With regard to memory, as with regard to sensation, we seem in waking life to be dealing with a selection made for purposes of earthly use. From the pre-conscious unselective memory which depends on the mere organisation of living matter, it is the task of consciousness, as it dawns in each higher organism, to make its own appropriate selection and to develop into distinctness certain helpful lines of reminiscence. The question of self-preservation--What must I needs be aware of in order to escape my foes?--involves the question, What must I needs remember in order to act upon the facts of which I am aware? The selected currents of memory follow the selected avenues of sensation; what by disuse I lose the power of noticing at the time, I also lose the power of recalling afterwards. For simpler organisms this rule may perhaps suffice. Man needs a more complex formula. For it may happen, as we have already seen, that two or more phases of personality in one man may each select from the mass of potential reminiscences a special group of memories of its own. These special groups, moreover, may bear to one another all kinds of relations; one may include another, or they may alternate and may be apparently co-exclusive. From these dissociations and alternations of memory there will be many lessons to learn. The lesson which here presents itself is not the least important. What is the relation of the sleeping state to these dissociated, these parallel or concentric memories? Is it the case that when one memory includes another it is the waking memory--as one might expect from that state's apparently superior vividness--which shows itself the deeper, the more comprehensive record?