1. Foreword. Man's every act is an act of faith; generally, an unconscious faith; seldom, an examined faith; never, a faith that by taking thought could have been replaced by assured certainties. All this appears whenever we stop to “rationalize” our conduct. Whatever that conduct may have been, it can be made to appear reasonable only if some propositions are taken to be true, others false. Or, perhaps the more experienced and less exacting mind would feel the reasonableness of its actions to have been as well established as might be, if it could show the premises with which its acts were consistent to be more likely true than false; those with whose truth they were inconsistent, more likely false than true. Either way of putting it points to the one moral; namely, that, apart from incalculable chance, there is nothing but one's judgment in the matter of premises to determine all that one's life can come to know of fruition or frustration. Nothing, then, can be more important to any man than to do all he can to assure the soundness of his reasons for the faith that is in him; i.e. to test the weight of evidence supporting the working hypotheses on which he is willing to act. But to have considered the evidence for the hypotheses on which he has acted or is prepared to act, could be practically useful only to one who had in one way or another come by a sound idea of the evidence on which one should act, if one's actions are not to end in disappointment. The result at which a man's thought on this matter of evidence may have arrived, will constitute his theory of evidence; and the task of constructing the soundest possible theory of evidence is the undertaking of philosophy.