The full significance as well as the validity of this theory can be properly estimated only in the light of the facts which have been revealed by modern technical methods of investigation. After all, it is the consequences of a theory which count, and this will be seen to be true particularly as respects memory. The pragmatic point of view of counting the consequences, of determining the difference that the theory makes in the understanding of the mental processes of normal and abnormal life, reveals the importance to us of validating the theory. The consequences of the psycho-physiological theory are so far-reaching, in view of its bearing upon a large number of problems in normal and abnormal psychology, that it is worthy of sustained and exhaustive examination. I will, therefore, briefly résumé the various classes of facts which support the theory and which any adequate theory of memory must satisfactorily explain. For, as will appear, besides the common facts of memory pertaining to everyday life, there are a large number of other facts which can be observed only when the mind is dissected, so to speak, by pathological processes, and by the production of artificial conditions, and when investigations are carried out by special technic. Irrespective of any theory of explanation, a knowledge of these facts is extremely important for an understanding of many phenomena in the domain of both normal and abnormal psychology. =The meaning of conservation.=—We all know, as an everyday experience of mankind, that at one time we can recall what happened to us at some particular moment in the past, and at another time we cannot. We know that when we have forgotten some experience if we stimulate or refresh our memory, as the lawyers say to us on the witness-stand, by reference to our notes, appropriately called memoranda, the original experience may come back to mind. Often at one moment we cannot recall a verse, or a name, or a piece of acquired knowledge, while at another time, a little later, we can. We have a feeling, a perhaps justifiable belief, that a desired piece of knowledge is not lost, that it is back somewhere in our minds but we cannot get at it. If, sooner or later, under one circumstance or another, with or without the aid of some kind of stimulus, we can recall the desired knowledge we say it was preserved (or conserved). If we continue, under all circumstances and at all moments, to be unable to recall it we say it is lost, that our memory of it is not conserved. So the notion of conservation of knowledge being something apart from recollection enters even into popular language. What sort of thing conservation is, popular language does not attempt to define. It is clear, however, that we may with propriety speak of the conservation of experiences, using this term in a descriptive sense without forming any definite concept of the nature of conservation. Provisionally, then, I shall speak of conservation of a given experience