Immanuel Kant Critique of Pure Reason
§1
Intuition is that by which a cognition refers to objects directly, and at which all thought aims as a means. Intuition, however, takes place only insofar as the object is given to us; but that in turn, is possible only by the mind’s being affected in a certain manner.
The capacity to acquire presentations as a result of the way in which we are affected by objects is called sensibility. Hence by means of sensibility objects are given to us, and it alone supplies us with intuitions.
Through understanding, on the other hand, objects are thought, and from it arise concepts. But all thought must, by means of certain characteristics, refer ultimately to intuitions, whether it does so straightforwardly (directe) or circuitously (indirecte); and hence it must, in us human beings, refer ultimately to sensibility, because no object can be given to us in any other manner than through sensibility.
The effect of an object on our capacity for presentation is sensation. The undetermined object of an empirical intuition is called appearance. Whatever in an appearance corresponds to sensation I call its matter; but whatever in an appearance brings about the fact that the manifold of the appearance can be ordered in certain relations I call the from of appearance.
Now, that in which alone sensation can be ordered and put into a certain from cannot itself be sensation again. Therefore, although the matter of all appearance is given to us only a posteriori, the form of all appearance must altogether lie ready for the sensations a priori in the mind; and hence that form must be capable of being examined apart from all sensation.
There must, therefore, be a science of all principles of a priori sensibility; I call such a science transcendental aesthetic. We shall in the transcendental aesthetic, first of all, isolate sensibility, by separating from it everything that the understanding through its concepts thinks with it, so that nothing other than empirical intuition will remain.
In the course of that inquiry it will be found that there are two pure forms of sensible intuition, which are principles for a priori cognition: viz., space and time.
§2 Metaphysical Exposition of this concept
By means of outer sense we present objects as outside us, and present them one and all in space. In space their shape, magnitude, and relation to one another are determined or determinable. By means of inner sense the mind intuits itself, or its inner state. Although inner sense provides no intuition of the soul itself as an object, yet there is a determinate from under which alone we can intuit the soul’s inner state.
That form is time. Thus everything belonging to our inner determinations is presented in relations of time. Time cannot be intuited outwardly, any more than space can be intuited as something within us. What, then, are space and time? Are they actual beings? Are they only determinations of things, or, for that matter, relations among them?
Let us first of all give an exposition of the concept of space. Now, by exposition I mean clear presentation of what belongs to a concept; and such exposition is metaphysical if it contains what exhibits the concept as given a priori.
Space is not an empirical concept that has been abstracted from outer experiences. For the presentation of space must already lie at the basis in order for certain sensations to be referred to something outside me. And it must similarly already lie at the basis in order for me to be able to present these sensations as outside and alongside one another. Accordingly, the presentation of space cannot be one that we take from the relations of outer appearance by means of experience; rather only through the presentation of space is that outer experience possible in the first place.
We can never have a presentation of there being no space, even though we are quite able to think of there being no objects encountered in it. Hence space must be regarded as the condition for the possibility of appearances. Space is an a priori presentation that necessarily underlies outer appearances. The original presentation of space is an a priori intuition, not a concept.
§3 Transcendental exposition of the concept of space
By a transcendental exposition I mean the explication of a concept as a principle that permits insight into the possibility of other synthetic a priori cognitions.
Geometry is a science that determines the properties of space synthetically and yet a priori. For geometric propositions are one and all apodeictic, i.e., linked with the consciousness of their necessity.
How then, can the mind have an outer intuition which precedes he objects themselves, and in which the concept of these objects can be determined a priori? Obviously, this can be only insofar as this intuition resides merely in the subject, as the subject’s formal character of being affected by objects and of thereby acquiring from them direct presentations, i.e., intuition.
Our explication of the concept of space is, therefore, the only one that makes comprehensible the possibility of geometry as a synthetic a priori cognition.
Conclusions from the above concepts
Space represents no property whatever of any things in themselves, nor does it represent things in themselves in their relation to one another.
Space is nothing but the mere form of all appearances of outer senses; i.e., it is the subjective condition of sensibility under which alone outer intuition is possible for us.
