transcendental deduction we have shown their possibility as a priori modes of knowledge of objects of an intuition in general (cf. §§20, 21). We have now to explain the possibility of knowing a priori, by means of categories, whatever objects may present themselves to our senses, not indeed in respect of the form of their intuition, but in respect of the laws of their combination, and so, as it were, of prescribing laws to nature, and even of making nature possible. For unless the categories discharged this function, there could be no explaining why everything that can be presented to our senses must be subject to laws which have their origin a priori in the understanding alone.

First of all, I may draw attention to the fact that by synthesis of apprehension I understand that combination of the manifold in an empirical intuition, whereby perception, that is, empirical consciousness of the intuition (as appearance), is possible.

In the representations of space and time we have a priori forms of outer and inner sensible intuition; and to these the synthesis of apprehension of the manifold of appearance must always conform, because in no other way can the synthesis take place at all. But space and time are represented a priori not merely as forms of sensible intuition, but as themselves intuitions which contain a manifold [of their own], and therefore are represented with the determination of the unity of this manifold (see the Transcendental Aesthetic).

[Space, represented as object (as we are required to do in geometry), contains more than mere form of intuition; it also contains combination of the manifold, given according to the form of sensibility, in an intuitive representation, so that the form of intuition gives only a manifold, the formal intuition gives unity of representation. In the Aesthetic I have treated this unity as belonging merely to sensibility, simply in order to emphasise that it precedes any concept, although, as a matter of fact, it presupposes a synthesis which does not belong to the senses but through which all concepts of space and time first become possible. For since by its means (in that the understanding determines the sensibility) space and time are first given as intuitions, the unity of this a priori intuition belongs to space and time, and not to the concept of the understanding (cf. §24).]

Thus unity of the synthesis of the manifold, without or within us, and consequently also a combination to which everything that is to be represented as determined in space or in time must conform, is given a priori as the condition of the synthesis of all apprehension -- not indeed in, but with these intuitions. This synthetic unity can be no other than the unity of the combination of the manifold of a given intuition in general in an original consciousness, in accordance with the categories, in so far as the combination is applied to our sensible intuition. All synthesis, therefore, even that which renders perception possible, is subject to the categories; and since experience is knowledge by means of connected perceptions, the categories are conditions of the possibility of experience, and are therefore valid a priori for all objects of experience.

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When, for instance, by apprehension of the manifold of a house I make the empirical intuition of it into a perception, the necessary unity of space and of outer sensible intuition in general lies at the basis of my apprehension, and I draw as it were the outline of the house in conformity with this synthetic unity of the manifold in space. But if I abstract from the form of space, this same synthetic unity has its seat in the understanding, and is the category of the synthesis of the homogeneous in an intuition in general, that is, the category of quantity. To this category, therefore, the synthesis of apprehension, that is to say, the perception, must completely conform.

[In this manner it is proved that the synthesis of apprehension, which is empirical, must necessarily be in conformity with the synthesis of apperception, which is intellectual and is contained in the category completely a priori. It is one and the same spontaneity, which in the one case, under the title of imagination, and in the other case, under the title of understanding, brings combination into the manifold of intuition.]

When, to take another example, I perceive the freezing of water, I apprehend two states, fluidity and solidity, and these as standing to one another in a relation of time. But in time, which I place at the basis of the appearance [in so far] as [it is] inner intuition, I necessarily represent to myself synthetic unity of the manifold, without which that relation of time could not be given in an intuition as being determined in respect of time-sequence. Now this synthetic unity, as a condition a priori under which I combine the manifold of an intuition in general, is -- if I abstract from the constant form of my inner intuition, namely, time -- the category of cause, by means of which, when I apply it to my sensibility, I determine everything that happens in accordance with the relation which it prescribes, and I do so in time in general. Thus my apprehension of such an event, and therefore the event itself, considered as a possible perception, is subject to the concept of the relation of effects and causes, and so in all other cases.

Categories are concepts which prescribe laws a priori to appearances, and therefore to nature, the sum of all appearances (natura materialiter spectata). The question therefore arises, how it can be conceivable that nature should have to proceed in accordance with categories which yet are not derived from it, and do not model themselves upon its pattern; that is, how they can determine a priori the combination of the manifold of nature, while yet they are not derived from it. The solution of this seeming enigma is as follows.

