When, however, such a notion of memory is examined in the light of all the facts which have to be explained it will be found to be descriptive only of our conscious experiences. It does not explain memory; it does not answer the question of the ordinary man, “How can ideas which have ceased to exist be reproduced again as memory?” For, putting aside various psychological difficulties such as, How can I determine the reproduction of a former phase of consciousness—that is, memory—without first _remembering_ what I want to determine?, or, if this be answered, “By the association of phases (ideas),” how can there be any bond of association between an existing idea and one that does not exist?, and, therefore, how can association bring back that which has ceased to exist?—putting aside such questions, there are a number of psycho-physiological facts which this conception of memory will be found inadequate to meet. As a matter of fact, investigations into the behavior of mental processes, particularly under artificial and pathological conditions, have disclosed certain phenomena which can be adequately explained only on the supposition that ideas as they pass out of mind—the mental experiences of the moment—leave something behind, some residuum which is preserved, stored up as it were, and which plays a subsequent part in the process of memory. These phenomena seem to require what may be called a psycho-physiological theory of memory. Although the theory has long been one of the concepts of normal psychology it can be said to have been satisfactorily validated only by the investigations of recent years in abnormal psychology.