Looking, then, at one hemisphere only of the brain, it is difficult to infer that one or more parts of it are sleeping while the other parts are awake. May the solution of the problem be found in the fact that we have _two_ brains? Can it be that in the condition of dream one hemisphere—that is, one mind—is awake while the other is asleep? To answer this it is necessary to inquire what is the action of _two_ brains working, like the two eyes, together or separately? For the common purposes of life the two brains act in complete accord. Like the two nerves of vision, they co-ordinate. Either can act alone for the ordinary uses of existence, just as one eye will do the usual work of sight. But as we see more perfectly, extensively, and roundly with two eyes than with one—so it may be reasonably concluded that we think more truly and clearly, and feel more strongly, when the two brains act together than when one is working alone. The faculty of _comparison_ is one of the most important of the mental powers, for it is the basis of accurate knowledge. But it is doubtful if this faculty can do its work in one brain unless co-ordinated with the same faculty in the other brain. Unlike the other mental faculties, “comparison” can exercise itself only upon _two_ ideas. Its very purpose is to make us conscious of the resemblances and differences between any two ideas presented to it. All mental processes are successive—that is to say, no two mental actions are performed by the same mental faculty at the same instant of time. Consequently, the faculty of comparison cannot exercise itself without having before it _two_ ideas to contrast. As one brain can present only one idea at any one moment, one brain cannot provide the materials wherewith comparison can work. The process of comparison cannot therefore be effected without the aid of the other brain. This, in healthy waking life, is done instantly, perfectly and unconsciously, by means of the power of co-ordination possessed by the two hemispheres. Such being the action of the waking brain, does sleep present any conditions that might be explained in like manner? Suppose the state of dream to be the slumber of one hemisphere only, the other being awake. May not this solve the problem? In dream we believe shadows to be substances, ideas to be things, incongruities to be natural, and impossibilities to be realities; and so believing, we have no sense of surprise and reason is not shocked. Nothing of these results presents itself when we are awake. Why? Waking, the faculty of _Comparison_ is enabled to do its work. It compares the idea with the reality, the shadow with the substance, the dream within with the impression without, the present picture of the mind with the stored knowledge of the past. The differences being thus discovered, the mind dismisses them as being the mere visions that they are.