Truth, indeed, is something that is believed in completely only by persons who have never tried personally to pursue it to its fastnesses and grab it by the tail. It is the adoration of second-rate men—men who always receive it at second-hand. Pedagogues believe in immutable truths and spend their lives trying to determine them and propagate them; the intellectual progress of man consists largely of a concerted effort to block and destroy their enterprise. Nine times out of ten, in the arts as in life, there is actually no truth to be discovered; there is only error to be exposed. In whole departments of human inquiry it seems to me quite unlikely that the truth ever will be discovered. Nevertheless, the rubber-stamp thinking of the world always makes the assumption that the exposure of an error is identical with the discovery of the truth—that error and truth are simple opposites. They are nothing of the sort. What the world turns to, when it has been cured of one error, is usually simply another error, and maybe one worse than the first one. This is the whole history of the intellect in brief. The average man of to-day does not believe in precisely the same imbecilities that the Greek of the fourth century before Christ believed in, but the things that he does believe in are often quite as idiotic. Perhaps this statement is a bit too sweeping. There is, year by year, a gradual accumulation of what may be called, provisionally, truths—there is a slow accretion of ideas that somehow manage to meet all practicable human tests, and so survive. But even so, it is risky to call them absolute truths. All that one may safely say of them is that no one, as yet, has demonstrated that they are errors. Soon or late, if experience teaches us anything, they are likely to succumb too. The profoundest truths of the Middle Ages are now laughed at by schoolboys. The profoundest truths of democracy will be laughed at, a few centuries hence, even by school-teachers.Foreshadowing Thomas Kuhn's The Structure of Scientific Revolutions. All knowledge is provisional.
And there's that Mencken sotto voce twist of the dagger, describing an idea so foolish it will be laughed at "even by school-teachers."