In the transcendental aesthetic we proved that everything intuited in space and time, all objects of a possible experience, are nothing but phenomena, that is, mere representations; and that these, as presented to us- as extended bodies, or as series of changes- have no self-subsistent existence apart from human thought. This doctrine I call Transcendental Idealism.* The realist in the transcendental sense regards these modifications of our sensibility, these mere representations, as things subsisting in themselves.
*I have elsewhere termed this theory formal idealism, to distinguish it from material idealism, which doubts or denies the existence of external things. To avoid ambiguity, it seems advisable in many cases to employ this term instead of that mentioned in the text.
It would be unjust to accuse us of holding the long-decried theory of empirical idealism, which, while admitting the reality of space, denies, or at least doubts, the existence of bodies extended in it, and thus leaves us without a sufficient criterion of reality and illusion. The supporters of this theory find no difficulty in admitting the reality of the phenomena of the internal sense in time; nay, they go the length of maintaining that this internal experience is of itself a sufficient proof of the real existence of its object as a thing in itself.
Transcendental idealism allows that the objects of external intuition- as intuited in space, and all changes in time- as represented by the internal sense, are real. For, as space is the form of that intuition which we call external, and, without objects in space, no empirical representation could be given us, we can and ought to regard extended bodies in it as real. The case is the same with representations in time. But time and space, with all phenomena therein, are not in themselves things. They are nothing but representations and cannot exist out of and apart from the mind. Nay, the sensuous internal intuition of the mind (as the object of consciousness), the determination of which is represented by the succession of different states in time, is not the real, proper self, as it exists in itself- not the transcendental subject- but only a phenomenon, which is presented to the sensibility of this, to us, unknown being. This internal phenomenon cannot be admitted to be a self-subsisting thing; for its condition is time, and time cannot be the condition of a thing in itself. But the empirical truth of phenomena in space and time is guaranteed beyond the possibility of doubt, and sufficiently distinguished from the illusion of dreams or fancy- although both have a proper and thorough connection in an experience according to empirical laws. The objects of experience then are not things in themselves, but are given only in experience, and have no existence apart from and independently of experience. That there may be inhabitants in the moon, although no one has ever observed them, must certainly be admitted; but this assertion means only, that we may in the possible progress of experience discover them at some future time. For that which stands in connection with a perception according to the laws of the progress of experience is real. They are therefore really existent, if they stand in empirical connection with my actual or real consciousness, although they are not in themselves real, that is, apart from the progress of experience.
There is nothing actually given- we can be conscious of nothing as real, except a perception and the empirical progression from it to other possible perceptions. For phenomena, as mere representations, are real only in perception; and perception is, in fact, nothing but the reality of an empirical representation, that is, a phenomenon. To call a phenomenon a real thing prior to perception means either that we must meet with this phenomenon in the progress of experience, or it means nothing at all. For I can say only of a thing in itself that it exists without relation to the senses and experience. But we are speaking here merely of phenomena in space and time, both of which are determinations of sensibility, and not of things in themselves. It follows that phenomena are not things in themselves, but are mere representations, which if not given in us- in perception- are non-existent.
The faculty of sensuous intuition is properly a receptivity- a capacity of being affected in a certain manner by representations, the relation of which to each other is a pure intuition of space and time- the pure forms of sensibility. These representations, in so far as they are connected and determinable in this relation (in space and time) according to laws of the unity of experience, are called objects. The non-sensuous cause of these representations is completely unknown to us and hence cannot be intuited as an object. For such an object could not be represented either in space or in time; and without these conditions intuition or representation is impossible. We may, at the same time, term the non-sensuous cause of phenomena the transcendental object- but merely as a mental correlate to sensibility, considered as a receptivity. To this transcendental object we may attribute the whole connection and extent of our possible perceptions, and say that it is given and exists in itself prior to all experience. But the phenomena, corresponding to it, are not given as things in themselves, but in experience alone. For they are mere representations, receiving from perceptions alone significance and relation to a real object, under the condition that this or that perception- indicating an object- is in complete connection with all others in accordance with the rules of the unity of experience. Thus we can say: "The things that really existed in past time are given in the transcendental object of experience." But these are to me real objects, only in so far as I can represent to my own mind, that a regressive series of possible perceptions- following the indications of history, or the footsteps of cause and effect- in accordance with empirical laws- that, in one word, the course of the world conducts us to an elapsed series of time as the condition of the present time. This series in past time is represented as real, not in itself, but only in connection with a possible experience. Thus, when I say that certain events occurred in past time, I merely assert the possibility of prolonging the chain of experience, from the present perception, upwards to the conditions that determine it according to time.
If I represent to myself all objects existing in all space and time, I do not thereby place these in space and time prior to all experience; on the contrary, such a representation is nothing more than the notion of a possible experience, in its absolute completeness. In experience alone are those objects, which are nothing but representations, given. But, when I say they existed prior to my experience, this means only that I must begin with the perception present to me and follow the track indicated until I discover them in some part or region of experience. The cause of the empirical condition of this progression- and consequently at what member therein I must stop, and at what point in the regress I am to find this member- is transcendental, and hence necessarily incognizable. But with this we have not to do; our concern is only with the law of progression in experience, in which objects, that is, phenomena, are given. It is a matter of indifference, whether I say, "I may in the progress of experience discover stars, at a hundred times greater distance than the most distant of those now visible," or, "Stars at this distance may be met in space, although no one has, or ever will discover them." For, if they are given as things in themselves, without any relation to possible experience, they are for me non-existent, consequently, are not objects, for they are not contained in the regressive series of experience. But, if these phenomena must be employed in the construction or support of the cosmological idea of an absolute whole, and when we are discussing a question that oversteps the limits of possible experience, the proper distinction of the different theories of the reality of sensuous objects is of great importance, in order to avoid the illusion which must necessarily arise from the misinterpretation of our empirical conceptions.