6.1223 Now it becomes clear why it has often been felt as if “logical truths” were “postulated” by us: We can indeed [nämlich] postulate them in so far as we can postulate an adequate notation.

Why “postulated by” instead of “exacted from”? Black (p. 325) suggests ‘require’ or ‘demand.’ Anyway, the sense is clear enough. The form of logical propositions depends solely on the notation we use, which is invented by us. Which is why logic can seem to be invented, although of course it has a kind of (non-metaphysical) independence.