That is a physiological or biological problem concerning which much is still in dispute. We can come to no decisive answer, but I think we may attempt to define one psychological characteristic of sleep. Sleep is a condition in which I refuse to have anything to do with the outer world and have withdrawn my interest from it. I go to sleep by retreating from the outside world and warding off the stimuli proceeding from it. Again, when I am tired by that world I go to sleep. I say to it as I fall asleep: “Leave me in peace, for I want to sleep.” The child says just the opposite: “I won’t go to sleep yet; I’m not tired, I want more things to happen to me!” Thus the biological object of sleep seems to be recuperation, its psychological characteristic the suspension of interest in the outer world. Our relationship with the world which we entered so unwillingly seems to be endurable only with intermission; hence we withdraw again periodically into the condition prior to our entrance into the world: that is to say, into intra-uterine existence. At any rate, we try to bring about quite similar conditions—warmth, darkness and absence of stimulus—characteristic of that state. Some of us still roll ourselves tightly up into a ball resembling the intra-uterine position. It looks as if we grown-ups do not belong wholly to the world, but only by two-thirds; one-third of us has never yet been born at all. Every time we wake in the morning it is as if we were newly born. We do, in fact, speak of the condition of waking from sleep in these very words: we feel “as if we were newly born,”—and in this we are probably quite mistaken in our idea of the general sensations of the new-born infant; it may be assumed on the contrary that it feels extremely uncomfortable. Again, in speaking of birth we speak of “seeing the light of day.”