Necessity of the form 'if A exists, B must necessarily exist' cannot be comprehended through pure a priori reasoning from concepts alone

Hume proceeded primarily from a single but important concept of metaphysics, namely, that of the connection of cause and effect … , and he challenged reason, which here pretends to have generated this concept in her womb, to give him an account of by what right she thinks that something could be so constituted that, if it is posited, something else must necessarily also be posited thereby; for this is what the concept of cause says. He proved indisputably that it is completely impossible for rea