I say, Chesterton is a true genius in first philosophy as Bosanquet and Green are in political philosophy!
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. . . . The whole secret of mysticism is this: that man can understand everything by the help of what he does not understand. The morbid logician seeks to make everything lucid, and succeeds in making everything mysterious. The mystic allows one thing to be mysterious, and everything else becomes lucid. The determinist makes the theory of causation quite clear, and then finds that he cannot say “if you please” to the housemaid. The Christian permits free will to remain a sacred mystery; but because of this his relations with the housemaid become of a sparkling and crystal clearness. He puts the seed of dogma in a central darkness; but it branches forth in all directions with abounding natural health. As we have taken the circle as the symbol of reason and madness, we may very well take the cross as the symbol at once of mystery and of health. Buddhism is centripetal, but Christianity is centrifugal: it breaks out. For the circle is perfect and infinite in its nature; but it is fixed for ever in its size; it can never be larger or smaller. But the cross, though it has at its heart a collision and a contradiction, can extend its four arms for ever without altering its shape. Because it has a paradox in its centre it can grow without changing. The circle returns upon itself and is bound. The cross opens its arms to the four winds; it is a signpost for free travellers.