In the use of its functions the mind manifests certain powers and certain modes of expression which can act as powerful allies or as damaging enemies of health. We speak of man as adaptable, but also as a being of habits. We speak of him as "feeling" when we wish to express the fact that his emotions influence his body. We expect of the average man a certain amount of suggestibility. We say that he is tremendously affected by his environment, which simply means that his attention, naturally centered chiefly on the things at hand, largely determines what he is. But we recognize that a man of trained mind can choose and will to substitute for his present surroundings thoughts upon more constructive things from past experience, or from future possibilities, or from within the mind's own storehouse. His ability to largely modify his life by his will, we recognize as man's greatest power. _Adaptability_, emotional response, _suggestibility_, _attention_, _thought-substitution_, _habit-formation_, and _will_ can minister vitally to health, or can prove damaging avenues of disease. NECESSITY OF ADAPTABILITY Adaptability is as essential to life of mind as to life of body; and health of mind as well as health of body is determined by the individual ability to adjust himself to environment. There are dreamers who have lived in their ideal world so long that they cannot meet the stern realities of life when they come. The shock is too great for the mind that has accepted only the fantastic, the real as the dreamer would have it; and he lets go altogether his hold on the actual, accepting the would-be world as present fact. And we call him insane. Other visionaries wakened rudely to life as it is, accept it as unchangeable fate, lose all their true ideals and become cynical, or victims of utter depression for whom life holds nothing that matters. Still others go on through the years self-satisfied and serene because they simply refuse to believe unpleasant truths; they "pretend" that their wishes are realities, and acknowledge as facts only the pleasant things of existence. The first two groups have failed to adapt self to life as it is, and the mind is lost or so damaged as to no longer serve its body properly. The "pretenders" have adjusted themselves, and so long as they can remain happily self-deceived all goes well for them, though they complicate living for others. However, they have made an adaptation, a defective one, it is true, but one through which the mind may survive. Some of this class, however, finally build up a more and more elaborate system of self-deception until they, too, are insane.