From this it follows incontestably, that the pure concepts of the understanding never admit of a transcendental, but only of an empirical use, and that the principles of the pure understanding can only be referred, as general conditions of a possible experience, to objects of the senses, never to things by themselves (without regard to the manner in which we have to look at them).
Transcendental Analytic has therefore yielded us this important result, that the understanding a priori can never do more than anticipate the form of a possible experience; and as nothing can be an object of experience except the phenomenon, it follows that the understanding can never go beyond the limits of sensibility, within which alone objects are given to us. Its principles are principles for the exhibition of phenomena only; and the proud name of Ontology, which presumes to supply in a systematic form different kinds of synthetical knowledge a priori of things by themselves (for instance the principle of causality), must be replaced by the more modest name of a mere Analytic of the pure understanding.
Thought is the act of referring a given intuition to an object. If the mode of such intuition is not given, the object is called transcendental, and the concept of the understanding admits then of a transcendental use only, in producing a unity in the thought of the manifold in general. A pure category therefore, in which every condition of sensuous intuition, the only one that is possible for us, is left out, cannot determine an object, but only the thought of an object in general, according to different modes. Now, if we want to use a concept, we require in addition some function of the faculty of judgment, by which an object is subsumed under a concept, consequently the at least formal condition under which something can be given in intuition. If this condition of the faculty of judgment (schema) is wanting, all subsumption is impossible, because nothing is given that could be subsumed under the concept. The purely transcendental use of categories therefore is in reality of no use at all, and has no definite or even, with regard to its form only, definable object. Hence it follows that a pure category is not fit for any synthetical a priori principle, and that the principles of the pure understanding admit of empirical only, never of transcendental application, nay, that no synthetical principles a priori are possible beyond the field of possible experience.
It might therefore be advisable to express ourselves in the following way: the pure categories, without the formal conditions of sensibility, have a transcendental character only, but do not admit of any transcendental use, because such use in itself is impossible, as the categories are deprived of all the conditions of being used in judgments, that is, of the formal conditions of the subsumption of any possible object under these concepts. As therefore (as pure categories) they are not meant to be used empirically, and cannot be used transcendentally, they admit, if separated from sensibility, of no use at all; that is, they cannot be applied to any possible object, and are nothing but the pure form of the use of the understanding with reference to objects in general, and of thought, without ever enabling us to think or determine any object by their means alone.
[Appearances,5 so far as they are thought as objects under the unity of the categories, are called phenomena. But if I admit things which are objects of the understanding only, and nevertheless can be given as objects of an intuition, though not of sensuous intuition (as coram intuitu intellectuali), such things would be called Noumena (intelligibilia).
One might feel inclined to think that the concept of Phenomena, as limited by the transcendental æsthetic, suggested by itself the objective reality of the Noumena, and justified a division of objects into phenomena and noumena, and consequently of the world into a sensible and intelligible world (mundus sensibilis et intelligibilis); and this in such a way that the distinction between the two should not refer to the logical form only of a more or less clear knowledge of one and the same object, but to a difference in their original presentation to our knowledge, which makes them to differ in themselves from each other in kind. For if the senses only represent to us something as it appears, that something must by itself also be a thing, and an object of a non-sensuous intuition, i.e. of the understanding. That is, there must be a kind of knowledge in which there is no sensibility, and which alone possesses absolute objective reality, representing objects as they are, while through the empirical use of our understanding we know things only as they appear. Hence it would seem to follow that, beside the empirical use of the categories (limited by sensuous conditions), there was another one, pure and yet objectively valid, and that we could not say, as we have hitherto done, that our knowledge of the pure understanding contained nothing but principles for the exhibition of phenomena, which, even a priori, could not apply to anything but the formal possibility of experience. Here, in fact, quite a new field would seem to be open, a world, as it were, realised in thought (nay, according to some, even in intuition), which would be a more, and not a less, worthy object for the pure understanding.
All our representations are no doubt referred by the understanding to some sort of object, and as phenomena are nothing but representations, the understanding refers them to a something, as the object of our sensuous intuition, this something being however the transcendental object only. This means a something equal to x, of which we do not, nay, with the present constitution of our understanding, cannot know anything, but which6 can only serve, as a correlatum of the unity of apperception, for the unity of the manifold in sensuous intuition, by means of which the understanding unites the manifold into the concept of an object. This transcendental object cannot be separated from the sensuous data, because in that case nothing would remain by which it could be thought. It is not therefore an object of knowledge in itself, but only the representation of phenomena, under the concept of an object in general, which can be defined by the manifold of sensuous intuition.
