Critique of Pure Reason

by Immanuel Kant

  That metaphysic has hitherto remained in so vacillating a state of ignorance and contradiction is entirely due to people not having thought sooner of this problem, or perhaps even of a distinction between analytical and synthetical judgments. The solution of this problem, or a sufficient proof that a possibility which is to be explained does in reality not exist at all, is the question of life or death to metaphysic. David Hume, who among all philosophers approached nearest to that problem, though he was far from conceiving it with sufficient definiteness and universality, confining his attention only to the synthetical proposition of the connection of an effect with its causes (principium causalitatis), arrived at the conclusion that such a proposition a priori is entirely impossible. According to his conclusions, everything which we call metaphysic would turn out to be a mere delusion of reason, fancying that it knows by itself what in reality is only borrowed from experience, and has assumed by mere habit the appearance of necessity. If he had grasped our problem in all its universality, he would never have thought of an assertion which destroys all pure philosophy, because he would have perceived that, according to his argument, no pure mathematical science was possible either, on account of its certainly containing synthetical propositions a priori; and from such an assertion his good sense would probably have saved him.

  On the solution of our problem depends, at the same time, the possibility of the pure employment of reason, in establishing and carrying out all sciences which contain a theoretical knowledge a priori of objects, i.e. the answer to the questions

  How is pure mathematical science possible?

  How is pure natural science possible?

  As these sciences really exist, it is quite proper to ask, How they are possible? for that they must be possible, is proved by their reality.10

  But as to metaphysic, the bad progress which it has hitherto made, and the impossibility of asserting of any of the metaphysical systems yet brought forward that it really exists, so far as its essential aim is concerned, must fill every one with doubts as to its possibility.

  Yet, in a certain sense, this kind of knowledge also must be looked upon as given, and though not as a science, yet as a natural disposition (metaphysica naturalis) metaphysic is real. For human reason, without being moved merely by the conceit of omniscience, advances irresistibly, and urged on by its own need, to questions such as cannot be answered by any empirical employment of reason, or by principles thence derived, so that we may really say, that all men, as soon as their reason became ripe for speculation, have at all times possessed some kind of metaphysic, and will always continue to possess it. And now it will also have to answer the question

  How is metaphysic possible, as a natural disposition? that is how does the nature of universal human reason give rise to questions which pure reason proposes to itself, and which it is urged on by its own need to answer as well as it can?

  As, however, all attempts which have hitherto been made at answering these natural questions (for instance, whether the world has a beginning, or exists from all eternity) have always led to inevitable contradictions, we cannot rest satisfied with the mere natural disposition to metaphysic, that is, with the pure faculty of reason itself, from which some kind of metaphysic (whatever it may be) always arises; but it must be possible to arrive with it at some certainty as to our either knowing or not knowing its objects; that is, we must either decide that we can judge of the objects of these questions, or of the power or want of power of reason, in deciding anything upon them,—therefore that we can either enlarge our pure reason with certainty, or that we have to impose on it fixed and firm limits. This last question, which arises out of the former more general problem, would properly assume this form,

  How is metaphysic possible, as a science?

  The critique of reason leads, therefore, necessarily, to true science, while its dogmatical use, without criticism, lands us in groundless assertions, to which others, equally specious, can always be opposed, that is, in scepticism.

  Nor need this science be very formidable by its great prolixity, for it has not to deal with the objects of reason, the variety of which is infinite, but with reason only, and with problems, suggested by reason and placed before it, not by the nature of things, which are different from it, but by its own nature; so that, if reason has only first completely understood its own power, with reference to objects given to it in experience, it will have no difficulty in determining completely and safely the extent and limits of its attempted application beyond the limits of all experience.

  We may and must therefore regard all attempts which have hitherto been made at building up a metaphysic dogmatically, as non-avenu. For the mere analysis of the concepts that dwell in our reason a priori, which has been attempted in one or other of those metaphysical systems, is by no means the aim, but only a preparation for true metaphysic, namely, the answer to the question, how we can enlarge our knowledge a priori synthetically; nay, it is utterly useless for that purpose, because it only shows what is contained in those concepts, but not by what process a priori we arrive at them, in order thus to determine the validity of their employment with reference to all objects of knowledge in general. Nor does it require much self-denial to give up these pretensions, considering that the undeniable and, in the dogmatic procedure, inevitable contradictions of reason with itself, have long deprived every system of metaphysic of all authority. More firmness will be required in order not to be deterred by difficulties from within and resistance from without, from trying to advance a science, indispensable to human reason (a science of which we may lop off every branch, but will never be able to destroy the root), by a treatment entirely opposed to all former treatments, which promises, at last, to ensure the successful and fruitful growth of metaphysical science.

