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.