It is best to begin by looking into the mental constituents of waking. There is then passing before us an endless current of images and reflections, furnished from our recollections, and suggested by our hopes and our fears, by pursuits that interest us, or by their own inter-associations. This current of thought is continually being changed or modified, through impressions made upon our senses. It is further liable to be still more importantly and systematically modified by the exercise of the faculty of Attention. The attention operates in a twofold manner. It enables us to detain at pleasure any subject of thought before the mind; and, when not on such urgent duty, it vigilantly inspects every idea which presents itself, and reports if it be palpably unsound or of questionable tendency. To speak with more precision, it is a power we have of controlling our thoughts, which we drill to warn us whenever the suggested ideas conflict with our experience or our principles. Then of sleep. We catch glimpses of its nature at the moments of falling asleep and of waking. When it is the usual time for sleep, if our attention happen to be livelily excited, it is in vain we court sleep. When we are striving to contend against the sense of overwhelming fatigue, what we feel is, that we can no longer command our attention. Then we are lost, or are asleep. Then the head and body drop forwards; we have ceased to attend to the maintenance of our equilibrium. Any iteration of gentle, impressions, enough to divert attention from other objects, without arousing it, promotes sleep. Thus we recognise as the psychical basis of sleep the suspension of the attention. Are any other mental faculties suspended in sleep? Sensation and the influence of the will over the muscular system are not; for our dreams are liable to be shaped by what we hear. The sleeper, without waking, will turn his head away from a bright light, will withdraw his arm if you pinch it, will utter loud words which he dreams he is employing. The seeming insensibility in sleep, the apparent suspension of the influence of the will, are simply consequences of the suspension of attention.