For this very reason the categories do not represent a peculiar object, given to the understanding only, but serve only to define the transcendental object (the concept of something in general) by that which is given us through the senses, in order thus to know empirically phenomena under the concepts of objects.
What then is the cause why people, not satisfied with the substratum of sensibility, have added to the phenomena the noumena, which the understanding only is supposed to be able to realise? It is this, that sensibility and its sphere, that is the sphere of phenomena, is so limited by the understanding itself that it should not refer to things by themselves, but only to the mode in which things appear to us, in accordance with our own subjective qualification. This was the result of the whole transcendental æsthetic, and it really follows quite naturally from the concept of a phenomenon in general, that something must correspond to it, which in itself is not a phenomenon, because a phenomenon cannot be anything by itself, apart from our mode of representation. Unless therefore we are to move in a constant circle, we must admit that the very word phenomenon indicates a relation to something the immediate representation of which is no doubt sensuous, but which nevertheless, even without this qualification of our sensibility (on which the form of our intuition is founded) must be something by itself, that is an object independent of our sensibility.
Hence arises the concept of a noumenon, which however is not positive, nor a definite knowledge of anything, but which implies only the thinking of something, without taking any account of the form of sensuous intuition. But in order that a noumenon may signify a real object that can be distinguished from all phenomena, it is not enough that I should free my thought of all conditions of sensuous intuition, but I must besides have some reason for admitting another kind of intuition besides the sensuous, in which such an object can be given; otherwise my thought would be empty, however free it may be from contradictions. It is true that we were not able to prove that the sensuous is the only possible intuition, though it is so for us: but neither could we prove that another kind of intuition was possible; and although our thought may take no account of any sensibility, the question always remains whether
, after that, it is not a mere form of a concept, and whether any real object would thus be left.
The object to which I refer the phenomenon in general is the transcendental object, that is, the entirely indefinite thought of something in general. This cannot be called, the noumenon, for I know nothing of what it is by itself, and have no conception of it, except as the object of sensuous intuition in general, which is therefore the same for all phenomena. I cannot lay hold of it by any of the categories, for these are valid for empirical intuitions only, in order to bring them under the concept of an object in general. A pure use of the categories is no doubt possible, that is, not self-contradictory, but it has no kind of objective validity, because it refers to no intuition to which it is meant to impart the unity of an object. The categories remain for ever mere functions of thought by which no object can be given to me, but by which I can only think whatever may be given to me in intuition.]
If all thought (by means of categories) is taken away from empirical knowledge, no knowledge of any object remains, because nothing can be thought by mere intuition, and the mere fact that there is within me an affection of my sensibility, establishes in no way any relation of such a representation to any object. If, on the contrary, all intuition is taken away, there always remains the form of thought, that is, the mode of determining an object for the manifold of a possible intuition. In this sense the categories may be said to extend further than sensuous intuition, because they can think objects in general without any regard to the special mode of sensibility in which they may be given; but they do not thus prove a larger sphere of objects, because we cannot admit that such objects can be given, without admitting the possibility of some other but sensuous intuition, for which we have no right whatever.
I call a concept problematic, if it is not self-contradictory, and if, as limiting other concepts, it is connected with other kinds of knowledge, while its objective reality cannot be known in any way. Now the concept of a noumenon, that is of a thing which can never be thought as an object of the senses, but only as a thing by itself (by the pure understanding), is not self-contradictory, because we cannot maintain that sensibility is the only form of intuition. That concept is also necessary, to prevent sensuous intuition from extending to things by themselves; that is, in order to limit the objective validity of sensuous knowledge (for all the rest to which sensuous intuition does not extend is called noumenon, for the very purpose of showing that sensuous knowledge cannot extend its domain over everything that can be thought by the understanding). But, after all, we cannot understand the possibility of such noumena, and whatever lies beyond the sphere of phenomena is (to us) empty; that is, we have an understanding which problematically extends beyond that sphere, but no intuition, nay not even the conception of a possible intuition, by which, outside the field of sensibility, objects could be given to us, and our understanding could extend beyond that sensibility in its assertory use. The concept of a noumenon is therefore merely limitative, and intended to keep the claims of sensibility within proper bounds, therefore of negative use only. But it is not a mere arbitrary fiction, but closely connected with the limitation of sensibility, though incapable of adding anything positive to the sphere of the senses.
