Front Page Titles (by Subject) SUPPLEMENT XXI - Critique of Pure Reason
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SUPPLEMENT XXI - Friedrich Max Müller, Critique of Pure Reason 
Immanuel Kant’s Critique of Pure Reason. In Commemoration of the Centenary of its First Publication. Translated into English by F. Max Mueller (2nd revised ed.) (New York: Macmillan, 1922).
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[See page 184]
An important protest, however, against these rules for proving existence mediately is brought forward by Idealism, and this is therefore the proper place for its refutation.
* * * * * * * *
Refutation of Idealism
Idealism (I mean material idealism) is the theory which declares the existence of objects in space, without us, as either doubtful only and not demonstrable, or as false and impossible. The former is the problematical idealism of Descartes, who declares one empirical assertion only to be undoubted, namely, that of I am; the latter is the dogmatical idealism of Berkeley, who declares space and all things to which it belongs as an inseparable condition, as something impossible in itself, and, therefore, the things in space as mere imaginations. Dogmatic idealism is inevitable, if we look upon space as a property belonging to things by themselves, for in that case space and all of which it is a condition, would be a non-entity. The ground on which that idealism rests has been removed by us in the transcendental Æsthetic. Problematical idealism, which asserts nothing, but only pleads our inability of proving any existence except our own by means of immediate experience, is reasonable and in accordance with a sound philosophical mode of thought, which allows of no decisive judgment, before a sufficient proof has been found. The required proof will have to demonstrate that we may have not only an imagination, but also an experience of external things, and this it seems can hardly be effected in any other way except by proving that even our internal experience, which Descartes considers as andoubted, is possible only under the supposition of external experience.
I am conscious of my own existence as determined in time, and all determination in time presupposes something permanent in the perception.1 That permanent, however, cannot be an intuition within me, because all the causes which determine my existence, so far as they can be found within me, are representations, and as such require themselves something permanent, different from them, in reference to which their change, and therefore my existence in time in which they change, may be determined. The perception of this permanent, therefore, is possible only through a thing outside me, and not through the mere representation of a thing outside me, and the determination of my existence in time is, consequently, possible only by the existence of real things, which I perceive outside me. Now, as the consciousness in time is necessarily connected with the consciousness of the possibility of that determination of time, it is also necessarily connected with the existence of things outside me, as the condition of the determination of time. In other words, the consciousness of my own existence is, at the same time, an immediate consciousness of the existence of other things.
Note 1. — It will have been perceived that in the foregoing proof the trick played by idealism has been turned against it, and with greater justice. Idealism assumed that the only immediate experience is the internal, and that from it we can no more than infer external things, though in an untrustworthy manner only, as always happens if from given effects we infer definite causes: it being quite possible that the cause of the representations, which are ascribed by us, it may be wrongly, to external things, may lie within ourselves. We, however, have proved that external experience is really immediate,1 and that only by means of it, though not the consciousness of our own existence, yet its determination in time, that is, internal experience, becomes possible. No doubt the representation of I am, which expresses the consciousness that can accompany all thought, is that which immediately includes the existence of a subject: but it does not yet include a knowledge of it, and therefore no empirical knowledge, that is, experience. For that we require, besides the thought of something existing, intuition also, and in this case internal intuition in respect to which, that is, to time, the subject must be determined. For that purpose external objects are absolutely necessary, so that internal experience itself is possible, mediately only, and through external experience.
Note 2. — This view is fully confirmed by the empirical use of our faculty of knowledge, as applied to the determination of time. Not only are we unable to perceive any determination of time, except through a change in external relations (motion) with reference to what is permanent in space (for instance, the movement of the sun with respect to terrestrial objects), but we really have nothing permanent to which we could refer the concept of a substance, as an intuition, except matter only: and even its permanence is not derived from external experience, but presupposed a priori as a necessary condition of all determination of time, and therefore also of1 the determination of the internal sense with respect to our own existence through the existence of external things. The consciousness of myself, in the representation of the ego, is not an intuition, but a merely intellectual representation of the spontaneity of a thinking subject. Hence that ego has not the slightest predicate derived from intuition, which predicate, as permanent, might serve as the correlate of the determination of time in the internal sense: such as is, for instance, impermeability in matter, as an empirical intuition.
Note 3. — Because the existence of external objects is required for the possibility of a definite consciousness of ourselves, it does not follow that every intuitional representation of external things involves, at the same time, their existence; for such a representation may well be the mere effect of the faculty of imagination (in dreams as well as in madness); but it can be such an effect only through the reproduction of former external perceptions, which, as we have shown, is impossible without the reality of external objects. What we wanted to prove here was only that internal experience in general is possible only through external experience in general. Whether this or that supposed experience be purely imaginary, must be settled according to its own particular determinations, and through a comparison with the criteria of all real experience.
[1 ]This passage has been translated as amended by Kant himself in the Preface to the Second Edition (p. 386).
[1 ]The immediate consciousness of the existence of external things is not simply assumed in the preceding theorem, but proved, whether we can understand the possibility of this consciousness or not. The question with regard to that possibility would come to this, whether we have an internal sense only, and no external sense, but merely an external imagination. It is clear, however, that, even in order to imagine only something as external, that is, to represent it to the senses in intuition, we must have an external sense, and thus distinguish immediately the mere receptivity of an external intuition from that spontaneity which characterizes every act of imagination. For merely to imagine an external sense would really be to destroy the faculty of intuition, which is to be determined by the faculty of imagination.
[1 ]Read der instead of als.