  Supplement VII

  Still less ought we to except here a criticism on the books and systems treating of pure reason, but only on the faculty of pure reason itself. It is only if we are in possession of this, that we possess a safe criterion for estimating the philosophical value of old and new works on this subject. Otherwise, an unqualified historian and judge does nothing but criticise the groundless assertions of others by means of his own, which are equally groundless.

  Supplement VIII

  4. Space is represented as an infinite given quantity. Now it is quite true that every concept is to be thought as a representation, which is contained in an infinite number of different possible representations (as their common characteristic), and therefore comprehends them: but no concept, as such, can be thought as if it contained in itself an infinite number of representations. Nevertheless, space is so thought (for all parts of infinite space exist simultaneously). Consequently, the original representation of space is an intuition a priori, and not a concept.

  § 3

  Transcendental Exposition of the Concept of Space

  I understand by transcendental exposition (Erörterung), the explanation of a concept, as of a principle by which the possibility of other synthetical cognitions a priori can be understood. For this purpose it is necessary, 1. That such cognitions really do flow from the given concept. 2. That they are possible only under the presupposition of a given mode of explanation of such concept.

  Geometry is a science which determines the properties of space synthetically, and yet a priori. What then must be the representation of space, to render such a knowledge of it possible? It must be originally intuitive; for it is impossible from a mere concept to deduce propositions which go beyond that concept, as we do in geometry (Introduction V. See Suppl. VI). That intuition, however, must be a priori, that is, it must exist within us before any perception of the object, and must therefore be pure, not empirical intuition. For all geometrical propositions are apodictic, that is, connected with the consciousness of their necessity, as for instance the proposition, that space has only three dimensions; and such propositions cannot be empirical judgments, nor conclusions from them (Introduction II. See Suppl. IV. II).r />
  How then can an external intuition dwell in the mind anterior to the objects themselves, and in which the concept of objects can be determined a priori? Evidently not otherwise than so far as it has its seat in the subject only, as the formal condition under which the subject is affected by the objects and thereby is receiving an immediate representation, that is, intuition of them; therefore as a form of the external sense in general.

  It is therefore by our explanation only that the possibility of geometry as a synthetical science a priori becomes intelligible. Every other explanation, which fails to account for this possibility, can best be distinguished from our own by that criterion, although it may seem to have some similarity with it.

  Supplement IX

  With the exception of space there is no other subjective representation, referring to something external, that could be called a priori objective. For from none of them can we derive synthetical propositions a priori, as we can from the intuition in space § 3. (See Suppl. VIII.) Strictly speaking, therefore, they can claim no ideality at all, though they agree with the representation of space in this, that they belong only to the subjective nature of sensibility, for instance, of sight, of hearing, and feeling, through the sensations of colours, sounds, and heat. All these, however, being sensations only, and not intuitions, do not help us by themselves to know any object, least of all a priori.

  Supplement X

  §5

  Transcendental Exposition of the Concept of Time

  I can here refer to No. III. p. 27, where, for the sake of brevity, I have placed what is properly transcendental under the head of metaphysical exposition. Here I only add that the concept of change, and with it the concept of motion (as change of place), is possible only through and in the representation of time; and that, if this representation were not intuitive (internal) a priori, no concept, whatever it be, could make us understand the possibility of a change, that is, of a connection of contradictorily opposed predicates (for instance, the being and not-being of one and the same thing in one and the same place) in one and the same object. It is only in time that both contradictorily opposed determinations can be met with in the same object, that is, one after the other. Our concept of time, therefore, exhibits the possibility of as many synthetical cognitions a priori as are found in the general doctrine of motion, which is very rich in them.

