This chapter is purely practical and is concerned with what actually is the chief mark and element of insanity; we may say in summary that it is reason used without root, reason in the void. The man who begins to think without the proper first principles goes mad, the man who begins to think at the wrong end. And for the rest of these pages we have to try and discover what is the right end. But we may ask in conclusion, if this be what drives men mad, what is it that keeps them sane? By the end of this book I hope to give a definite, some will think a far too definite, answer. But for the moment it is possible in the same solely practical manner to give a general answer touching what in actual human history keeps men sane. Mysticism keeps men sane. As long as you have mystery you have health; when you destroy mystery you create morbidity. The ordinary man has always been sane because the ordinary man has always been a mystic. He has permitted the twilight. He has always had one foot in earth and the other in fairyland. He has always left himself free to doubt his gods; but (unlike the agnostic of today) free also to believe in them. He has always cared more for truth than for consistency.