=Ancient Knowledge.= People have always known that mind in some strange way carries its moods over into the body. The writer of the Book of Proverbs tells us, from that far-off day, that “A merry heart doeth good like a medicine, but a broken spirit drieth the bones.” Jesus in His healing ministry always emphasized the place of faith in the cure of the body. “Thy faith hath made thee whole,” is a frequent word on His lips, and ever since His day people have been rediscovering the truth that faith, even in the absence of a worthy object, does often make whole. Faith in the doctor, the medicine, the charm, the mineral waters, the shrine, and in the good God, has brought health to many thousands of sufferers. People have always reckoned on this bodily result from a mental state. They have intuitively known better than to tell a sick person that he is looking worse, but they have not always known why. They have known that a fit of anger is apt to bring on a headache, but they have not stopped to look for the reason, or if they have, they have often gotten themselves into a tangle. This is because there has always been, until recently, a missing link. Now the link has been found. After the last chapter, it will not be hard to understand that this connecting link, this go-between of body and mind, is nothing else than the subconscious mind. When we remember that it has the double power of knowing our thoughts and of controlling our bodies, it is not hard to see how an idea can translate itself into a pain, nor to realize with new vividness the truth of the statement that healthy mental states make for health, and unhealthy mental states for illness.
=Suggestion and Emotion.= There are still many gaps in our knowledge of the ways of the subconscious, but investigation has thrown a good deal of light on the problem. Two of the principles already discussed are sufficient to explain most of the phenomena. These are, first, that the subconscious is amenable to control by suggestion, and secondly, that it is greatly influenced by emotion. Tracing back the principles behind any example of the power of mind over body, one finds at the root of the matter either a suggestion or an emotion, or both. If, then, the stimulating and depressing effects of mental states are to be understood, the first Step must be a fuller understanding of the laws governing suggestion and emotion.
One of the most important points about the subconscious mind is its openness to suggestion. It likes to believe what it is told and to act accordingly. The conscious mind, too,—proud seat of reason though it may be,—shares this habit of accepting ideas without demanding too much proof of their truth. Even at his best, man is extremely susceptible to the contagion of ideas. Most of us are even less immune to this mental contagion than we are to colds or influenza; for ideas are catching. They are such subtle, insinuating things that they creep into our minds without our knowing it at all; and once there, they are as powerful as most germs.