 Rational psychology also takes up the question of what best explains the agreement between the states of the soul and the body. Wolff considers three possible systems that purport to explain this agreement: (i) the system of physical influx, according to which one substance produces a state in another directly through its own activity (RP: §§558–60), (ii) the (Cartesian) system of occasional causes, according to which God modifies one substance on the occasion of some state arising in another (§§589–91); and (iii) the (Leibnizian) system of pre-established harmony, where the agreement between states of substances is the result of God’s initial activity in actualizing this world of substances (§§612–13). Wolff provides a number of familiar objections to the first two systems, claiming for instance, that physical influx conflicts with the laws of physics (cf. §§578–9), and that occasionalism relies on what amounts to a perpetual miracle (cf. §603), while defending the pre-established harmony from similar criticisms (cf. Watkins 2005: 45–51). Even so, given that any possible explanation cannot be confirmed or rejected by experience (as was disclosed at the conclusion of empirical psychology), each of these systems amounts to a mere hypothesis, and Wolff’s conclusion is only that the pre-established harmony is a more probable hypothesis than the other two (RP: §§503, 685; cf. Dyck 2014: 34–6), though he thinks that nothing significant turns on settling this contentious issue.