One of the most remarkable of man's characteristics is his capacity for becoming used to conditions of almost any kind, whether good or bad, both in the self and in the environment, and once he has become used to such conditions they seem to him both right and natural. This capacity is a boon when it enables him to adapt himself to conditions which are desirable, but it may prove a great danger when the conditions are undesirable. When his sensory appreciation is untrustworthy, it is possible for him to become so familiar with seriously harmful conditions of misuse of himself that these malconditions will feel right and comfortable.