Leibniz sometimes describes thought as an internal action (see this earlier Modsquad discussion). Moreover, in a couple of places he says that we can know this by experience. Indeed, he suggests we can know enough in this area by experience to establish some substantive philosophical truths about the mind.
Thus, in “On Nature Itself” (1698):
Indeed, if this view [occasionalism] were extended so far as to eliminate even the immanent actions of substances … then it would be as distant as it could possibly be from reason. For who would call into doubt that the mind thinks and wills, that we elicit in ourselves many thoughts and volitions, and that there is a spontaneity that belongs to us? If this were called into doubt, then not only would human liberty be denied and the cause of evil things be thrust into God, but it would also fly in the face of the testimony of our innermost experience and consciousness, testimony by which we ourselves sense that the things my opponents have transferred to God, without even a pretense of reason, are ours (ONI 10).
And later in a 1704 letter to Masham:
In this [pre-established harmony] I am doing no more than attributing to souls and bodies always and everywhere what we experience in them whenever the experience is distinct, that is to say, mechanical laws to bodies, and internal actions to souls (WFNS 206).
As well as using different terminology, these arguments have different purposes. The first is directed against a particular version of occasionalism, which denies all causal power to the human mind. To oppose this, Leibniz appeals to experience that he takes to show that we do have such power: “we elicit in ourselves many thoughts and volitions”. The comment to Masham suggests, however, the possibility of something stronger: of an argument from experience for the pre-established harmony.
That argument, if it is such would begin from a premise about our experience: when our experience is distinct, we see our souls working by internal action (rather than mechanical laws or external action). How it would proceed from there is less clear. Leibniz appeals to distinct experience. Perhaps too he is relying on a principle of uniformity according to which our minds always work in the way we see they sometimes – those distinctly experienced times? – work. This appeal to uniformity fits well with the general themes of Leibniz’s letter.
It would be intriguing if Leibniz really was suggesting an argument from experience for the pre-established harmony here. And such a thing would not be entirely unprecedented for him. Recall the way he appeals to the evidence of “Swammerdam, Malpighi, and Leeuwenhoek, the best observers of our time” in the “New System”. But perhaps this is straining the text of the letter to Masham too much, and all Leibniz aims to do is indicate that his system, in which souls only ever cause changes in themselves, is not opposed to all experience. We know at least, he might just be saying, that our souls sometimes cause changes in themselves. So however strange my view might seem, it does not go against all experience. This more modest reading would also tie back in to the comment in “On Nature Itself” – for the occasionalist there has gone against the very thing that experience tells us and Leibniz’s system upholds.
A lack of supporting texts making the bolder argument inclines me to the more modest reading – watch me talk myself out of the more exciting bit of the post even while I’m still writing it – but the bolder reading is not completely ungrounded. Any thoughts out there?