whether our mental images answer to present realities or not.  On the other hand, when asleep, this reference to a fixed objective standard is clearly impossible.  Secondly, we may fairly argue that the mental images of sleep approximate in character to external impressions.  This they do to some extent in point of intensity, for, in spite of the diminished excitability of the centres, the mode of stimulation which occurs in sleep may, as I have hinted, involve an energetic cerebral action.  And, however this be, it is plain that the image will gain a preternatural force through the greatly narrowed range of attention.  When the mind of the sleeper is wholly possessed by an image or group of images, and the attention kept tied down to these, there is a maximum reinforcement of the images.  But this is not all.  When the attention is thus held captive by the image, it approximates in character to an external impression in another way.  In our waking state, when our powers of volition are intact, the external impression is characterized by its fixity or its obdurate resistance to our wishes.  On the other hand, the mental image is fluent, accommodating, and disappears and reappears according to the direction of our volitions.  In sleep, through the suspension of the higher voluntary power of attention, the mental image seems to lord it over our minds just as the actual impression of waking life.

This much may suffice, perhaps, by way of a general description of the sleeping and dreaming state.  Other points will make themselves known after we have studied the contents and structure of dreams in detail.

Dreams are commonly classified (e.g. by Wundt) with hallucinations, and this rightly, since, as their common appellation of “vision” suggests, they are for the most part the semblance of percepts in the absence of external impressions.  At the same time, recent research goes to show that in many dreams something answering to the “external impression” in waking perception is the starting-point.  Consequently, in order to be as accurate as possible, I shall divide dreams into illusions (in the narrow sense) and hallucinations.

Dream-Illusions.

By dream-illusions I mean those dreams which set out from some peripheral nervous stimulation, internal or external.  That the organic processes of digestion, respiration, etc., act as stimuli to the centres in sleep is well known.  Thus, David Hartley assigns as the second great source of dreams “states of the body."[79] But it is not so well known to what an extent our dreams may be influenced by stimuli acting on the exterior sense-organs.  Let us first glance at the action of such external stimuli.

Action of External Stimuli.