e
presupposition of their interaction; this is therefore also the
condition of the possibility of things themselves as objects of
experience."[54]

[51] _Anschauung._

[52] _Wahrnehmung._

[53] _Verstandesbegriff._

[54] B. 257-8, M. 156-7.

The proof begins, as we should expect, in a way parallel to that of
causality. Just as Kant had apparently argued that we learn that a
succession of perceptions is the perception of a sequence when we find
the order of the perceptions to be irreversible, so he now definitely
asserts that we learn that certain perceptions are the perceptions of
a coexistence of bodies in space when we find that the order of the
perceptions is reversible, or, to use Kant's language, that there can
be a reciprocal sequence of the perceptions. This beginning, if read
by itself, seems as though it should also be the end. There seems
nothing more which need be said. Just as we should have expected Kant
to have completed his account of the apprehension of a succession when
he pointed out that it is distinguished by the irreversibility of the
perceptions, so here we should expect him to have said enough when he
points out that the earth and the moon are said to be coexistent
because our perceptions of them can follow one another reciprocally.

The analogy, however, has in some way to be brought in, and to this
the rest of the proof is devoted. In order to consider how this is
done, we must first consider the nature of the analogy itself. Kant
speaks of 'a conception-of-the-understanding of the reciprocal
sequence of the determinations of things which coexist externally to
one another'; and he says that 'that relation of substances in which
the one contains determinations, the ground of which is contained in
the other substance, is the relation of influence'. His meaning can be
illustrated thus. Suppose two bodies, A, a lump of ice, and B, a fire,
close together, yet at such a distance that they can be observed in
succession. Suppose that A passes through changes of temperature a_{1}
a_{2} a_{3} ... in certain times, the changes ending in states
[alpha]_{1} [alpha]_{2} [alpha]_{3} ..., and that B passes through
changes of temperature b_{1} b_{2} b_{3} ... in the same times, the
changes ending in states [beta]_{1} [beta]_{2} [beta]_{3}. Suppose
also, as we must, that A and B interact, i. e. that A in passing
through its changes conditions the changes through which B passes, and
theref