But, as always happens, when we reason from given effects to determined causes, idealism bas reasoned with too much haste and uncertainty, for it is quite possible that the cause of our representations may lie in ourselves, and that we ascribe it falsely to external things.But our proof shows that external experience is properly immediate,* that only by virtue of it- not, indeed, the consciousness of our own existence, but certainly the determination of our existence in time, that is, internal experience- is possible.It is true, that the representation "I am," which is the expression of the consciousness which can accompany all my thoughts, is that which immediately includes the existence of a subject.But in this representation we cannot find any knowledge of the subject, and therefore also no empirical knowledge, that is, experience.For experience contains, in addition to the thought of something existing, intuition, and in this case it must be internal intuition, that is, time, in relation to which the subject must be determined.But the existence of external things is absolutely requisite for this purpose, so that it follows that internal experience is itself possible only mediately and through external experience.
*The immediate consciousness of the existence of external things is, in the preceding theorem, not presupposed, but proved, by the possibility of this consciousness understood by us or not.The question as to the possibility of it would stand thus: "Have we an internal sense, but no external sense, and is our belief in external perception a mere delusion?" But it is evident that, in order merely to fancy to ourselves anything as external, that is, to present it to the sense in intuition we must already possess an external sense, and must thereby distinguish immediately the mere receptivity of an external intuition from the spontaneity which characterizes every act of imagination.For merely to imagine also an external sense, would annihilate the faculty of intuition itself which is to be determined by the imagination.
Remark II.Now with this view all empirical use of our faculty of cognition in the determination of time is in perfect accordance.Its truth is supported by the fact that it is possible to perceive a determination of time only by means of a change in external relations (motion) to the permanent in space (for example, we become aware of the sun's motion by observing the changes of his relation to the objects of this earth).But this is not all.We find that we possess nothing permanent that can correspond and be submitted to the conception of a substance as intuition, except matter.This idea of permanence is not itself derived from external experience, but is an a priori necessary condition of all determination of time, consequently also of the internal sense in reference to our own existence, and that through the existence of external things.In the representation "I," the consciousness of myself is not an intuition, but a merely intellectual representation produced by the spontaneous activity of a thinking subject.It follows, that this "I" has not any predicate of intuition, which, in its character of permanence, could serve as correlate to the determination of time in the internal sense- in the same way as impenetrability is the correlate of matter as an empirical intuition.
Remark III.From the fact that the existence of external things is a necessary condition of the possibility of a determined consciousness of ourselves, it does not follow that every intuitive representation of external things involves the existence of these things, for their representations may very well be the mere products of the imagination (in dreams as well as in madness); though, indeed, these are themselves created by the reproduction of previous external perceptions, which, as has been shown, are possible only through the reality of external objects.The sole aim of our remarks has, however, been to prove that internal experience in general is possible only through external experience in general.Whether this or that supposed experience be purely imaginary must be discovered from its particular determinations and by comparing these with the criteria of all real experience.