That the laws of appearances in nature must agree with the understanding and its a priori form, that is, with its faculty of combining the manifold in general, is no more surprising than that the appearances themselves must agree with the form of a priori sensible intuition. For just as appearances do not exist in themselves but only relatively to the subject in which, so far as it has senses, they inhere, so the laws do not exist in the appearances but only relatively to this same being, so far as it has understanding. Things in themselves would necessarily, apart from any understanding that knows them, conform to laws of their own. But appearances are only representations of things which are unknown as regards what they may be in themselves. As mere representations, they are subject to no law of connection save that which the connecting faculty prescribes. Now it is imagination that connects the manifold of sensible intuition; and imagination is dependent for the unity of its intellectual synthesis upon the understanding, and for the manifoldness of its apprehension upon sensibility. All possible perception is thus dependent upon synthesis of apprehension, and this empirical synthesis in turn upon transcendental synthesis, and therefore upon the categories. Consequently, all possible perceptions, and therefore everything that can come to empirical consciousness, that is, all appearances of nature, must, so far as their connection is concerned, be subject to the categories. Nature, considered merely as nature in general, is dependent upon these categories as the original ground of its necessary conformity to law (natura formaliter spectata). Pure understanding is not, however, in a position, through mere categories, to prescribe to appearances any a priori laws other than those which are involved in a nature in general, that is, in the conformity to law of all appearances in space and time. Special laws, as concerning those appearances which are empirically determined, cannot in their specific character be derived from the categories, although they are one and all subject to them. To obtain any knowledge whatsoever of these special laws, we must resort to experience; but it is the a priori laws that alone can instruct us in regard to experience in general, and as to what it is that can be known as an object of experience.

Outcome of this Deduction of the Concepts of Understanding

We cannot think an object save through categories; we cannot know an object so thought save through intuitions corresponding to these concepts. Now all our intuitions are sensible; and this knowledge, in so far as its object is given, is empirical. But empirical knowledge is experience. Consequently, there can be no a priori knowledge, except of objects of possible experience.

[Lest my readers should stumble at the alarming evil consequences which may over-hastily be inferred from this statement, I may remind them that for thought the categories are not limited by the conditions of our sensible intuition, but have an unlimited field. It is only the knowledge of that which we think, the determining of the object, that requires intuition. In the absence of intuition, the thought of the object may still have its true and useful consequences, as regards the subject's employment of reason. The use of reason is not always directed to the determination of an object, that is, to knowledge, but also to the determination of the subject and of its volition -- a use which cannot be here dealt with.]

But although this knowledge is limited to objects of experience, it is not therefore all derived from experience. The pure intuitions [of receptivity] and the pure concepts of understanding are elements in knowledge, and both are found in us a priori. There are only two ways in which we can account for a necessary agreement of experience with the concepts of its objects: either experience makes these concepts possible or these concepts make experience possible. The former supposition does not hold in respect of the categories (nor of pure sensible intuition); for since they are a priori concepts, and therefore independent of experience, the ascription to them of an empirical origin would be a sort of generatio aequivoca. There remains, therefore, only the second supposition -- a system, as it were, of the epigenesis of pure reason -- namely, that the categories contain, on the side of the understanding, the grounds of the possibility of all experience in general. How they make experience possible, and what are the principles of the possibility of experience that they supply in their application to appearances, will be shown more fully in the following chapter on the transcendental employment of the faculty of judgment.

A middle course may be proposed between the two above mentioned, namely, that the categories are neither self-thought first principles a priori of our knowledge nor derived from experience, but subjective dispositions of thought, implanted in us from the first moment of our existence, and so ordered by our Creator that their employment is in complete harmony with the laws of nature in accordance with which experience proceeds -- a kind of preformation-system of pure reason.

Apart, however, from the objection that on such an hypothesis we can set no limit to the assumption of predetermined dispositions to future judgments, there is this decisive objection against the suggested middle course, that the necessity of the categories, which belongs to their very conception, would then have to be sacrificed. The concept of cause, for instance, which expresses the necessity of an event under a presupposed condition, would be false if it rested only on an arbitrary subjective necessity, implanted in us, of connecting certain empirical representations according to the rule of causal relation. I would not then be able to say that the effect is connected with the cause in the object, that is to say, necessarily, but only that I am so constituted that I cannot think this representation otherwise than as thus connected. This is exactly what the sceptic most desires. For if this be the situation, all our insight, resting on the supposed objective validity of our judgments, is nothing but sheer illusion; nor would there be wanting people who would refuse to admit this subjective necessity, a necessity which can only be felt. Certainly a man cannot dispute with anyone regarding that which depends merely on the mode in which he is himself organised.

Brief Outline of this Deduction

The deduction is the exposition of the pure concepts of the understanding, and therewith of all theoretical a priori knowledge, as principles of the possibility of experience -- the principles being here taken as the determination of appearances in space and time in general, and this determination, in turn, as ultimately following from the original synthetic unity of apperception, as the form of the understanding in its relation to space and time, the original forms of sensibility.

I consider the division by numbered paragraphs as necessary up to this point, because thus far we have had to treat of the elementary concepts. We have now to give an account of their employment, and the exposition may therefore proceed in continuous fashion, without such numbering.