I hadn't noticed this passage in Hume's A Treatise onf Human Nature
In common life 'tis established as a maxim, that the streightest way is always the shortest; which would be as absurd as to say, the shortest way is always the shortest, if our idea of a right line was not different from that of the shortest way betwixt two points (Book I, Part II, Section IV)
Hume is arguing against the suggestion that the definition of straight line is the shortest path between two points
. It can't be, Hume says, because if it was saying that the straight line is the shortest path between two points wouldbe as uninformative as saying that the shortest path between two points is the shortest path between two points, and it isn't.
This is reminiscent of Kripke's so-called epistemic argument
against descriptivism: "Plato" cannot mean Socrates's protegé
because, if it did, it would be a priori (uninformative) that Plato is Socrates's protegé, and it isn't. Hume's example, though, would not pass another prominent Kripkean test: it is impossible for a straight line not to be the shortest path between two points -- assuming Euclidean geometry. Anyway, it seems to me now that a Kripkean would have to agree with Hume that the shortest path between two points
does not give the meaning of "straight line". Which is, maybe, unexpected.