From Orthodoxy, Chapter 2 "The Maniac":

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; he 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 too definite, answer.

But for the moment it is possible in the same solely practical manner to give a general answer touching with 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 the same 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 the agnostic of today) free also to believe in them.  He has always cared more for truth than consistency.

If he saw two truths that seemed to contradict each other, he would take the two truths and the contradiction along with them.  His spiritual sight is stereoscopic, like his physical sight: he sees two different pictures at once and yet sees all the better for that.  Thus he has always believed that there was such a thing as fate, but such as thing as free will also.  Thus he believed that children were indeed the kingdom of heaven, but nevertheless ought to be obedient to the kingdom of earth.  He admired youth because it was young, and age because it was not.

It is exactly this balance of apparent contradictions that has been the whole buyoancy of the healthy man.  The whole secret of mysticism is this: that man can understand everything by the help of what he does not understand.

PLEASE buy Orthodoxy or Heretics this week.  Read them and learn.