But though it may be admitted that the only possible deduction of pure knowledge a priori must be transcendental, it has not yet been proved that such a deduction is absolutely necessary. We have before, by means of a transcendental deduction, followed up the concepts of space and time to their very sources, and explained and defined their objective validity a priori. Geometry, however, moves along with a steady step, through every kind of knowledge a priori, without having to ask for a certificate from philosophy as to the pure legitimate descent of its fundamental concept of space. But it should be remarked that in geometry this concept is used with reference to the outer world of sense only, of which space is the pure form of intuition, and where geometrical knowledge, being based on a priori intuition, possesses immediate evidence, the objects being given, so far as their form is concerned, through their very knowledge a priori in intuition. When we come, however, to the pure concepts of the understanding, it becomes absolutely necessary to look for a transcendental deduction, not only for them, but for space also, because they, not being founded on experience, apply to objects generally, without any of the conditions of sensibility; and, speaking of objects, not through predicates of intuition and sensibility, but of pure thought a priori, are not able to produce in intuition a priori any object on which, previous to all experience, their synthesis was founded. These concepts of pure understanding, therefore, not only excite suspicion with regard to the objective validity and the limits of their own application, but render even the concept of space equivocal, because of an inclination to apply it beyond the conditions of sensuous intuition, which was the very reason that made a transcendental deduction of it, such as we gave before, necessary. Before the reader has made a single step in the field of pure reason, he must be convinced of the inevitable necessity of such a transcendental deduction, otherwise he would walk on blindly and, after having strayed in every direction, he would only return to the same ignorance from which he started. He must at the same time perceive the inevitable difficulty of such a deduction, so that he may not complain about obscurity where the object itself is obscure, or weary too soon with our removal of obstacles, the fact being that we have either to surrender altogether all claims to the knowledge of pure reason—the most favourite field of all philosophers, because extending beyond the limits of all possible experience—or to bring this critical investigation to perfection.
It was easy to show before, when treating of the concepts of space and time, how these, though being knowledge a priori, refer necessarily to objects, and how they make a synthetical knowledge of them possible, which is independent of all experience. For, as no object can appear to us, that is, become an object of empirical intuition, except through such pure forms of sensibility, space and time are pure intuitions which contain a priori the conditions of the possibility of objects as phenomena, and the synthesis in these intuitions possesses objective validity.
The categories of the understanding, on the contrary, are not conditions under which objects can be given in intuition, and it is quite possible therefore that objects should appear to us without any necessary reference to the functions of the understanding, thus showing that the understanding contains by no means any of their conditions a priori. There arises therefore here a difficulty, which we did not meet with in the field of sensibility, namely, how subjective conditions of thought can have objective validity, that is, become conditions of the possibility of the knowledge of objects. It cannot be denied that phenomena may be given in intuition without the functions of the understanding. For if we take, for instance, the concept of cause, which implies a peculiar kind of synthesis, consisting in placing according to a rule after something called A something totally different from it, B, we cannot say that it is a priori clear why phenomena should contain something of this kind. We cannot appeal for it to experience, because what has to be proved is the objective validity of this concept a priori. It would remain therefore a priori doubtful whether such a concept be not altogether empty, and without any corresponding object among phenomena. It is different with objects of sensuous intuition. They must conform to the formal conditions of sensibility existing a priori in the mind, because otherwise they could in no way be objects to us. But why besides this they should conform to the conditions which the understanding requires for the synthetical unity of thought, does not seem to follow quite so easily. For we could quite well imagine that phenomena might possibly be such that the understanding should not find them conforming to the conditions of its synthetical unity, and all might be in such confusion that nothing should appear in the succession of phenomena which could supply a rule of synthesis, and correspond, for instance, to the concept of cause and effect, so that this concept would thus be quite empty, null, and meaningless. With all this phenomena would offer objects to our intuition, because intuition by itself does not require the functions of thought.
It might be imagined that we could escape from the trouble of these investigations by saying that experience offers continually examples of such regularity of phenomena as to induce us to abstract from it the concept of cause, and it might be attempted to prove thereby the objective validity of such a concept. But it ought to be seen that in this way the concept of cause cannot possibly arise, and that such a concept ought either to be founded a priori in the understanding or be surrendered altogether as a mere hallucination. For this concept requires strictly that something, A, should be of such a nature that something else, B, follows from it necessarily and according to an absolutely universal rule. Phenomena no doubt supply us with cases from which a rule becomes possible according to which something happens usually, but never so that the result should be necessary. There is a dignity in the synthesis of cause and effect which cannot be expressed empirically, for it implies that the effect is not only an accessory to the cause, but given by it and springing from it. Nor is the absolute universality of the rule a quality inherent in empirical rules, which by means of induction cannot receive any but a relative universality, that is, a more or less extended applicability. If we were to treat the pure concepts of the understanding as merely empirical products, we should completely change their character and their use.
