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._ During sleep the eyes are closed, and consequently the action of external light on the retina impeded.

Yet it is found that even under these circumstances any very bright light suddenly introduced is capable of stimulating the optic fibres, and of affecting consciousness.