Kant says somewhere: "The lunatic is a dreamer in the waking state." According to Krauss, "Insanity is a dream in which the senses are awake." Schopenhauer terms the dream a brief insanity, and insanity a long dream. Hagen describes delirium as a dream-life which is inducted not by sleep but by disease. Wundt, in his Physiologische Psychologie, declares: "As a matter of fact we ourselves may in dreams experience almost all the manifestations which we observe in the asylums for the insane."
Sigmund Freud, The Interpretation of Dreams, p. 66.