“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. …
“To live continually in thoughts of ill will, cynicism, suspicion, and envy, is to be confined in a self-made prison-hole. But to think well of all, to be cheerful with all, to patiently learn to find the good in all—such unselfish thoughts are the very portals of heaven; and to dwell day by day in thoughts of peace toward every creature will bring abounding peace to their possessor.” —James Allen, As A Man Thinketh (emphasis mine)
You can check out other quotes from As A Man Thinketh by clicking here.