3. _Loss of Control over the Mind._--Analogous to this is the loss of voluntary control over the mental operations, which is in fact, so far as the mind is concerned, the essential feature and characteristic of sleep. Mental action still goes on, there is reason to suppose; in many cases we know that it does; but the thoughts come and go at their own pleasure, without regulation or control. It is not in our power to arrest a certain thought, and fix our minds upon it for the time, to the exclusion of others, as we can do in the waking moments, and which constitutes, in fact, the chief control and power we have over our thoughts, nor can we dismiss, and throw off, an unpleasant train of thought, a disagreeable impression, however much we may desire to be rid of it. We are at the mercy of our own thoughts and casual associations, which, in the ungoverned, spontaneous play of the mind's own inherent energy, and guided only by its own native laws, produce the wildest and strangest phantasmagoria, having to us all the semblance of reality, while we are, in truth, mere passive spectators of the scene. _Faculties of Mind not suspended in Sleep._--It has been supposed by some that the faculties of the mind are, in part or wholly, suspended in sleep, especially the higher faculties more immediately dependent on the will. So long as mental activity goes on, however,--and there is no evidence that it ever entirely ceases in sleep--so long there is thought, and so long must that thought and activity be exerted in some particular direction, and on some particular object. We cannot conceive of the mind as acting or thinking, and not exercising any of its faculties, for what is a faculty of the mind but its capacity of acting in this or that way or mode, and on this or that class of subjects. It may be perception, or conception, or memory, or imagination, or judgment, or reasoning, or any other faculty that is for the moment active; it _must_ be some one of the known faculties of the mind, unless, indeed, we suppose some new faculties to be then developed, of whose existence we are at other times unconscious.