The mental operation is performed somewhat in this manner. Two ideas are present in the mind, which compares them and traces their resemblances and differences. The sense-borne idea being thus brought face to face, as it were, with the brain-born idea, the distinction is discovered, and the latter is relegated to the limbo of visions, the former is accepted as a reality and made the basis of action. But inasmuch as two ideas cannot be presented at the same instant of time by one brain hemisphere, the presence of the two ideas requisite to the process of comparison can be had only by the combined action of both hemispheres. Hence the usual inability of persons afflicted with hemiplegia to compare or reason accurately. If the action of the faculty of comparison were paralysed, we should dream when awake. The suspension of the action of this faculty in dream would suffice to account for the accepted incongruities of dream, without assuming the sleep of the entire hemisphere. But, as observed above, it is difficult to assume the slumber of one mental faculty alone, packed as all are among many with which they are intimately united. It is more probable that in dream the entire of one hemisphere sleeps. The facts are in accordance with such a suggestion. But, however this may be, it does not disturb the conclusion, that the seat of sleep is in the ganglia at the base of the brain. That portion of the brain which directs the motions of the body sleeps always. Sleep reigns more or less perfectly in the portions of the brain that receive the impressions of the senses. Sleep is very partial in the cerebrum, the duplex organ of the intelligence, and probably—(for it is as yet only conjectural)—partial sleep prevails there, if at all, by the contrivance of slumber by one hemisphere while the other is awake. Such being the _Physiology_ of Dream—so far as science has yet succeeded in tracing it—we proceed now to investigate its _Psychology_. [1] _The Functions of the Brain._ By DAVID FERRIER, M.D., F.R.S. London: Smith, Elder, & Co., 1876. CHAPTER VII. THE PSYCHOLOGY OF DREAM. The base of the brain being quite asleep, the central ganglia being partially asleep, the cerebral hemispheres or some part of them being awake, we have the physiological condition in which occur the Phenomena of Dream. The first coming on of Dream is found at the moment of “falling asleep,” before actual sleep has begun. _Then_ we _are_ conscious for an instant that we are dreaming—that the mental impressions are not external realities. But this consciousness is for a moment only. Either we start into waking life and the incipient dream is banished, or we fall into actual sleep and the condition of complete dream is established.