Only form the human standpoint, therefore, can we speak of space, of extended beings, etc. if we depart from the subjective condition under which alone we can acquire outer intuition, then the presentation of space means nothing whatsoever.
The constant form of this receptivity which we call sensibility is a necessary condition of all relations in which objects are intuited as outside us; and if we abstract from these objects, then the from of that receptivity is a pure intuition that bears the name of space.
Hence we assert that space is empirically real, despite asserting that space is transcendentally ideal, i.e., that it is nothing as soon as we omit that space is the condition of the possibility of all experience and suppose space to be something underlying things in themselves.
Nothing whatever that is intuited in space is a thing in itself, and space is not a form of things, one that might belong to them as they are in themselves. Rather, what we call external objects are nothing but mere presentations of our sensibility.
The form of this sensibility is space, but its true correlate, i.e., the thing in itself, is not cognized at all through these presentations, and cannot be. Nor, on the other hand, is the thing in itself ever at issue in experience.
§4 Metaphysical exposition of the concept of time
Time in not an empirical concept that has been abstracted from any experience. Only on the presupposition of this presentation can we present this and that as being at one and the same time (simultaneously) or at different times (sequentially).
Time is a necessary presentation that underlies all intuition. As regards appearances in general, we cannot annul time itself, though we can quite readily remove appearances from time. Hence time is given a priori. All actuality of appearances is possible only in time. Appearances, one and all, may go away; but time itself as the universal condition of their possibility cannot be annulled.
Time has only one dimension; different times are not simultaneously but sequential (just as different spaces are not sequential but simultaneous).
§5 Transcendental exposition of the concept of time
The concept of change, and with it the concept of motion, is possible only through and in the presentation of time; and that if this presentation were not (inner) a priori intuition, no concept whatsoever could make comprehensible the possibility of a change, i.e. of a combination, in one and the same object, of contradictorily opposed predicates (one and the same thing’s being in a place and not being in that same place).
§6 Conclusions from these concepts
If time were self-subsistent, then it would be something that without there being an actual object would yet be actual.
Time is an a priori condition of all appearance generally: it is the direct condition of inner appearances (of our souls), and precisely thereby also, indirectly, a condition of outer appearances. If I can say a priori that all outer appearances are in space and are determined a priori according to spatial relations, then the principle of inner sense allows me to say, quite universally, that all appearances generally, i.e. all objects of the senses, are in time and stand necessarily in relations of time.
Time is merely a subjective condition of our human intuition; in itself, i.e., apart from the subject, time is nothing.
If we now add the condition to the concept, and say that all things as appearances are in time, then this principle has all its objective correctness and a priori universality. Hence the doctrine we are asserting is that time is empirically real, i.e. objectively valid in regard to all objects that might ever be given to our senses.
§7 Elucidation
Time is indeed something actual, viz., the actual form of inner intuition. It therefore has subjective reality in regard to inner experience; i.e., I actually have the presentation of time and of my determinations in time. Hence time is to be regarded as actual, though not as an object but as the way of presenting that I myself have as an object.
Time is nothing but the mere form of our inner intuition. If we take away from time the special condition of our sensibility, then the concept of time vanishes as well; time attaches not to objects themselves, but merely to the subject intuiting them.
By contrast, the actuality of the object of inner sense is directly evident through consciousness. External objects might be a mere illusion; but the object of inner sense is, in the opinion of men of insight, undeniably something actual.
They failed to bear in mind, however, that both of them, though their actuality as presentations is indisputable, still belong only to appearance. Appearance always has two sides. One is the side where the object is regarded in itself. The other is the side where we take account of the form of the intuition of this object. The form must be sought not in the object in itself, but in the subject to whom the object appears. Yet this form belongs to the appearance of this object actually and necessarily.
Finally, transcendental aesthetic cannot contain more than these two elements, i.e., space and time. This is evident from the fact that all other concepts belonging to sensibility presuppose something empirical. This holds even for the concept of motion which unites the two components.
§8 General comments on transcendental aesthetic, I.
