In a complete example of memory the following factors may be noted: 1. The original presentation--as the first perception of an object or scene, the reading of a new story, the hearing of a particular voice, etc. 2. Retention--this involves the permanent changes wrought in the nervous tissue as a result of the presentation or learning process and, as mentioned above, is really physiological. 3. Recall--this implies the re-establishment of the nervous movements involved in the original experiences and an accompanying revival of the mental imagery. 4. Recognition--under this heading is included the sense of familiarity experienced in consciousness, and the consequent belief that the present experience actually occurred at some certain time as an element in our past experience. CONDITIONS OF MEMORY =A. Physical Conditions.=--One of the first conditions for an effective recollection of any particular experience will be, evidently, the strength of the co-ordinations set up in the nervous system during the learning process. The permanent changes brought about in the nervous tissue as a result of conscious experience is often spoken of as the physical basis of memory. The first consideration, therefore, relative to the memorizing of knowledge is to decide the conditions favourable to establishing such nervous paths during the learning process. First among these may be mentioned the condition of the nervous tissue itself. As already seen, the more plastic and active the condition of this tissue, the more susceptible it is to receive and retain impressions. For this reason anything studied when the body is tired and the mind exhausted is not likely to be remembered. It is for the same reason, also, that knowledge acquired in youth is much more likely to be remembered than things learned late in life. The intensity and the clearness of the presentation also cause it to make a stronger impression upon the system and thus render its retention more permanent. This demands in turn that attention should be strongly focused upon the presentations during any learning process. By adding to the clearness and intensity of any impressions, attention adds to the likelihood of their retention. The evident cause of the scholar's ability to learn even relatively late in life is the fact that he brings a much greater concentration of attention to the process than is usually found in others. Repetition also, since it tends to break down any resistance to the paths which are being established in the nervous system during the learning process, is a distinct aid to retention. For this reason any knowledge acquired should be revived at intervals. This is especially true of the school knowledge being acquired by young children, and their acquisitions must be occasionally reviewed and used in various ways, if the knowledge is to become a permanent possession. A special application of the law of repetition may be noted in the fact that we remember better any topic