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Reality testing, one of the most important of the functions of the ego, is relatively easy to define but quite difficult to comprehend. It is part of a conglomerate of ego functions which include such activities as perception, memory, object relations, sense of reality, superego, and the more recently discussed concept of reality constancy (19).
As used in psychoanalysis, reality testing refers to the ability to distinguish between perceptions and ideas. It is quite different from the philosopher's concept of the nature of reality. As defined in analytic terms, emphasis is placed upon the differentiation between representations of what is external—of the object world—from representations of what is internal—of the self or of mental life. The feeling of reality is not necessarily a part of perceptual experience. It does not have the sense of immediacy that characterizes consciousness. There is nothing in the quality of the perceptual experience which makes it apparent at once whether a mental representation is external or internal, real or unreal. An additional mental function, perhaps a set of mental functions, have to be called upon in order to make this decision. This operation has to be applied to all data registered at that station of mental experience that we call awareness.
A great deal has already been learned concerning how the function of reality testing develops but much still remains to be understood. Reality testing develops gradually. The early stages of this process are particularly difficult to study. In addition to the maturation of the essential ego apparatuses, the vicissitudes of development are very important. All workers in