3 - Spinoza only put Leibniz on the track of [his theory of] pre-established harmony (1763)

Summary

I begin with the first dialogue. I still share the opinion you express in it that it was Spinoza who led Leibniz to [his theory of] pre-established harmony. For Spinoza was the first whose system confronted him with the possibility that all changes to the body might result purely and simply from its own mechanical forces. It was this possibility which put Leibniz on the track of his hypothesis. But it only put him on the track: its further elaboration was the work of his own sagacity.

For all that I think I have recently grasped of Spinoza's system leads me to doubt whether he can have believed in, or even had a remote inkling of, pre-established harmony itself, if only as it exists in the divine intellect prior to the divine decree [antecedenter ad decretum].

Just tell me this: if Spinoza expressly declares that body and soul are one and the same particular thing, which we merely conceive of at times under the attribute of thought and at times under the attribute of extension (Ethics, Pt II, Prop. 21, note), what kind of harmony can he have had in mind? The greatest, it could be said, that can possibly exist, namely the harmony which a thing has with itself. But is this not just playing with words? The harmony which a thing has with itself!