Transition to a Transcendental Deduction of the Categories
Two ways only are possible in which synthetical representations and their objects can agree, can refer to each other with necessity, and so to say meet each other. Either it is the object alone that makes the representation possible, or it is the representation alone that makes the object possible. In the former case their relation is empirical only, and the representation therefore never possible a priori. This applies to phenomena with reference to whatever in them belongs to sensation. In the latter case, though representation by itself (for we do not speak here of its2 causality by means of the will) cannot produce its object so far as its existence is concerned, nevertheless the representation determines the object a priori, if through it alone it is possible to know anything as an object. To know a thing as an object is possible only under two conditions. First, there must be intuition by which the object is given us, though as a phenomenon only, secondly, there must be a concept by which an object is thought as corresponding to that intuition. From what we have said before it is clear that the first condition, namely, that under which alone objects can be seen, exists, so far as the form of intuition is concerned, in the soul a priori. All phenomena therefore must conform to that formal condition of sensibility, because it is through it alone that they appear, that is, that they are given and empirically seen.
Now the question arises whether there are not also antecedent concepts a priori, forming conditions under which alone something can be, if not seen, yet thought as an object in general; for in that case all empirical knowledge of objects would necessarily conform to such concepts, it being impossible that anything should become an object of experience without them. All experience contains, besides the intuition of the senses by which something is given, a concept also of the objec
t, which is given in intuition as a phenomenon. Such concepts of objects in general therefore must form conditions a priori of all knowledge produced by experience, and the objective validity of the categories, as being such concepts a priori, rests on this very fact that by them alone, so far as the form of thought is concerned, experience becomes possible. If by them only it is possible to think any object of experience, it follows that they refer by necessity and a priori to all objects of experience.
There is therefore a principle for the transcendental deduction of all concepts a priori which must guide the whole of our investigation, namely, that all must be recognized as conditions a priori of the possibility of experience, whether of intuition, which is found in it, or of thought. Concepts which supply the objective ground of the possibility of experience are for that very reason necessary. An analysis of the experience in which they are found would not be a deduction, but a mere illustration, because they would there have an accidental character only. Nay, without their original relation to all possible experience in which objects of knowledge occur, their relation to any single object would be quite incomprehensible.
[There are three original sources, or call them faculties or powers of the soul, which contain the conditions of the possibility of all experience, and which themselves cannot be derived from any other faculty, namely, sense, imagination, and apperception. On them is founded—
1. The synopsis of the manifold a priori through the senses.
2. The synthesis of this manifold through the imagination.
3. The unity of that synthesis by means of original apperception.
Besides their empirical use all these faculties have a transcendental use also, referring to the form only and possible a priori. With regard to the senses we have discussed that transcendental use in the first part, and we shall now proceed to an investigation of the remaining two, according to their true nature.3]
Deduction of the Pure Concepts of the Understanding
Section II
Of the a priori Grounds for the Possibility of Experience
[That a concept should be produced entirely a priori and yet refer to an object, though itself neither belonging to the sphere of possible experience, nor consisting of the elements of such an experience, is self-contradictory and impossible. It would have no contents, because no intuition corresponds to it, and intuitions by which objects are given to us constitute the whole field or the complete object of possible experience. An a priori concept therefore not referring to experience would be the logical form only of a concept, but not the concept itself by which something is thought.
If therefore there exist any pure concepts a priori, though they cannot contain anything empirical, they must nevertheless all be conditions a priori of a possible experience, on which alone their objective reality depends.
If therefore we wish to know how pure concepts of the understanding are possible, we must try to find out what are the conditions a priori on which the possibility of experience depends, nay, on which it is founded, apart from all that is empirical in phenomena. A concept expressing this formal and objective condition of experience with sufficient generality might properly be called a pure concept of the understanding. If we once have these pure concepts of the understanding, we may also imagine objects which are either impossible, or, if not impossible in themselves, yet can never be given in any experience. We have only in the connection of those concepts to leave out something which necessarily belongs to the conditions of a possible experience (concept of a spirit), or to extend pure concepts of the understanding beyond what can be reached by experience (concept of God). But the elements of all knowledge a priori, even of gratuitous and preposterous fancies, though not borrowed from experience (for in that case they would not be knowledge a priori) must nevertheless contain the pure conditions a priori of a possible experience and its object, otherwise not only would nothing be thought by them, but they themselves, being without data, could never arise in our mind.