All our intuition is nothing but the presentation of appearance. The things that we intuit are not in themselves what we intuit them as being. Nor do their relations in themselves have the character that they appear to us as having. And if we annul ourselves as subject, or even annul only the subjective character of the senses generally, then this entire character of objects and all their relations in space and time –indeed even space and time themselves- would vanish; being appearances, they cannot exist in themselves, but can exist only in us. All we know is the way in which we perceive them.
Even if we could bring this intuition of ours to the highest degree of distinctness, that would still not get us closer to the character of objects in themselves. For what we would cognize, and cognize completely, would still be only our way of intuiting, i.e., our sensibility; and we would always cognize it only under the conditions attaching to the subject originally: space and time. What objects may be in themselves would still never become known to us, not even through the most enlightened cognition of what alone is given to us, viz., their appearance.
When a body is presented in intuition, this presentation contains nothing whatever that could belong to an object in itself. It contains, rather, merely the appearance of something, and the way we are affected by that something.
We then believe after all that we cognize things in themselves, even though in the world of sense, however deeply we explore its objects, we deal with nothing whatever but appearances.
II.
This theory according to which both outer and inner sense are ideal and hence all objects of the senses are mere appearances, can be confirmed superbly by the following observation. Whatever in our cognition belongs to intuition contains nothing but mere relations: of places in an intuition (extension), of change of places (motion), and of laws according to which this change is determined (motive forces).
Now through mere relations we do not, of course, cognize a thing in itself. Presentation that can precede all acts of thinking anything is intuition; and if this intuition contains nothing but relations then it is the form of intuition. But this form does not present anything except insofar as something is being placed within the mind.
Therefore this form can be nothing but the way in which the mind is affected by its own activity. Whatever is presented through a sense is, to that extent, always appearance.
What underlies this whole difficulty is this: how can a subject inwardly intuit himself? The consciousness of oneself (apperception) is the simple presentation of the I. If the power to become conscious of oneself is to locate (apprehend) what lies in the mind, then it must affect the mind; and only in that way can it produce an intuition of itself.
This power does not intuit itself as it would if it presented itself directly and self-actively; rather, it intuits itself according to the way in which it is affected form within, and hence intuits itself as it appears to itself, not as it is.
III.
I am saying, then, that the intuition of external objects and the self- intuition of the mind both present these objects and the mind, in space and in time, as they affect our senses, i.e., as they appear. But I do not mean by this that these objects are a mere illusion. For when we deal appearance, the objects, and indeed even the properties that we ascribe to them, are always regarded as something actually given – except that we do also distinguish this object as appearance from the same object as object in itself. It would be my own fault if I turned into mere illusion what I ought to class with appearance. It is when we attribute objective reality to those forms of presentation that we cannot prevent everything from being thereby transformed into mere illusion.
IV.
In natural theology we think an object [viz., God] that not only cannot possibly be an object of intuition for us, but that cannot in any way be an object of sensible intuition even to itself. We take great care to remove the conditions of time and space from all his intuition. But what right do we have to do this if we have beforehand turned space and time into forms of things in themselves – such forms, moreover, as are a priori conditions of the existence of things and hence would remain even if we ha annulled the things themselves? For as conditions of all existence in general, they would have to be conditions also of the existence of God.
If we are not to make space and time objective forms of all things, then we are left with only one alternative: we must make them subjective forms of our kind of intuition, inner and outer. Our kind of intuition is called sensible because it is not original. I.e., it is not such that through this intuition itself the existence of its object is given. Rather, our kind of intuition is dependent on the existence of the object, and hence is possible only by the object’s affecting the subject’s capacity to present.
Concluding the transcendental aesthetic
Thus in our pure a priori intuitions, space and time, we now have one of the components required for solving the general problem of the transcendental philosophy: How can synthetic propositions be possible a priori? When in an a priori judgement about space and time we want to go beyond the given concept, we encounter what cannot be discovered a priori in the given concept, but can indeed be so discovered a priori in the intuition corresponding to that concept and can be combined with it synthetically. Because of this, however, such judgements can never reach beyond objects of the senses, and can hold only for objects of possible experience.