The normal illusions of the senses, which are believed to be real by primitive men, and by those ignorant of physical laws, have a similar origin. The objection of such phenomena as a mirage, or the tremulous effect produced in tropical regions by the refraction and reflection of light on trees, rocks, and mountains, so well described by Humboldt, is due to ignorance of the laws of nature, and this is in fact an entification of the phenomenon, occasioned by the innate tendency to animation which is proper to the perception. In this it is easy to trace the genesis both of myth and dreams. The fact of hallucination is more complex, even in its normal state, that is, in those general conditions of mind and body in which reason has complete command over us. 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.