The faculty of imagination, that shapes to the dream ideal pictures of things, is not sleeping. The faculties that perform the process of reasoning are not sleeping. _Comparison_—the power to compare the ideal with the real—alone is wanting. We mistake the shadows of the mind for substances. We accept the brain-born visions as realities. Why? Because we are unable to compare them. In brief, Comparison is the faculty, paralysed in sleep, whose absence causes the credulity of dream. Of this fact there can be no doubt. But a very formidable difficulty here presents itself. How and why is it that this faculty alone is found to slumber when the greater part of the mental mechanism is awake and active? It has been one of the most perplexing problems of Psychology. A solution of it has occurred to me which I submit to the consideration of the reader, but as a suggestion merely. It is too novel to be offered as anything more than a suggestion. Each mental faculty can perform only one act at the same instant of time. It is one of the conditions of existence here that all consciousness shall be in succession. Hence indeed our conception of time. If any other being could obtain many perceptions simultaneously, and not in succession, to that being there would be no _time_, in our sense of the term. But the process of comparison involves the contemplation together of the two things (or ideas of things) to be compared. This difficulty is removed by the double brain. Each brain presents one of the ideas to be compared and upon these the faculty of comparison employs itself, discerning their resemblances and differences. If so it be, the cause of our incapacity to discover the absurdities of dream is the partial paralysis (or sleep) of one of the two mental faculties that present the ideas of objects and the consequent incapacity of the faculty of comparison to discharge its proper function of informing us what of our mental impressions are real and what illusory.