And this raises a curious question as to the relative functions and operations of the two brains. In profound slumber, when both brains are sleeping, there is no consciousness—time is annihilated to such a sleeper and awakening seems to follow immediately upon falling asleep, although in reality many hours may have passed. When the brain is sleeping but partially there is some consciousness of time in sleep and of the lapse of time upon awaking. Is such partial sleep the slumber of _one brain only_, and are these phenomena of dream due to the action of that one brain deprived of the correcting influence of the other brain? Does the faculty of comparison fail to show us that our mental impressions are subjective and not objective because it is not assisted by the normal action of the duplicate faculty of the other brain? Comparison is the foundation of the process of reasoning. It has been noticed that persons suffering from hemiplegia—that is, from disease of one brain only—often lose the power to compare and consequently the capacity for reasoning readily and correctly. May it not be that a similar condition is produced by temporary paralysis of the brain in sleep? As already stated, the power to reason is not absent in dream. We often reason elaborately and well, taking the ideal pictures as real incidents. We accept as objective facts what are merely mental impressions and thus build an argument on an incorrect assumption. The reasoning is right, but the basis of it is false. Question each mental faculty in turn and it will appear that but one is at fault in dream—namely, _comparison_. We are unable to discern the difference between the mental and the sensual impression—the self-created and the sense-borne idea—because we are incompetent to compare them and it is by comparison alone that we can distinguish the false from the true. I throw out this, as a suggestion merely, to Mental Philosophers and Psychologists. Indeed, the fact that we have two perfect brains with every mental faculty in duplicate (as contended by Sir HENRY HOLLAND and now conclusively established by the experiments of BROWN-SEQUARD and Professor FERRIER), has opened a new field to the Mental Philosopher and Psychologist. It must have the most intimate relationship, not to the phenomena of Sleep and Dream alone but to all the phenomena of Mind. In this great fact will doubtless be found the obvious solution of many problems hitherto insoluble. Foremost among those philosophical puzzles has been the instantaneous lapse of the Mind into _insanity_ in dream, and the no less marvellous manner in which upon waking we pass almost as quickly out of that insane condition into sanity.