Between sensation and conception, says M. Leuret, there exists an intermediate phenomenon, that practitioners call hallucination. Hallucination resembles sensation, inasmuch as, like sensation, it gives the idea of a body actually acting on the organs. It differs from it, inasmuch as it exists without an external object Like conception, it is creative, but it produces images, not ideas; images, however, which are of the same worth to the hallucinated as the objects themselves.*

* Bland, Note pour servir a Phistoire des hallucinations, Revue Medi-cale, Juin, 1842.

Again, M. Aubanel, in his excellent treatise on hallucinations, † looks on this phenomenon as a form or variety of mental alienation, in which a man transforms the delirious conceptions of his mind into sensations, or, by means of these same conceptions, perverts real sensations by assimilating them to his delirious ideas.

M. Baillarger admits two kinds of hallucination, the one complete, which are the result of the double action of imagination and the senses; these are psycho-senaorial: the other produced only by the involuntary exercise of memory and imagination, which are psychical hallucinations. He defined psycho-sensorial hallucination as a sensorial perception, independent of all external excitation of the sensuous organs, and having its starting-point in the involuntary exercise of memory and imagination. Psychical hallucinations may be defined as perceptions purely intellectual, having their starting-point in the involuntary exercise of memory and imagination, and in a less degree than hallucinations of the former kind - the internal excitation of the sensorial apparatus.‡

According to M. Michea, hallucination is the transformation, sometimes voluntary, but more frequently involuntary, of memory and imagination, either isolated or combined, resembling sensorial perception.§ Sometimes it is cerebral, and idiopathic; sometimes sensorial, peripheric, or symptomatic.

M. Szalkowski defines hallucination as a cerebral or psychical phenomenon perfected independently of the senses, and consisting in external sensations experienced by the individual, although no external agent acts materially on the senses.||

According to M. Parchappe, the inherence of the illusory characteristic in the psychical phenomenon, considered as an essential condition of the special change of modality which engenders illusions, necessarily excludes from this category of symptoms those phenomena which result immediately and exclusively from the operations of the soul as much as intellectual strength. In this opinion, notions and ideas can be true or false, and never illusory. Illusory sensations are distinguished from other psychical phenomena, inasmuch as they have an act of the senses for their condition, and an illusion for their effect.

* Leuret, Fragments psychologique sur la folie, p. 133, Paris, 1834.

† Aubanel, Essai sur les hallucinations, these, Paris, 1839.

‡ Baillarger, Des hallucinations, des causes qui les produisent, et des maladies qu'elles caracterisent (Memoires de l'Academie de Medecine, t. 12, Paris, 1846).

§ Michea, Du delire des sensations, p. 82, Paris, 1846.

|| Louis Rutin Szalkowski, Recherche sur les hallucinations au point de vue de la psychologie, de I'histoire, et de la me'decine legale, p. 8, 1849.

The illusory sensations are of two orders, objective and subjective. The essential character of the former is to have a sensation created by an external object, and, as a necessary effect, to engender an illusion relative to the sensible properties of that object. The subjective have for their essential character a sensation without a real object, caused by an exterior modification of the organs of the subject, and causing an illusion relative to the existence of an external object.*

Dendy explains hallucination as a past recollection - illusion, as a present recollection.†

For ourselves, building on the symptomatology of hallucinations and illusions, we will define hallucination as the perception of the sensible signs of an idea; and illusion, a false appreciation of real sensations. In considering this phenomenon in a psychological point of view, we will explain our opinion, by stating that the spiritual character of the idea, or its essence, never forms a part of the hallucination, but that the sensible sign forms its only foundation.

The division of hallucinations ought to rest on a previous and deep study of their states of complication and simplicity. But this method, suitable for classification, would offer serious drawbacks to the comprehension of this work; the reader would not at once seize on the whole of the plan, his mind would lose itself in details, and the result would be a painful impression, which would nullify all our efforts. Under this conviction, we will proceed to sketch our classification, after having exhibited the most generally adopted divisions.

* Max Parchappe, Symptomatologie de la folie, Annales Medico-Psy-chologiques (see numbers for January and April, 1850, and January, 1851, p. 268, et seq.).

† Walter Cooper Dendy, The Philosophy of Mystery, London, 1841.

Esquirol recognizes only cerebral, mental, or idiopathic hallucinations. M. Calmeil says that in theory we may suppose that symptomatic hallucinations exist, and that the peripheric nervous system is the starting-point for the most varied and perhaps the greatest number of hallucinations. But, whether we lean to the theories of Gapron, or rest upon those of Meyer, the sensations brought by the sentient extremities, which come from the nerves of sight, of hearing, of taste, of smell, of external and visceral touch, and of the spinal marrow, impress only a shock on the brain, and it requires the action of that organ to constitute the operations of hallucination.

M. Leuret divides hallucinations into those which occur during waking hours, and those which take place during sleep, and which are ordinarily known as visions. He includes incubus and succubus in the hallucinations of sleep.