Preface: I ask only about the syntax and not semantics; I comprehend the meaning behind the following quote (for a paraphrase in 20C English; see p 27 of 35), but I am inexperienced with Early Modern English syntax.
Source: Section 12, Part 1, An Enquiry Concerning Human Understanding (1748) by D Hume
There is a species of scepticism, antecedent to all study and philosophy, which is much inculcated by Des Cartes and others, as a sovereign preservative against error and precipitate judgement. It recommends an universal doubt, not only of all our former opinions and principles, but also of our very faculties; of whose veracity, say they, we must assure ourselves, by a chain of reasoning, deduced from some original principle, which cannot possibly be fallacious or deceitful. But neither is there any such original principle, which has a prerogative above others, that are self-evident and convincing: or if there were, could we advance a step beyond it, but by the use of those very faculties, of which we are supposed to be already diffident. The Cartesian doubt, therefore, were it ever possible to be attained by any human creature (as it plainly is not) would be entirely incurable; and no reasoning could ever bring us to a state of assurance and conviction upon any subject.
Please see the bold above: Why might have Hume inverted the subject and modal auxiliary verb?
My conjecture: Is that declarative statement (with the bolded) meant as a rhetorical question? I am unsure because if so, would Hume have concluded it with a question mark?
PS: Initially I did not notice the significance of 'neither', but the comments and answer have since motivated me to bold it.