That such action does prompt a large number of dream-images may be regarded as fairly certain. First of all, it seems impossible to account for all the images of dream-fancy as secondary phenomena connected by links of association with the foregoing classes of sensation. However fine and invisible many of the threads which hold together our ideas may be, they will hardly explain the profusion and picturesque variety of dream-imagery. Secondly, we are able in certain cases to infer with a fair amount of certainty that a dream-image is due to such central stimulation. The common occurrence that we dream of the more stirring events, the anxieties and enjoyments of the preceding day, appears to show that when the cerebral elements are predisposed to a certain kind of activity, as they are after having been engaged for some time in this particular work, they are liable to be excited by some stimulus brought directly to bear on them during sleep. And if this is so, it is not improbable that many of the apparently forgotten images of persons and places which return with such vividness in dreams are excited by a mode of stimulation which is for the greater part confined to sleep. I say "for the greater part," because even in our indolent, listless moments of waking existence such seemingly forgotten ideas sometimes return as though by a spontaneous movement of their own and by no discoverable play of association. It may be well to add that this immediate revival of impressions previously received by the brain includes not only the actual perceptions of waking life, but also the ideas derived from others, the ideal fancies supplied by works of fiction, and even the images which our unaided waking fancy is wont to shape for itself. Our daily conjectures as to the future, the communications to us by others of their thoughts, hopes, and fears,--these give rise to numberless vague fugitive images, any one of which may become distinctly revived in sleep.[92] This throws light on the curious fact that we often dream of experiences and events quite unlike those of our individual life. Thus, for example, the common construction by the dream-fancy of the experience of flight in mid-air, and the creation of those weird forms which the terror of a nightmare is wont to bring in its train, seem to point to the past action of waking fancy. To imagine one's self flying when looking at a bird is probably a common action with all persons, at least in their earlier years, and images of preternaturally horrible beings are apt to be supplied to most of us some time during life by nurses or by books. _Indirect Central Stimulation._