Mark Twain's most delightful trick as a popular humorist was to strike out some comic epigram, that passed currency with the masses whose fancy it tickled, and also had upon it the minted stamp of the classic aphorism. These epigrams were frequently pseudo-moral in their nature; and their humour usually lay in the assumption that everybody is habitually addicted to prevarication--which is just precisely true enough and reprehensible enough to validate the epigram. His method was humorous inversion; and he told a story whose morals are so ludicrously twisted that the right moral, by contrast, spontaneously springs to light. “Never tell a lie--except for practice,” is less successful than the more popularly known “When in doubt, tell the truth.” Out of the latter maxim he succeeded in extracting a further essence of humour. He admitted inventing the maxim, but never expected it to be applied to himself. His advice, he said, was intended for other people; when he was in doubt himself, he used more sagacity! Mark Twain has made no more delightful epigram than that one in which he recognizes that a lie, morally reprehensible as it may be, is undoubtedly an ever present help in time of need: “Never waste a lie. You never know when you may need it.”
Sometimes in a humorous, sometimes in a grimly serious way, Mark Twain was fond of drawing the distinction between theoretical and practical morals. Theoretical morals, he would point out, are the sort you get on your mother's knee, in good books, and from the pulpit. You get them into your head, not into your heart. Only by the commission of crime can anyone acquire real morals. Commit all the crimes in the decalogue, take them in rotation, persevere in this stern determination--and after awhile you will thereby attain to moral perfection! It is not enough to commit just one crime or two--though every little bit helps. Only by committing them all can you achieve real morality! It is interesting to note this distinction between Mark Twain, the humorous moralist, and Bernard Shaw, the ethical thinker. Each teaches precisely the same thing--the one not even half seriously, the other with all the sharp sincerity of conviction. Shaw unhesitatingly declares that trying to be wicked is precisely the same experiment as trying to be good, viz., the discovery of character.