David Hume, who among all philosophers came closest to this problem, still did not conceive of it anywhere near deter minately enough and in its universality, but rather stopped with the syn thetic proposition of the connection of the effect with its cause (Principium causalitatis), believing himself to have brought out that such an a priori proposition is entirely impossible, and according to his in ferences everything that we call metaphysics would come down to a mere delusion of an alleged insight of reason into that which has in fact merely been borrowed from experience and from habit has taken on the appearance of necessity; an assertion, destructive of all pure philosophy, on which he would never have fallen if he had had our problem in its generality before his eyes, since then he would have comprehended that according to his argument there could also be no pure mathematics, since this certainly contains synthetic a priori propositions, an assertion a from which his sound understanding would surely have protected him.