 Yet another sense in which it has been thought that truths like (1)-(3), and logical truths quite generally, “could” not be false or “must” be true is epistemic. It is an old observation, going at least as far back as Plato, that some truths count as intuitively known by us even in cases where we don't seem to have any empirical grounds for them. Truths that are knowable on non-empirical grounds are called a priori (an expression that begins to be used with this meaning around the time of Leibniz; see e.g. his “Primæ Veritates”, p. 518). The axioms and theorems of mathematics, the lexicographic and stipulative definitions, and also the paradigmatic logical truths, have been given as examples. If it is accepted that logical truths are a priori, it is natural to think that they must be true or could not be false at least partly in the strong sense that their negations are incompatible with what we are able to know non-empirically.