This problem is the most difficult and the last to be solved by mankind. - Immanuel Kant

...whose true object is to shed the clearest light on every step which reason takes.

One age cannot bind itself, and thus conspire, to place a succeeding one in a condition whereby it would be impossible for the later age to expand its knowledge (particularly where it is so very important), to rid itself of errors, and generally to increase its enlightenment. That would be a crime against human nature, whose essential destiny lies precisely in such progress; subsequent generations are thus completely justified in dismissing such agreements as unauthorized and criminal. The criterion of everything that can be agreed upon as a law by a people lies in this question: Can a people impose such a law on itself?

Chisels and hammers may suffice to work a piece of wood, but for etching we require an etcher's needle. Thus common sense and speculative understanding are both useful, but each in its own way: the former in judgments which apply immediately to experience; the latter when we judge universally from mere concepts, as in metaphysics, where sound common sense, so called in spite of the inappropriateness of the word, has no right to judge at all.