  Supplement XI

  II. As a confirmation of this theory of the ideality both of the external and of the internal sense, and therefore of all objects of the senses as mere phenomena, we may particularly remark, that everything in our knowledge which belongs to intuition (excluding therefore the feelings of pain and pleasure, and the will, which are no knowledge at all) contains nothing but mere relations, namely, of the places in an intuition (extension), change of places (motion), and laws, according to which that change is determined (moving forces). Nothing is told us thereby as to what is present in the place, or what, besides the change of place, is active in the things. A thing by itself, however, cannot be known by mere relations, and we may, therefore, fairly conclude that, as the external sense gives us nothing but representations of relations, that sense can contain in its representation only the relation of an object to the subject, and not what is inside the object by itself. The same applies to internal intuition. Not only do the representations of the external senses constitute its proper material with which we fill our mind, but time, in which these representations are placed, and which precedes even our consciousness of them in experience, nay, forms the formal condition of the manner in which we place them in the mind, contains itself relations of succession, coexistence, and that which must be coexistent with succession, namely, the permanent. Now that which, as a representation, can precede every act of thinking something, is the intuition: and, if it contains nothing but relations, then the form of intuition. As this represents nothing except what is being placed in the mind, it can itself be the manner only in which the mind, through its own activity, that is, by this placing of its representation, is affected by itself, in other words, an internal sense with respect to its form. Whatever is represented by a sense is so far always phenomenal, and we should therefore have either to admit no internal sense at all, or the subject, which is its object, could be represented by it as phenomenal only, and not, as it might judge of itself, if its intuition were spontaneous only, that is, if it were intellectual. The difficulty here lies wholly in this, how a subject can 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 it alone all the manifold (representations) in the subject were given spontaneously, the inner intuition would be intellectual. In man this consciousness requires internal perception of the manifold, which is previously given in the subject, and the manner in which this is given in the mind without spontaneity, must, on account of this difference, be called sensibility. If the faculty of self-consciousness is to seek for, that is, to apprehend, what lies in the mind, it must affect the mind, and can thus only produce an intuition of itself. The form of this, which lay antecedently in the mind, determines the manner in which the manifold exists together in the mind, namely, in the representation of time. The intuition of self, therefore, is not, as if it could represent itself immediately and as spontaneously and independently active, but according to the manner in which it is internally affected, consequently as it appears to itself, not as it is.

  III. If I say that the intuition of external objects and the self-intuition of the mind, represent both (viz. the objects and the mind) in space and time, as they affect our senses, that is, as they appear, I do not mean, that these objects are mere illusion. For the objects, as phenomena, nay, even the properties which we ascribe to them, are always looked upon as something really given: and all we do is, that, as their quality depends only on the manner of intuition on the part of the subject in relation to a given object, we distinguish the object, as phenomenon, from itself, as an object by itself. Thus, if I assert that the quality of space and time, according to which, as a condition of their existence, I accept both external objects and my own soul, lies in my manner of intuition and not in these objects by themselves, I do not mean to say that bodies seem only to exist outside me, or that my soul seems only to be given in my self-consciousness. It would be my own fault, if I changed that, which I ought to count as phenomenal, into mere illusion.11

  This cannot happen, however, according to our principle of the ideality of all sensuous intuitions; on the contrary, it is only when we attribute objective reality to those forms of intuition that everything is changed inevitably into mere illusion. For if we take space and time as properties that ought to exist as possible in things by themselves, and then survey the absurdities in which we should be involved in having to admit that two infinite things, which are not substances, nor something inherent in substances, but nevertheless must be something existing, nay, the necessary condition of the existence of all things, would remain, even if all existing things were removed, we really cannot blame the good Bishop Berkeley for degrading bodies to mere illusion. Nay, it would follow that even our own existence, which would thus be made dependent on the independent reality of such a non-entity as time, must become a mere illusion, an absurdity which hitherto no one has been guilty of.

  IV. In natural theology, where we think of an object which not only can never be an object of intuition to us, but which even to itself can never be an object of sensuous intuition, great care is taken to remove all conditions of space and time from its intuition (for all its knowledge must be intuitive, and not thought, which always involves limitation). But how are we justified in doing this, when we have first made space and time forms of things by themselves, such as would remain as conditions of the existence of things a priori, even if the things themselves had been removed? If conditions of all existence, they would also be conditions of the existence of God. If we do not wish to change space and time into ob
jective forms of all things, nothing remains but to accept them as subjective forms of our external as well as internal intuition, which is called sensuous, for the very reason that it is not originally spontaneous, that is such, that it could itself give us the existence of the objects of intuition (such an intuition, so far as we can understand, can belong to the First Being only), but dependent on the existence of objects, and therefore possible only, if the faculty of representation in the subject is affected by them.

  It is not necessary, moreover, that we should limit this intuition in space and time to the sensibility of man; it is quite possible that all finite thinking beings must necessarily agree with us on this point (though we cannot decide this). On account of this universal character, however, it does not cease to be sensibility, for it always is, and remains derivative (intuitus derivativus), not original (intuitus originarius), and therefore not intellectual intuition. For the reason mentioned before, the latter intuition seems only to belong to the First Being, and never to one which is dependent, both in its existence and its intuition (which intuition determines its existence with reference to given objects). This latter remark, however, must only be taken as an illustration of our æsthetic theory, and not as a proof.

  Conclusion of the Transcendental Æsthetic

  Here, then, we have one of the requisites for the solution of the general problem of transcendental philosophy, How are synthetical propositions a priori possible? namely, pure intuitions a priori, space and time. In them we find, if in a judgment a priori we want to go beyond a given concept, that which can be discovered a priori, not in the concept, but in the intuition corresponding to it, and can be connected with it synthetically. For this very reason, however, such judgments can never go beyond the objects of the senses, but are valid only for objects of possible experience.