A real division of objects into phenomena and noumena, and of the world into a sensible and intelligible world (in a positive sense),7 is therefore quite inadmissible, although concepts may very well be divided into sensuous and intellectual. For no objects can be assigned to these intellectual concepts, nor can they be represented as objectively valid. If we drop the senses, how are we to make it conceivable that our categories (which would be the only remaining concepts for noumena) have any meaning at all, considering that, in order to refer them to any object, something more must be given than the mere unity of thought, namely, a possible intuition, to which the categories could be applied? With all this the concept of a noumenon, if taken as problematical only, remains not only admissible, but, as a concept to limit the sphere of sensibility, indispensable. In this case, however, it is not a particular intelligible object for our understanding, but an understanding to which it could belong is itself a problem, if we ask how it could know an object, not discursively by means of categories, but intuitively, and yet in a non-sensuous intuition,—a process of which we could not understand even the bare possibility. Our understanding thus acquires a kind of negative extension, that is, it does not become itself limited by sensibility, but, on the contrary, limits it, by calling things by themselves (not considered as phenomena) noumena. In doing this, it immediately proceeds to prescribe limits to itself, by admitting that it cannot know these noumena by means of the categories, but can only think of them under the name of something unknown.
In the writings of modern philosophers, however, I meet with a totally different use of the terms of mundus sensibilis and intelligibilis8, totally different from the meaning assigned to these terms by the ancients. Here all difficulty seems to disappear. But the fact is, that there remains nothing but mere word-mongery. In accordance with this, some people have been pleased to call the whole of phenomena, so far as they are seen, the world of sense; but so far as their connection, according to general laws of the understanding, is taken into account, the world of the understanding. Theoretical astronomy, which only teaches the actual observation of the starry heavens, would represent the former; contemplative astronomy, on the contrary (taught according to the Copernican system, or, it may be, according to Newton's laws of gravitation), the latter, namely, a purely intelligible world. But this twisting of words is a mere sophistical excuse, in order to avoid a troublesome question, by changing its meaning according to one's own convenience. Understanding and reason may be applied to phenomena, but it is very questionable whether they can be applied at all to an object which is not a phenomenon, but a noumenon; and it is this, when the object is represented as purely intelligible, that is, as given to the understanding only, and not to the senses. The question therefore is whether, besides the empirical use of the understanding (even in the Newtonian view of the world), a transcendental use is possible, referring to the noumenon, as its object; and that question we have answered decidedly in the negative.
When we therefore say that the senses represent objects to us as they appear, and the understanding as they are, the latter is not to be taken in a transcendental, but in a purely empirical meaning, namely, as to how they, as objects of experience, must be represented, according to the regular connection of phenomena, and not according to what they may be, as objects of the pure understanding, apart from their relation to possible experience, and therefore to our senses. This will always remain unknown to us; nay, we shall never know whether such a transcendental and exceptional knowledge is possible at all, at least as comprehended under our ordinary categories. With us understanding and sensibility cannot determine objects, unless they are joined together. If we separate them, we have intuitions without concepts, or concepts without intuitions, in both cases representations which we cannot refer to any definite object.
If, after all these arguments, anybody should still hesitate to abandon the purely transcendental use of the categories, let him try an experiment with them for framing any synthetical proposition. An analytical proposition does not in the least advance the understanding, which, as in such a proposition it is only concerned with what is already thought in the concept, does not ask whether the concept in itself has any reference to objects, or expresses only the unity of thought in general (this completely ignoring the manner in which an object may be given). The understanding in fact is satisfied if it knows what it contained in the concept of an object; it is indifferent as to the object to which the concept may refer. But let him try the experiment with any synthetical and so-called transcendental proposition, as for instance, 'Everything that exists, exists as a substance, or as a determination inherent in it,' or 'Everything contingent exists as an effect of some other thing, namely, its cause,' etc. Now I ask, whence can the understa
nding take these synthetical propositions, as the concepts are to apply, not to some possible experience, but to things by themselves (noumena)? Where is that third term to be found which is always required for a synthetical proposition, in order thus to join concepts which have no logical (analytical) relation with each other? It will be impossible to prove such a proposition, nay even to justify the possibility of any such pure assertion, without appealing to the empirical use of the understanding, and thus renouncing entirely the so-called pure and nonsensuous judgment. There are no principles therefore according to which the concepts of pure and merely intelligible objects could ever be applied, because we cannot imagine any way in which they could be given, and the problematic thought, which leaves a place open to them, serves only, like empty space, to limit the sphere of empirical principles, without containing or indicating any other object of knowledge, lying beyond that sphere.
1 Additions of the Second Edition.
2 I am treating here of the real definition, which not only puts in place of the name of a thing other and more intelligible words, but that which contains a clear mark by which the object (definitum) can at all times be safely recognised, and by which the defined concept becomes fit for practical use. A real definition (Realarklärung) must therefore render clear the concept itself, and its objective reality also. Of this kind are the mathematical explanations which represent an object in intuition, according to its concept.
Critique of Pure Reason Page 24
