There exists a law, not written down anywhere but inborn in our hearts; a law which comes to us not by training or custom or reading but by derivation and absorption and adoption from nature itself; a law which has come to us not from theory but from practice, not by instruction but by natural intuition. I refer to the law which lays it down that, if our lives are endangered by plots or violence or armed robbers or enemies, any and every method of protecting ourselves is morally right.