"But it so happens that everything on this planet is, ultimately, irrational; there is not, and cannot be, any reason for the causal connexion of things, if only because our use of the word "reason" already implies the idea of causal connexion. But, even if we avoid this fundamental difficulty, Hume said that causal connexion was not merely unprovable, but unthinkable; and, in shallower waters still, one cannot assign a true reason why water should flow down hill, or sugar taste sweet in the mouth. Attempts to explain these simple matters always progress into a learned lucidity, and on further analysis retire to a remote stronghold where every thing is irrational and unthinkable.