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 timedetermination; and it is therefore necessarily bound up with the
existence of things outside me, as the condition of the timedetermination.
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.”