James Allen's treatise, As a Man Thinketh, regarding the effect of thought and health on the body, tells us that:
"The body is the servant of the mind. It obeys the operations of the mind, whether they be deliberately chosen or automatically expressed. At the bidding of unlawful thoughts the body sinks rapidly into disease and decay; at the command of glad and beautiful thoughts it becomes clothed with youthfulness and beauty. Disease and health, like circumstances, are rooted in thought. Sickly thoughts will express themselves through a sickly body. Thoughts of fear have been known to kill a man as speedily as a bullet, and they are continually killing thousands of people just as surely though less rapidly. The people who live in fear of disease are the people who get it. Anxiety quickly demoralizes the whole body, and lays it open to the entrance of disease; while impure thoughts, even if not physically indulged, will soon shatter the nervous system.
Strong, pure, and happy thoughts build up the body in vigor and grace. The body is a delicate and plastic instrument, which responds readily to the thoughts by which it is impressed, and habits of thought will produce their own effects, good or bad, upon it. Men will continue to have impure and poisoned blood so long as they propagate unclean thoughts. Out of a clean heart comes a clean life and a clean body. Out of a defiled mind proceeds a defiled life and corrupt body. Thought is the fount of action, life and manifestation; make the fountain pure, and all will be pure.
Change of diet will not help a man who will not change his thoughts. When a man makes his thoughts pure, he no longer desires impure food. If you would perfect your body, guard your mind. If you would renew your body, beautify your mind. Thoughts of malice, envy, disappointment, despondency, rob the body of its health and grace. A sour face does not come by chance; it is made by sour thoughts. "