Such concepts, then, which comprehend the pure thinking a priori involved in every experience, are discovered in the categories, and it is really a sufficient deduction of them and a justification of their objective validity, if we succeed in proving that by them alone an object can be thought. But as in such a process of thinking more is at work than the faculty of thinking only, namely, the understanding, and as the understanding, as a faculty of knowledge which is meant to refer to objects, requires quite as much an explanation as to the possibility of such a reference, it is necessary for us to consider the subjective sources which form the foundation a priori for the possibility of experience, not according to their empirical, but according to their transcendental character.
If every single representation stood by itself, as if isolated and separated from the others, nothing like what we call knowledge could ever arise, because knowledge forms a whole of representations connected and compared with each other. If therefore I ascribe to the senses a synopsis, because in their intuition they contain something manifold, there corresponds to it always a synthesis, and receptivity can make knowledge possible only when joined with spontaneity. This spontaneity, now, appears as a threefold synthesis which must necessarily take place in every kind of knowledge, namely, first, that of the apprehension of representations as modifications of the soul in intuition, secondly, of the reproduction of them in the imagination, and, thirdly, that of their recognition in concepts. This leads us to three subjective sources of knowledge which render possible the understanding, and through it all experience as an empirical product of the understanding.
Preliminary Remark
The deduction of the categories is beset with so many difficulties and obliges us to enter so deeply into the first grounds of the possibility of our knowledge in general, that I thought it more expedient, in order to avoid the lengthiness of a complete theory, and yet to omit nothing in so essential an investigation, to add the following four paragraphs with a view of preparing rather than instructing the reader. After that only I shall in the third section proceed to a systematical discussion of these elements of the understanding. Till then the reader must not allow himself to be frightened by a certain amount of obscurity which at first is inevitable on a road never trodden before, but which, when we come to that section, will give way, I hope, to a complete comprehension.
I
Of the Synthesis of Apprehension in Intuition
Whatever the origin of our representations may be, whether they be due to the influence of external things or to internal causes, whether they have arisen a priori or empirically as phenomena, as modifications of the mind they must always belong to the internal sense, and all our knowledge must therefore finally be subject to the formal condition of that internal sense, namely, time, in which they are all arranged, joined, and brought into certain relations to each other. This is a general remark which must never be forgotten in all that follows.
Every representation contains something manifold, which could not be represented as such, unless the mind distinguished the time in the succession of one impression after another; for as contained in one moment, each representation can never be anything but absolute unity. In order to change this manifold into a unity of intuition (as, for instance, in the representation of space), it is necessary first to run through the manifold and then to hold it together. It is this act which I call the synthesis of apprehension, because it refers directly to intuition which no doubt offers something manifold, but which, without a synthesis, can never make it such, as it is contained in one representation.
This synthesis of apprehension must itself be carried out a priori also, that is, with reference to representations which are not empirical. For without it we should never be able to have the representations either of space or time a priori, because these cannot be produced except by a synthesis of the manifold which the senses offer in their original receptivity. It follows therefore that we have a pure synthesis of apprehension.
II
/> Of the Synthesis of Reproduction in Imagination
It is no doubt nothing but an empirical law according to which representations which have often followed or accompanied one another, become associated in the end and so closely united that, even without the presence of the object, one of these representations will, according to an invariable law, produce a transition of the mind to the other. This law of reproduction, however, presupposes that the phenomena themselves are really subject to such a rule, and that there is in the variety of these representations a sequence and concomitancy subject to certain rules; for without this the faculty of empirical imagination would never find anything to do that it is able to do, and remain therefore buried within our mind as a dead faculty, unknown to ourselves. If cinnabar were sometimes red and sometimes black, sometimes light and sometimes heavy, if a man could be changed now into this, now into another animal shape, if on the longest day the fields were sometimes covered with fruit, sometimes with ice and snow, the faculty of my empirical imagination would never be in a position, when representing red colour, to think of heavy cinnabar. Nor, if a certain name could be given sometimes to this, sometimes to that object, or if that the same object could sometimes be called by one, and sometimes by another name, without any rule to which representations are subject by themselves, would it be possible that any empirical synthesis of reproduction should ever take place.
Critique of Pure Reason Page 13