Without entering into any analysis of the various forms of hallucination of which many able psychologists and physicians of the insane have treated, let us turn to the more ordinary cases in which an image of the mind is projected on the external world so as to appear real.  The roots of such a phenomenon are strictly organic, and belong to the centres in which the image is formed, as we have already observed; this image sometimes stands out in such vivid relief on the psychical space that it seems to be an external, not, as it usually appears in less vivid form, an internal intuition.  The hallucinations which Nicolai describes himself to have experienced may be taken as a classical example.  When Andral was returning from an autopsy, he clearly saw the corpse stretched before him as he entered his room.  Goethe, Byron, and many others, have been affected in the same way.  I myself have occasionally had hallucinations of the kind when in a perfectly healthy condition of mind and body; one, in particular, of a very vivid character, occurred when I awoke one morning and seemed to see a tall and venerable priest entering my chamber.  It is needless to multiply examples; similar facts abound in classic books in English, French, German, and other languages.  Let us rather study the phenomenon and trace its origin.

It is clear on the one side that the images of the hallucinations of sight or hearing appear to have a real existence, so that they may be observed and studied with ease; and it is also certain that this image has no external existence, and is simply a cerebral fact, due to the organs adapted for perception.  Without considering the cause of the external projection, to which I have already alluded, since perhaps its physiological and psychical genesis is not yet fully understood, we must consider the image, so far as it is believed to be real.

In cases of normal hallucination the reason is intact, and the observer is conscious of the illusion, yet notwithstanding this positive judgment the image has an appearance of complete reality.  The cause of this illusion is evidently the same as that of the illusions of dreams, and of the origin of myth; namely, that everywhere and always the mental or natural phenomenon and its image are respectively entified.  In the normal waking state, habit and other causes on which we have touched render our ideas of things altogether immaterial, as merely psychical forms and representative signs, but when the excitement of the organs increases, so as to present them to the consciousness as objective images, then, owing to the interruption of the ordinary process, they are suddenly entified, and appear as an external phenomenon.  Hallucinations are therefore explained by our theory, and it is further confirmed by the hallucinations of animals, and especially by the delirium of dogs and other animals affected by hydrophobia, or by cerebral excitement artificially produced by alcoholic and exhilarating drugs.