Now, it would sound absurd to say that each age must have its own geometry or its own physics. The fact that it has long been known that the sum of the interior angles of a plane triangle is equal to two right angles, does not warrant me in repudiating that truth; nor am I justified in doing so, and in believing the opposite, merely because I find the statement uninteresting or distasteful. When we are dealing with such matters as these, we recognize that truth is truth, and that, if we mistake it or refuse to recognize it, so much the worse for us. Is it otherwise in philosophy? Is it a perfectly proper thing that, in one age, men should be idealists, and in another, materialists; in one, theists, and in another, agnostics? Is the distinction between true and false nothing else than the distinction between what is in harmony with the spirit of the times and what is not? That it is natural that there should be such fluctuations of opinion, we may freely admit. Many things influence a man to embrace a given type of doctrine, and, as we have seen, verification is a difficult problem. But have we here, any more than in other fields, the right to assume that a doctrine was true at a given time merely because it _seemed_ to men true at that time, or because they found it pleasing? The history of science reveals that many things have long been believed to be true, and, indeed, to be bound up with what were regarded as the highest interests of man, and that these same things have later been discovered to be false--not false merely for a later age, but false for all time; as false when they were believed in as when they were exploded and known to be exploded. No man of sense believes that the Ptolemaic system was true for a while, and that then the Copernican became true. We say that the former only _seemed_ true, and that the enthusiasm of its adherents was a mistaken enthusiasm.