<r1> [B274] Refutation of Idealism
Idealism (by which I mean material idealism) is the theory that the existence of objects in space external to us is either merely doubtful and unprovable, or false and impossible. The first is the problematic idealism of Descartes, who held that there is only one undoubted empirical assertion, namely that ‘I am’. The second is the dogmatic idealism of Berkeley, who held that space, together with all the things it belongs to as an inseparable precondition, is impossible in itself, so that things in space are also mere images. Dogmatic idealism is unavoidable if you consider space to be a property which must belong to things in themselves, since in that case it is a nothing, along with everything which depends on it. However, we have already eliminated the grounds for this idealism in the Transcendental Aesthetic.
<r2> Problematic idealism makes no such claim, but merely holds [B275] that it is impossible to prove the existence of anything other than ourselves through immediate experience. This is reasonable, and in accordance with a fundamentally philosophical way of thinking, since it avoids making any decisive judgment until an adequate proof has been found. So the required proof must show that we have experience and not merely images of external things. This can be done only if we can prove that even our inner experience (which Descartes did not doubt) is possible only on the presupposition of outer experience.
<r3> Theorem
The pure, but empirically determined, consciousness of my own existence proves the existence of objects in space external to myself.
Proof
<r4> I am conscious of my existence as determined in time. Every determination in time presupposes something permanent in perception. However, that which is permanent cannot be an intuition in me. For the only grounds for determining my existence which can be met with in me are representations. But as such, they too need something permanent distinct from themselves, by reference to which their changes can be determined, and hence my existence at the time in which they change. So the perception of this permanent element is possible only through a thing external to me, and not through a mere representation of a thing external to me. Consequently, the determination of my existence in time is possible only through the existence of actual things which [B276] I perceive external to myself. But there is a necessary connection between consciousness in time and consciousness of the possibility of this determination in time. So it is also necessarily connected with the existence of things external to myself, as the precondition of determination in time — in other words, consciousness of my own existence is at the same time an immediate consciousness of the existence of other things external to myself.
<r5> Note 1. You will notice that, in the above proof, the game played by idealism has been turned against it — and with some justice. Idealism assumed that the only immediate experience is inner experience, and that outer things can only be inferred from it. But, as is always the case when we infer particular causes from given effects, the inference is unreliable, since the causes of our representations, which we ascribe (perhaps incorrectly) to external things, could also be found within ourselves. However, I have proved here that outer experience is essentially direct.*
[<r6> *In the above theorem, the direct consciousness of the existence of external things is not presupposed, but proved — whether or not we can comprehend how such a consciousness is possible. The question as to the possibility of such a consciousness would be as follows: would it be possible that we had only an inner sense, and that instead of an outer sense, we had only an outer imagination? But it is clear that we already have an outer sense, if we are even to imagine something as external — that is, to represent it to sense in intuition. [B277] And it is through this outer sense that we must immediately distinguish between the pure receptivity of an outer intuition from the spontaneity which characterises every act of the imagination. For even merely to imagine an outer sense would destroy the very faculty of intuition, which must be determined through the imagination.]
<r7> [B277] It is only by means of outer experience that inner experience is possible — not the consciousness of our own existence, but its determination in time. I accept that the representation ‘I am’, which gives expression to the consciousness which can accompany any thought, immediately includes within itself the existence of a subject. However, it does not include any knowledge of a subject, and hence no empirical knowledge, or experience of it. For experience involves intuition as well as the thought of something existing — in this case inner intuition, by reference to which the subject must be determined (i.e. by reference to time). External objects are absolutely necessary for this, so it follows that inner experience itself is only indirect, and possible only through outer experience.
<r8> Note 2. This accords perfectly with all our experience of using our faculty of knowledge to determine time. We can perform a time-determination only by means of the changing relationships of external things (motions) in relation to that which is permanent in space — for example, the motion of the sun by reference [B278] to objects on Earth. Furthermore, we do not even have anything permanent with which we could support the concept of a substance, as intuition, apart from mere matter. And even this permanence of matter is not derived from outer experience, but is presupposed apriori, as the necessary precondition of all determination in time, and hence also as the determination of inner sense in respect of our own existence through the existence of external things. The consciousness of my self in the representation ‘I’ is not an intuition at all, but a purely intellectual representation of the self-activity of a thinking subject. Hence this ‘I’ also has absolutely none of the characteristics of intuition which could, by being permanent, serve as a substitute for time determination in inner sense — somewhat as impenetrability, as an empirical intuition, does in matter.
<r9> Note 3. The existence of external objects is necessary for a determinate consciousness of our selves to be possible. But it does not follow that all representations of external things in intuition simultaneously include their existence, since they may well be merely the product of the imagination (in dreams as well as in mad delusions). However, imagination is possible only through the reproduction of previous outer perceptions, and, as I have shown, our perceptions are possible only [B279] through the actuality of external objects. Here, all I had to prove was that inner experience in general is possible only through outer experience in general. Whether this or that supposed experience is imaginary or not can be settled only by its particular determinations, and through its consistency with the criteria for all actual experience.