propositions a priori, but especially space — and for this reason we shall prefer it for investigation at present. As the propositions of geometry are cognized synthetically a priori, and with apodeictic certainty, I inquire: Whence do you obtain propositions of this kind, and on what basis does the understanding rest, in order to arrive at such absolutely necessary and universally valid truths?
There is no other way than through intuitions or conceptions, as such; and these are given either a priori or a posteriori. The latter, namely, empirical conceptions, together with the empirical intuition on which they are founded, cannot afford any synthetical proposition, except such as is itself also empirical, that is, a proposition of experience. But an empirical proposition cannot possess the qualities of necessity and absolute universality, which, nevertheless, are the characteristics of all geometrical propositions. As to the first and only means to arrive at such cognitions, namely, through mere conceptions or intuitions a priori, it is quite clear that from mere conceptions no synthetical cognitions, but only analytical ones, can be obtained. Take, for example, the proposition: “Two straight lines cannot enclose a space, and with these alone no figure is possible,” and try to deduce it from the conception of a straight line and the number two; or take the proposition: “It is possible to construct a figure with three straight lines,” and endeavour, in like manner, to deduce it from the mere conception of a straight line and the number three. All your endeavours are in vain, and you find yourself forced to have recourse to intuition, as, in fact, geometry always does. You therefore give yourself an object in intuition. But of what kind is this intuition? Is it a pure a priori, or is it an empirical intuition? If the latter, then neither a universally valid, much less an apodeictic proposition can arise from it, for experience never can give us any such proposition. You must, therefore, give yourself an object a priori in intuition, and upon that ground your synthetical proposition. Now if there did not exist within you a faculty of intuition a priori; if this subjective condition were not in respect to its form also the universal condition a priori under which alone the object of this external intuition is itself possible; if the object (that is, the triangle) were something in itself, without relation to you the subject; how could you affirm that that which lies necessarily in your subjective conditions in order to construct a triangle, must also necessarily belong to the triangle in itself? For to your conceptions of three lines, you could not add anything new (that is, the figure); which, therefore, must necessarily be found in the object, because the object is given before your cognition, and not by means of it. If, therefore, space (and time also) were not a mere form of your intuition, which contains conditions a priori, under which alone things can become external objects for you, and without which subjective conditions the objects are in themselves nothing, you could not construct any synthetical proposition whatsoever regarding external objects. It is therefore not merely possible or probable, but indubitably certain, that space and time, as the necessary conditions of all our external and internal experience, are merely subjective conditions of all our intuitions, in relation to which all objects are therefore mere phenomena, and not things in themselves, presented to us in this particular manner. And for this reason, in respect to the form of phenomena, much may be said a priori, whilst of the thing in itself, which may lie at the foundation of these phenomena, it is impossible to say anything.
II. In confirmation of this theory of the ideality of the external as well as internal sense, consequently of all objects of sense, as mere phenomena, we may especially remark that all in our cognition that belongs to intuition contains nothing more than mere relations. (The feelings of pain and pleasure, and the will, which are not cognitions, are excepted.) The relations, to wit, of place in an intuition (extension), change of place (motion), and laws according to which this change is determined (moving forces). That, however, which is present in this or that place, or any operation going on, or result taking place in the things themselves, with the exception of change of place, is not given to us by intuition. Now by means of mere relations, a thing cannot be known in itself; and it may therefore be fairly concluded, that, as through the external sense nothing but mere representations of relations are given us, the said external sense in its representation can contain only the relation of the object to the subject, but not the essential nature of the object as a thing in itself.
The same is the case with the internal intuition, not only because, in the internal intuition, the representation of the external senses constitutes the material with which the mind is occupied; but because time, in which we place, and which itself antecedes the consciousness of, these representations in experience, and which, as the formal condition of the mode according to which objects are placed in the mind, lies at the foundation of them, contains relations of the successive, the coexistent, and of that which always must be coexistent with succession, the permanent. Now that which, as representation, can antecede every exercise of thought (of an object), is intuition; and when it contains nothing but relations, it is the form of the intuition, which, as it presents us with no representation, except in so far as something is placed in the mind, can be nothing else than the mode in which the mind is affected by its own activity, to wit — its presenting to itself representations, consequently the mode in which the mind is affected by itself; that is, it can be nothing but an internal sense in respect to its form. Everything that is represented through the medium of sense is so far phenomenal; consequently, we must either refuse altogether to admit an internal sense, or the subject, which is the object of that sense, could only be represented by it as phenomenon, and not as it would judge of itself, if its intuition were pure spontaneous activity, that is, were intellectual. The difficulty here lies wholly in the question: How can the subject have an internal intuition of itself? But this difficulty is common to every theory. The consciousness of self (apperception) is the simple representation of the “ego”; and if by means of that representation alone, all the manifold representations in the subject were spontaneously given, then our internal intuition would be intellectual. This consciousness in man requires an internal perception of the manifold representations which are previously given in the subject; and the manner in which these representations are given in the mind without spontaneity, must, on account of this difference (the want of spontaneity), be called sensibility. If the faculty of self-consciousness is to apprehend what lies in the mind, it must all act that and can in this way alone produce an intuition of self. But the form of this intuition, which lies in the original constitution of the mind, determines, in the representation of time, the manner in which the manifold representations are to combine themselves in the mind; since the subject intuites itself, not as it would represent itself immediately and spontaneously, but according to the manner in which the mind is internally affected, consequently, as it appears, and not as it is.
III. When we say that the intuition of external objects, and also the self-intuition of the subject, represent both, objects and subject, in space and time, as they affect our senses, that is, as they appear — this is by no means equivalent to asserting that these objects are mere illusory appearances. For when we speak of things as phenomena, the objects, nay, even the properties which we ascribe to them, are looked upon as really given; only that, in so far as this or that property depends upon the mode of intuition of the subject, in the relation of the given object to the subject, the object as phenomenon is to be distinguished from the object as a thing in itself. Thus I do not say that bodies seem or appear to be external to me, or that my soul seems merely to be given in my self-consciousness, although I maintain that the properties of space and time, in conformity to which I set both, as the condition of their existence, abide in my mode of intuition, and not in the objects in themselves. It would be my own fault, if out of that which I should reckon as phenomenon, I made mere illusory appearance.14 But this will not happen, because of our principle of the ideality of all sensuous intuitions. On the contrary, if we ascribe objective reality to these forms of representation, it becomes impossible to avoid changing everything into mere appearance. For if we regard space and time as properties, which must be found in objects as things in themselves, as sine quibus non of the possibility of their existence, and reflect on the absurdities in which we then find ourselves involved, inasmuch as we are compelled to admit the existence of two infinite things, which are nevertheless not substances, nor anything really inhering in substances, nay, to admit that they are the necessary conditions of the existence of all things, and moreover, that they must continue to exist,