Emotion and repetition are the great welders of complexes. Emotion is the strongest cement in the world. A single emotional experience suffices to bind together ideas that were originally as far apart as the poles. Sometimes a complex includes not only ideas, movements, and emotions, but physiological disturbances and sensations. Some people cannot go aboard a stationary ship without vomiting, nor see a rose, even though it prove to be a wax one, without the sneezing and watery eyes of hay-fever. This is what is known as a "conditioned reflex." Past associations plus fear have so welded together idea and bodily manifestation that one follows the other as a matter of course, long after the real cause is removed. In such ways innumerable nervous symptoms arise. The same laws which form healthy complexes, and, indeed, which make all education possible, may thus be responsible for the unhealthy mal-adaptive association-habits which lie back of a neurosis. Fortunately, a knowledge of this fact furnishes the clue to the re-education that brings recovery. A complex may be either conscious or unconscious, but as it usually happens that either all or part of its elements are below the surface, the word is oftenest used to mean those buried systems of the subconscious mind that influence thought or behavior without themselves being open to scrutiny. It is these buried complexes, memory groups, gathered through the years of experience, that determine action in uniform and easily prophesied directions. Every individual has a definite complex about religion, about politics, about patriotism, about business, and it is the sum of these buried complexes which makes up his total personality. =Displacement.= Association or grouping is, then, an intrinsic power of mind; but as all life seems to be built on opposites--light and darkness, heat and cold, love and hate--so mind, which is capable of association, is capable also of displacement or the splitting apart of elements which belong together. There is such a thing as the simple breaking up of complexes, when education or experience or neglect separate ideas and emotions which had been previously welded together; but displacement is another matter. Here there is still a path between idea and emotion; they still belong to the same complex, but the connection is lost sight of. The impulse or emotion attaches itself to another substitute idea which is related to the first but which is more acceptable to the personality. Sometimes the original idea is forgotten; repressed, or dissociated into the subconscious, as in anxiety neurosis; and sometimes it is merely shorn of its emotional interest and remembered as an unrelated or insignificant idea, as in compulsion neurosis.