Mark Twain on Placebo effect

“The power which a man's imagination has over his body to heal it or make it sick is a force which none of us is born without. The first man had it, the last one will possess it. If left to himself, a man is most likely to use only the mischievous half of the force—the half which invents imaginary ailments for him and cultivates them; and if he is one of these—very wise people, he is quite likely to scoff at the beneficent half of the force and deny its existence.”

“No one doubts—certainly not I—that the mind exercises a powerful influence over the body. From the beginning of time, the sorcerer, the interpreter of dreams, the fortune-teller, the charlatan, the quack, the wild medicine-man, the educated physician, the mesmerist, and the hypnotist have made use of the client's imagination to help them in their work. They have all recognized the potency and availability of that force. Physicians cure many patients with a bread pill; they know that where the disease is only a fancy, the patient's confidence in the doctor will make the bread pill effective.”

“When I was a boy a farmer's wife who lived five miles from our village had great fame as a faith-doctor—that was what she called herself. Sufferers came to her from all around, and she laid her hand upon them and said, "Have faith—it is all that is necessary," and they went away well of their ailments. She was not a religious woman, and pretended to no occult powers. She said that the patient's faith in her did the work. Several times I saw her make immediate cures of severe toothaches. My mother was the patient. In Austria there is a peasant who drives a great trade in this sort of industry, and has both the high and the low for patients. He gets into prison every now and then for practising without a diploma, but his business is as brisk as ever when he gets out, for his work is unquestionably successful and keeps his reputation high. In Bavaria there is a man who performed so many great cures that he had to retire from his profession of stage-carpentering in order to meet the demand of his constantly increasing body of customers. He goes on from year to year doing his miracles, and has become very rich. He pretends to no religious helps, no supernatural aids, but thinks there is something in his make-up which inspires the confidence of his patients, and that it is this confidence which does the work, and not some mysterious power issuing from himself.”