I need hardly add that I refer to the illusions of sense dealt with in the foregoing chapters.
Dreams are to a large extent the semblance of external perceptions.
Other psychical phenomena, as self-reflection, emotional activity, and so on, appear in dream-life, but they do so in close connection with these quasi-perceptions.

The name "vision," given by old writers to dreams, sufficiently points out this close affinity of the mental phenomena to sense-perception; and so far as science is concerned, they must be regarded as a peculiar variety of sense-illusion.

Hence the appropriateness of studying them in close connection with the illusions of perception of the waking state.

Though marked off by the presence of very exceptional physiological conditions, they are largely intelligible by help of these physiological and psychological principles which we have just been considering.
_The State of Sleep._ The physiological explanation of dreams must, it is plain, set out with an account of the condition of the organism known as sleep.

While there is here much that is uncertain, there are some things which are fairly well known.