After all, though, you will say, Why such an ado about a matter concerning which, however we may theoretically differ, we all practically agree? In this age of toleration, no scientist will ever try actively to interfere with our religious faith, provided we enjoy it quietly with our friends and do not make a public nuisance of it in the market-place. But it is just on this matter of the market-place that I think the utility of such essays as mine may turn. If {xii} religious hypotheses about the universe be in order at all, then the active faiths of individuals in them, freely expressing themselves in life, are the experimental tests by which they are verified, and the only means by which their truth or falsehood can be wrought out. The truest scientific hypothesis is that which, as we say, 'works' best; and it can be no otherwise with religious hypotheses. Religious history proves that one hypothesis after another has worked ill, has crumbled at contact with a widening knowledge of the world, and has lapsed from the minds of men. Some articles of faith, however, have maintained themselves through every vicissitude, and possess even more vitality to-day than ever before: it is for the 'science of religions' to tell us just which hypotheses these are. Meanwhile the freest competition of the various faiths with one another, and their openest application to life by their several champions, are the most favorable conditions under which the survival of the fittest can proceed. They ought therefore not to lie hid each under its bushel, indulged-in quietly with friends. They ought to live in publicity, vying with each other; and it seems to me that (the régime of tolerance once granted, and a fair field shown) the scientist has nothing to fear for his own interests from the liveliest possible state of fermentation in the religious world of his time. Those faiths will best stand the test which adopt also his hypotheses, and make them integral elements of their own. He should welcome therefore every species of religious agitation and discussion, so long as he is willing to allow that some religious hypothesis _may_ be {xiii} true. Of course there are plenty of scientists who would deny that dogmatically, maintaining that science has already ruled all possible religious hypotheses out of court. Such scientists ought, I agree, to aim at imposing privacy on religious faiths, the public manifestation of which could only be a nuisance in their eyes. With all such scientists, as well as with their allies outside of science, my quarrel openly lies; and I hope that my book may do something to persuade the reader of their crudity, and range him on my side. Religious fermentation is always a symptom of the intellectual vigor of a society; and it is only when they forget that they are hypotheses and put on rationalistic and authoritative pretensions, that our faiths do harm. The most interesting and