a hard saying from Leibniz

I don’t quite understand the following from Leibniz.

“If we pretend that there is a machine whose structure makes it think, sense, and have perception, then we can conceive it enlarged, but keeping to the same proportions, so that we might go inside it as into a mill. Suppose that we do: then if we inspect the interior we shall find there nothing but parts that push one another, and never anything that would explain a perception. Thus perception must be sought in simple substances, not in what is composite or in machines (Monadology 17).”

There’s a weak reading that might be expressed as follows: There is an inexorable conceptual gap between mental and material concepts. No amount of “fact-mining” about matter will result in concepts and propositions of matter that entail concepts and propositions of mentality. There is thus an unbridgeable explanatory gap.

The aforementioned weak reading, true as it might be, is probably too weak to express what Leibniz had in mind. He’s going after John Locke and others who claim that it is possible that purely material beings could also be thinking things. A weak reading does not square with the more ambitious goal of proving that thought belongs to the province of immaterial substances (in Leibniz’s case, monads that qualify as souls).

While I understand what the stronger reading would be, I don’t really know what the strongest Leibnizian argument for it would be (other than Leibniz asserting that matter, understood in the manner of the corpuscularian mechanists, does not exist... and Leibniz did NOT believe in the existence of matter but not for the same reasons as Berkeley). I suppose that Leibniz would advert to the Cartesian premise that mentality is basic to its proper, simple substance and therefore not an ontological product of more basic elements. But that’s not so much an argument as it is a principle that already loads the game in one’s favor.

I think it’s this kind of stuff that Kant was after when he discusses the “rational psychologist” — viz., philosophers who write about the mind with only a priori considerations in view.