That the imagination may in its flights during sleep strike upon fancies which are subsequently developed by the reason into lucid and valuable ideas, is very probable. It would be strange if from among the innumerable absurdities and extravagancies to which it attains something fit to be appropriated by the mind should not occasionally be evolved, and thus there are many instances mentioned of the starting-point of important mental operations having been taken during sleep. Some of these may be based upon fact, but the majority are probably of the class of those just specified, or occurred at an age of the world when a belief in the supernatural exercised a greater power over men’s minds than it does at the present day. Among the most striking of them are the following: Galen declares that he owed a great part of his knowledge to the revelations made to him in dreams. Whether this was really the case or not we can in a measure determine by recalling the fact that he was a believer in the prophetic nature of dreams, and states that a man having dreamt that one of his legs was turned into stone, soon afterward became paralytic in this limb, although there was no evidence of approaching disease. Galen also conducted his practice by dreams, for an athlete, having dreamt that he saw red spots, and that the blood was flowing out of his body, was supposed by Galen to require blood-letting, which operation was accordingly performed. It has been said[41] that the idea of the _Divina Commedia_ occurred to Dante during sleep. There is nothing at all improbable in this supposition, though I have been unable to trace it to any definite source. Cabanis[42] states that Condillac assured him that often during the course of his studies he had to leave them unfinished in order to sleep, and that on awaking he had more than once found the work upon which he was engaged brought to a conclusion in his brain. These were clearly instances of “unconscious cerebration” of that power which the brain possesses to work out matters which have engaged its attention, without the consciousness of the individual being aroused to a knowledge of the labor being performed. It is not unlikely that this kind of mental activity goes on to some extent during sleep, but as it is of such a character that the mind does not take cognizance of its operations, I do not see how the exact period of its performance can be ascertained.