There is a very striking resemblance between dreams and waking perceptions. We see in dreams objects, persons, and events identical with those of the waking state. The belief in their reality is as complete as in that of what we see when awake; the emotions are as deep and vivid. Pleasures have a delicious savor, and pains are even more intense than those of the reality--as, for instance, those of nightmare, and the distresses to which we give ourselves up in full. In all cases these dream troubles seem as real as those of life, and are taken by us quite as seriously; and the existence of everything we see and feel is as evident as in life. Still we oppose the dream to the reality. The waking world is our true, our only world; the world of the dream seems to us purely interior and chimerical. The incoherence and absurdity of our dreams surprise and amuse us, and we are amazed to find that we have been able to believe, while asleep, in such foolish things. In short, dreaming is synonymous to us with illusion, phantasmagoria, and falsehood. The clearest of the prevailing theories about dreams rest upon the postulate that waking perceptions are the true ones, and the visions of the dream are false. They have answers to the three questions we are used to ask concerning dreams--Where do they come from? why are they incoherent? and why do we take their visions for realities? They explain dreams as former sensations reviving within us under different combinations, and as therefore simply confused reflexes of the reality. Dreams may, however, sometimes be produced by a present impression suffered by one of our senses, half awakened--a contact, the way we are lying, and the condition of the organic functions being thus the causes or occasions of dreams. The incoherence of dreams seems no more mysterious in these theories, and is explained as the result of two causes--the slumber of the "reflecting" faculties, judgment, reason, the will, the exercise of choice and control; and, secondly, the unrestricted reign of imagination and the association of ideas. Our faith in the reality of the things dreamed is accounted for by the mechanical play of the images, the law being set up that every image that is not opposed by stronger images appears to us a real object. The problem, therefore, resolves itself: the senses being asleep, the images that arise within us are not contradicted by normal sensations, and that is why we take them for realities. Further, our reflective faculties, being likewise dormant, can not contradict the images, in the absence of sensations, reasonings, or recollections. Hence a credence, as absolute as unreasonable. We purpose to show that there is something artificial and prejudiced in the classical theory of opposition between dreams and waking, which assigns illusion, confusion, and incoherence to the former, and solid and permanent reality to the latter, and that the difference between them is not so clean cut.