The riddle of life is simply this. For some mad reason in this mad world of ours, the things which men differ about most are exactly the things about which they must be got to agree. Men can agree on the fact that the earth goes round the sun. But then it does not matter a dump whether the earth goes around the sun or the Pleiades. But men cannot agree about morals: sex, property, individual rights, fixity and contracts, patriotism, suicide, public habits of health – these are exactly the things that men tend to fight about. And these are exactly the things that must be settled somehow on strict principles. Study each of them, and you will find each of them works back certainly to a philosophy, probably to a religion.
According to Larry Azar ("Evolution and Other Fairy Tales", AuthorHouse, 2005, p. 470), Chesterton made this statement on 16 March 1907