This well-known passage[1] practically replaces a long section,[2] contained only in the first edition, on the fourth paralogism of pure reason. Its aim is to vindicate against 'idealism' the reality of objects in space, and it is for this reason inserted after the discussion of the second postulate. The interest which it has excited is due to Kant's use of language which at least seems to imply that bodies in space are things in themselves, and therefore that here he really abandons his main thesis.

Idealism is the general name which Kant gives to any view which questions or denies the reality of the physical world; and, as has been pointed out before,[3] he repeatedly tries to defend himself against the charge of being an idealist in this general sense. This passage is the expression of his final attempt. Kant begins by distinguishing two forms which idealism can take according as it regards the existence of objects in space as false and impossible, or as doubtful and indemonstrable. His own view, which regards their existence as certain and demonstrable, and which he elsewhere[4] calls transcendental idealism, constitutes a third form. The first form is the dogmatic idealism of Berkeley. This view, Kant says, is unavoidable, if space be regarded as a property of things in themselves, and the basis of it has been destroyed in the Aesthetic. The second form is the problematic idealism of Descartes, according to which we are immediately aware only of our own existence, and belief in the existence of bodies in space can be only an inference, and an uncertain inference, from the immediate apprehension of our own existence. This view, according to Kant, is the outcome of a philosophical attitude of mind, in that it demands that a belief should be proved, and apparently—to judge from what Kant says of Berkeley—it does not commit Descartes to the view that bodies in space, if their reality can be vindicated, are things in themselves.

The assertion that the Aesthetic has destroyed the basis of Berkeley's view, taken together with the drift of the Refutation as a whole, and especially of Remark I, renders it clear that the Refutation is directed against Descartes and not Berkeley. Kant regards himself as having already refuted Berkeley's view, as he here states it, viz. that the existence of objects in space is impossible, on the ground that it arose from the mistake of supposing that space, if real at all, must be a property of things in themselves, whereas the Aesthetic has as he thinks, shown that space can be, and in point of fact is, a property of phenomena. He now wants to prove—compatibly with their character as phenomena—that the existence of bodies in space is not even, as Descartes contends, doubtful. To prove this he seeks to show that Descartes is wrong in supposing that we have no immediate experience of these objects. His method is to argue that reflection shows that internal experience presupposes external experience, i. e. that unless we were directly aware of spatial objects, we could not be aware of the succession of our own states, and consequently that it is an inversion to hold that we must reach the knowledge of objects in space, if at all, by an inference from the immediate apprehension of our own states.

An examination of the proof itself, however, forces us to allow that Kant, without realizing what he is doing, really abandons the view that objects in space are phenomena, and uses an argument the very nature of which implies that these objects are things in themselves. The proof runs thus:

Theorem. "The mere but empirically determined consciousness of my own existence proves the existence of objects in space external to me."

"Proof. I am conscious of my own existence as determined in time. All time-determination presupposes something permanent in perception.[5] This permanent, however, cannot be an intuition[6] in me. For all grounds of determination of my own existence, which can be found in me, are representations, and as such themselves need a permanent different from them, in relation to which their change and consequently my existence in the time in which they change can be determined.[7] The perception of this permanent, therefore, is possible only through a thing external to me, and not through the 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 I perceive external to me. Now consciousness in time is necessarily connected with the consciousness of the possibility of this time-determination; hence it is necessarily connected also with the existence of things external to me, as the condition of time-determination, i. e. the consciousness of my own existence, is at the same time an immediate consciousness of the existence of other things external to me."[8] The nature of the argument is clear. 'In order to be conscious, as I am, of a determinate succession of my states, I must perceive something permanent as that in relation to which alone I can perceive my states as having a definite order.[9] But this permanent cannot be a perception in me, for in that case it would only be a representation of mine, which, as such, could only be apprehended in relation to another permanent. Consequently, this permanent must be a thing external to me and not a representation of a thing external to me. Consequently, the consciousness of my own existence, which is necessarily a consciousness of my successive states, involves the immediate consciousness of things external to me.'

Here there is no way of avoiding the conclusion that Kant is deceived by the ambiguity of the phrase 'a thing external to me' into thinking that he has given a proof of the existence of bodies in space which is compatible with the view that they are only phenomena, although in reality the proof presupposes that they are things in themselves. In the 'proof', the phrase 'a thing external to me' must have a double meaning. It must mean a thing external to my body, i. e. any body which is not my body; in other words, it must be a loose expression for a body in space. For, though the 'proof' makes us appeal to the spatial character of things external to me, the Refutation as a whole, and especially Remark II, shows that it is of bodies in space that he is thinking throughout. The phrase must also, and primarily, mean a thing external to, in the sense of independent of, my mind, i. e. a thing in itself. For the nerve of the argument consists in the contention that the permanent the perception of which is required for the consciousness of my successive states must be a thing external to me in opposition to the representation of a thing external to me, and a thing external to me in opposition to a thing external to me can only be a thing in itself. On the other hand, in Kant's conclusion, 'a thing external to me' can only mean a body in space, this being supposed to be a phenomenon; for his aim is to establish the reality of bodies in space compatibly with his general view that they are only phenomena. The proof therefore requires that things external to me, in order that they may render possible the consciousness of my successive states, should have the very character which is withheld from them in the conclusion, viz. that of existing independently of me; in other words, if Kant establishes the existence of bodies in space at all, he does so only at the cost of allowing that they are things in themselves.[10]

Nevertheless, the Refutation may be considered to suggest the proper refutation of Descartes. It is possible to ignore Kant's demand for a permanent as a condition of the apprehension of our successive states, and to confine attention to his remark that he has shown that external experience is really immediate, and that only by means of it is the consciousness of our existence as determined in time possible.[11] If we do so, we may consider the Refutation as suggesting the view that Descartes' position is precisely an inversion of the truth; in other words, that our consciousness of the world, so far from being an uncertain inference from the consciousness of our successive states, is in reality a presupposition of the latter consciousness, in that this latter consciousness only arises through reflection upon the former, and that therefore Descartes' admission of the validity of self-consciousness implicitly involves the admission a fortiori of the validity of our consciousness of the world.[12]

Oxford: Printed at the Clarendon Press by Horace Hart, M.A.