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. If a man is habitually subject to many and various hallucinations, and his sane judgment esteems them to be such, they are undoubtedly unusual phenomena, but they do not in any way injure the rational exercise of the mind. It is only when he believes the images to be real that the abnormal state begins, termed delirium if it is of short duration, and madness if it is permanent. We must examine hallucination under these new conditions. In the delirium of fever, or in various forms of disease, the cerebral excitement is so great that not only the deliberate exercise of reason, but the power of estimating external objects is lost, and the organs of the senses are so completely altered, that the perceptions themselves are exaggerated and confused. In this state hallucination reaches its highest point, and the patient sees, hears, and feels, directly or indirectly, strange and terrible things: wild beasts, enemies of all kind, torments; or again, pleasing and agreeable images. Independently of the alteration in various sensations produced by the morbid alteration of the special organs which induce them, the real cause of this phenomenon consists in the objection of mental sensations and images. Such an objection of images or sensations, considered in the act which transforms them into a reality, depends on the same cause as all other acts of perception; there is always an entification of the phenomenon, which in this case is a vivid internal image, appearing to be external and real.