An illusion may be defined as a perception which alters the qualities of the object perceived and presents it to consciousness in a form other than its real one. One who hears insulting words in the singing of birds or in the noise of carriage-wheels experiences an illusion.

Illusions are of frequent occurrence in normal persons.

There is no one to whom the folds of a curtain seen in the dark have not appeared to assume more or less fantastic shapes. But the mind, aided by the testimony of the other senses, recognizes the abnormal character of the image; the illusion is recognized as such. In psychotic cases it is on the contrary taken for an exact perception and exercises a more or less marked influence upon all the psychic functions.

Illusions may affect any of the senses and present, in the case of each, features analogous to those of hallucinations; we shall therefore not describe them here. We shall say but a few words concerning illusions of sight which present certain peculiarities.

Illusions of sight may occur in most of the psychoses, but are chiefly found in the toxic psychoses and in the infectious deliria. When these illusions pertain to persons they lead to mistakes of identity. Many psychotics mistake fellow patients or employees of the institution for relatives or friends. This form of illusion sometimes attains such completeness that the subject may, while at a hospital, believe himself to be at his home.

Illusions are very apt to occur in the midst of vague impressions: those of hearing in the presence of confusing noises, and those of sight in partial darkness.

Like incomplete perceptions, inaccurate perceptions or illusions are the consequence of a disorder of ideation; abnormal associations replace normal ones, which are absent, and complete the image, altering it at the same time.