It thus seems proper to speak of the combined logical and psychological weaknesses that tend to the acceptance of unreal evidence and of irrelevant explanation as credulity; and the problem of problems, alike in the voyages of discovery and in everyday cruising on waters great and small, is to equip the pilot so that he may steer his course by right belief and not by credulity. The intellectual mariner’s compass, for all purposes alike, is the method of science; but none the less pilotage is an art. Many shores are imperfectly charted; there are reefs and shoals, storms and fogs; breakages in the machinery and lack of training in the crew. These are the dangers of the seas—and shipwrecks are not uncommon; but how much more imminent the dangers, and how almost impossible the traffic, without any compass or with a less reliable one! It is the worthy ambition that brightens the hopes of many a scholar to contribute, in what way it may be possible for each, some aid to the extension, the greater availability, the greater convenience and safety of the highways or of the equipment of intellectual navigation.
II
From what has been presented it might be inferred that the contemplation of error is more instructive than the story of the evolution of truth; and for certain purposes this is really the case. Error and credulity have thus far been utilized as a background against which to set off right belief; it will be useful to examine the background itself. Credulity is most generally exercised in regard to the acceptance of statements by others. When shall we believe what we are told, and when shall we suspect, discredit, deny? Again no golden rule; no automatic switch that makes connection when truth presses the button, but refuses to work for the touch of error; but also again, the possibility of reaching principles that may train the judgment and guide practice. Clifford has formulated these in a simple and acceptable fashion. You may believe another when “there is reasonable ground for supposing that he knows of the matter of which he speaks, and that he is speaking the truth so far as he knows it.” A man may deliberately lie; he
