On closer inspection the image of a cow resting may obtrude itself, and the presentation may finally resolve itself with certainty into a group of people sitting. The impressions which the mind receives during sleep through outer stimuli are of a similar indistinct nature; they give rise to illusions because the impression evokes a greater or lesser number of memory pictures through which the impression receives its psychic value. In which of the many spheres of memory to be taken into consideration the corresponding pictures are aroused, and which of the possible association connections thereby come into force, this, even according to Strümpell, remains indeterminable, and is left, as it were, to the caprice of the psychic life.
We may here take our choice. We may admit that the laws of the dream formation cannot really be traced any further, and therefore refrain from asking whether or not the interpretation of the illusion evoked by the sensory impression depends upon still other conditions; or we may suppose that the objective sensory stimulus encroaching upon sleep plays only a modest part as a dream source, and that other factors determine the choice of the memory picture to be evoked. Indeed, on carefully examining Maury's experimentally produced dreams, which I have purposely reported in detail, one is apt to think that the experiment really explains the origin of only one of the dream elements, and that the rest of the dream content appears in fact too independent, too much determined in detail, to be explained by the one demand, viz. that it must agree with the element experimentally introduced. Indeed one even begins to doubt the illusion theory, and the power of the objective impression to form the dream, when one learns that this impression at times experiences the most peculiar and far-fetched interpretations during the sleeping state. Thus B. M. Simon tells of a dream in which he saw persons of gigantic stature seated at a table, and heard distinctly the awful rattling produced by the impact of their jaws while chewing. On waking he heard the clacking of the hoofs of a horse galloping past his window. If the noise of the horse's hoofs had recalled ideas from the memory sphere of "Gulliver's Travels," the sojourn with the giants of Brobdingnag and the virtuous horse-creatures—as I should perhaps interpret it without any assistance on the author's part—should not the choice of a memory sphere so uncommon for the stimulus have some further illumination from other motives?
II. Internal (Subjective) Sensory Stimuli.—Notwithstanding all objections to the contrary, we must admit that the rôle of the objective sensory stimuli as a producer of dreams has been indisputably established, and if these stimuli seem perhaps insufficient in their nature and frequency to explain all dream pictures, we are then directed to look for other dream sources acting in an analogous manner. I do not know where the idea originated that along with the outer sensory stimuli the inner (subjective) stimuli should also be considered, but as a matter of fact this is done more or less fully in all the more recent descriptions of the etiology of dreams. "An important part is played in dream illusions," says Wundt (p. 363), "by those subjective sensations of seeing and hearing which are familiar to us in the waking state as a luminous chaos in the dark field of vision, ringing, buzzing, &c., of the ears, and especially irritation of the retina. This explains the remarkable tendency of the dream to delude the eyes with numbers of similar or identical objects. Thus we see spread before our eyes numberless birds, butterflies, fishes, coloured beads, flowers, &c. Here the luminous dust in the dark field of vision has taken on phantastic figures, and the many luminous points of which it consists are embodied by the dream in as many single pictures, which are looked upon as moving objects owing to the mobility of the luminous chaos. This is also the root of the great fondness of the dream for the most complex animal figures, the multiplicity of forms readily following the form of the subjective light pictures."
The subjective sensory stimuli as a source of the dream have the obvious advantage that unlike the objective stimuli they are independent of external accidents. They are, so to speak, at the disposal of the explanation as often as it needs them. They are, however, in so far inferior to the objective sensory stimuli that the rôle of dream inciter, which observation and experiment have proven for the latter, can be verified in their case only with difficulty or not at all. The main proof for the dream-inciting power of subjective sensory excitements is offered by the so-called hypnogogic hallucinations, which have been described by John Müller as "phantastic visual manifestations." They are those very vivid and changeable pictures which occur regularly in many people during the period of falling asleep, and which may remain for awhile even after the eyes have been opened. Maury, who was considerably troubled by them, subjected them to a thorough study, and maintained that they are related to or rather identical with dream pictures—this has already been asserted by John Müller. Maury states that a certain psychic passivity is necessary for their origin; it requires a relaxation of the tension of attention (p. 59). But in any ordinary disposition a hypnogogic hallucination may be produced by merging for a second into such lethargy, after which one perhaps awakens until this oft-repeated process terminates in sleep. According to Maury, if one awakens shortly thereafter, it is often possible to demonstrate the same pictures in the dream which one has perceived as hypnogogic hallucinations before falling asleep (p. 134). Thus it once happened to Maury with a group of pictures of grotesque figures, with distorted features and strange headdresses, which obtruded themselves upon him with incredible importunity during the period of falling asleep, and which he recalled having dreamed upon awakening. On another occasion, while suffering from hunger, because he kept himself on a rather strict diet, he saw hypnogogically a plate and a hand armed with a fork taking some food from the plate. In his dream he found himself at a table abundantly supplied with food, and heard the rattle made by the diners with their forks. On still another occasion, after falling asleep with irritated and painful eyes, he had the hypnogogic hallucination of seeing microscopically small characters which he was forced to decipher one by one with great exertion; having been awakened from his sleep an hour later, he recalled a dream in which there was an open book with very small letters, which he was obliged to read through with laborious effort.
Just as in the case of these pictures, auditory hallucinations of words, names, &c., may also appear hypnogogically, and then repeat themselves in the dream, like an overture announcing the principal motive of the opera which is to follow.
A more recent observer of hypnogogic hallucinations, G. Trumbull Ladd, takes the same path pursued by John Müller and Maury. By dint of practice he succeeded in acquiring the faculty of suddenly arousing himself, without opening his eyes, two to five minutes after having gradually fallen asleep, which gave him opportunity to compare the sensations of the retina just vanishing with the dream pictures remaining in his memory. He assures us that an intimate relation between the two can always be recognised, in the sense that the luminous dots and lines of the spontaneous light of the retina produced, so to speak, the sketched outline or scheme for the psychically perceived dream figures. A dream, e.g., in which he saw in front of him clearly printed lines which he read and studied, corresponded to an arrangement of the luminous dots and lines in the retina in parallel lines, or, to express it in his own words: "The clearly printed page, which he was reading in the dream, resolved itself into an object which appeared to his waking perception like part of an actual printed sheet looked at through a little hole in a piece of paper, from too great a distance to be made out distinctly." Without in any way under-estimating the central part of the phenomenon, Ladd believes that hardly any visual dream occurs in our minds that is not based on material furnished by this inner condition of stimulation in the retina. This is particularly true of dreams occurring shortly after falling asleep in a dark room, while dreams occurring in the morning near the period of awakening receive their stimulation from the objective light penetrating the eye from the lightened room. The shifting and endlessly variable character of the spontaneous luminous excitation of the retina corresponds exactly to the fitful succession of pictures presented to us in our dreams. If we attach any importance to Ladd's observations, we cannot underrate the productiveness of this subjective source of excitation for the dream; for visual pictures apparently form the principal constituent of our dreams. The share furnished from the spheres of the other senses, beside the sense of hearing, is more insignificant and inconstant.
III. Internal (Organic) Physical Excitation.—If we are disposed to seek dream sources not outside, but inside, the organism, we must remember that almost all our internal organs, which in their healthy state hardly remind us of their existence, may, in states of excitation—as we call them—or in disease, become for us a source of the most painful sensations, which must be put on an equality