1. I Am Conscious of My Own Existence As Determined in Time

The Proof

Kant reminds the reader that his Refutation of Idealism must “show that we have experience, and not merely imagination of outer things” [B275]. He says this will be accomplished by turning “the game played by idealism ... against itself” [B276]. It will show “that even our inner experience, which for Descartes is indubitable, is possible only on the assumption of outer experience.”[B275] The argument itself is given in six sentences at B275/276, beginning with a statement of the thesis to be proved:The mere, but empirically determined, consciousness of my own existence proves the existence of objects in space outside me. [B275]

Proof

1. I am conscious of my own existence as determined in time.

2. All determination of time presupposes something permanent in perception.

3. This permanent cannot, however, be something in me, since it is only through this permanent that my existence in time can itself be determined.[1]

4. Thus perception of this permanent is possible only through a thing outside me and not through the mere representation of a thing outside me; and consequently the detemrination of my existence in time is possible only through the existence of actual things which I perceive outside me.

5. Now consciousness [of my existence] in time is necessarily bound up with consciousness of the [condition of the] possibility of this time-determination; and it is therefore necessarily bound up with the existence of things outside me, as the condition of the time-determination.

6. In other words, the consciousness of my existence is at the same time an immediate consciousness of the existence of other things outside me.

[1] Kant amends this premise in the Preface to the B-Edition as follows: “But this permanent cannot be an intuition in me. For all grounds of determination of my existence which are to be met with in me are representations; and as representations themselves require a permanent distinct from them, in relation to which their change, and so my existence in the time wherein they change, may be determined.”