ore also the states in which B ends, and vice versa, so that
a_{2} and [alpha]_{2} will be the outcome not of a_{1} and [alpha]_{1}
alone, but of a_{1} and [alpha]_{1}, and b_{1} and [beta]_{1} jointly.
Then we can say (1) that A and B are in the relation of influence, and
also of interaction or reciprocal influence, in the sense that they
_mutually_ (not alternately) determine one another's states. Again, if
we first perceive A in the state [alpha]_1 by a perception A_{1}, then
B in the state [beta]_{2} by a perception B_{2}, then A in the state
[alpha]_{3} by a perception A_{3} and so on, we can speak (2) of a
reciprocal sequence of perceptions, in the sense of a sequence of
perceptions in which alternately a perception of B follows a
perception of A and a perception of A follows a perception of B; for
first a perception of B, viz. B_{2}, follows a perception of A, viz.
A_{1}, and then a perception of A, viz. A_{3}, follows a perception of
B, viz. B_{2}. We can also speak (3) of a reciprocal sequence of the
determinations of two things in the sense of a necessary succession of
states which _alternately_ are states of A and of B; for [alpha]_{1},
which is perceived first, can be said to contribute to determine
[beta]_{2}, which is perceived next, and [beta]_{2} can be said to
contribute to determine [alpha]_{3}, which is perceived next, and so
on; and this reciprocal sequence can be said to be involved in the
very nature of interaction. Further, it can be said (4) that if we
perceive A and B alternately, and so only in the states [alpha]_{1}
[alpha]_{3} ... [beta]_{2} [beta]_{4} ... respectively, we can
only fill in the blanks, i. e. discover the states [alpha]_{2}
[alpha]_{4} ... [beta]_{1} [beta]_{3} ... _coexistent_ with [beta]_{2}
[beta]_{4} ... and [alpha]_{1} [alpha]_{3} ... respectively, if we
presuppose the thought of interaction. For it is only possible to use
the observed states as a clue to the unobserved states, if we
presuppose that the observed states are members of a necessary
succession of which the unobserved states are also members and
therefore have partially determined and been determined by the
observed states. Hence it may be said that the determination of the
unobserved states coexistent with the observed states presupposes the
thought of interaction.

How then does Kant advance from the assertion that the apprehension of
a coexistence requires the knowledge that our _perceptions_ can be
recipro