If one would have to pick out a single most central topic in the formation of Wolff's philosophy, it might well be the notion of a pre-established harmony. It is this theory, borrowed from Leibniz, that was one of the main reasons why pietists attacked Wolffian philosophy and it was also a place that Wolff had to most carefully reconsider when answering the criticism. The problematic of this theory led Wolff to a careful demarcation between empirical and rational psychology. Empirical psychology is based on incontrovertible facts, like correspondence between sensations and certain movements of physical world and human freedom.
Rational psychology, on other hand tries to explain, among other things, why changes in world and consciousness correspond with one another and how human freedom is related to this correspondence. As Wolff has for a number of times explained, this explanation has only the status of a hypothesis that might be replaced by a better theory. The hypothetical nature of the explanation is still not detrimental, because this explanation serves only our interest to understand ourselves, but is of no concern in other fields of philosophy.
Wolff is also now more careful in explicating his reasons for abandoning the two other competing explanations of the correspondence, namely, the traditional influx theory and the occasionalist theory of many Cartesians. It is clearly the influx theory, with which Wolff engages more, probably because his main opponents, the pietists, endorsed it. In comparison, occasionalism Wolff dismisses quickly with the familiar remark that it breaks the principle of sufficient reason and replaces natural law with mere whims of God.
The main defense against influx theory is also familiar: true interaction between soul and body would contradict physical laws. Still, Wolff also has few other points of interest. He notes that influx theory actually explains nothing: the correspondence between motions of body and soul is just an appearance of an inexplicable interaction between soul and body and the influx theory just says that there really is an inexplicable interaction between the two entities. Influx theory is then no true theory, but just a denial of our capacity to explain anything, hence, of no use in rational psychology. On the other hand, since it is only the fact of correspondence that is of need in morality and theology and not any explanation (or lack of explanation) of that fact, the influx theory is of no use in philosophy.
Pre-established harmony is then left as the only viable option, that is, as the best hypothesis available. But even this is not enough, because Wolff is willing to emend this theory even more to make it a better fit with human freedom. Wolff's emendations are of such importance that they make some of my own comments on the pre-established harmony suspect. I proposed that pre-established harmony ties soul and body so tightly together that Wolffian philosophy becomes too close to materialistic theories of soul, which Wolff wants to avoid. Wolff's explanations serve to loosen the bonds of soul and body and so make my suspicions unfounded.
The essence of Wolff's emendations is that the pre-established harmony is only partial. We have seen that Wolff accepts correspondence of soul and body in case of sensations, imaginations and affects. Yet, when it comes to self-conscious states of thinking and volition, he has explicitly stated that nothing in a material body can correspond to such states – at most there can be correspondence between linguistic utterances expressing such self-conscious thoughts and images of such utterances. This means that soul and body are not exactly like two clocks showing the same time, or at least one clock has further features not present in the other clock.
What Wolff's emendation especially allows is the possibility of freedom – self-conscious actions can well be free and even not causally related to sensations corresponding to bodily events (remember that Wolff has explicitly also said that grounding in case of souls takes the shape of motivations, instead of causes: actions require motivations, but motivations do not necessitate actions). Indeed, the more free a soul is, the more independent its actions are of its body. It is then more that the God has looked upon the free actions of human souls and fashioned the material world to fit in with the actions, instead of God having made several mechanical machines that work in harmony. True, one might even now ask whether God's foreknowledge is detrimental to human freedom, but this is a question common to almost all philosophies of the time.
So much for pre-established harmony, next time I shall look at Wolff's general theory on spirits.