81. Sleep is a repose of the senses intended for the health of animals. -- Sleep is a cessation of action in the senses; but the mere inaction of one or several of the senses is not sleep, for this may be the effect of disease. Sleep is produced when the sensitive faculty called the common sense is impeded in its operation. Since the common sense (sensus cornmunis) depends on the activity of the external senses for its object, when they are no longer active it too is at rest. Sleep is necessary for animals, because the sensitive faculties, being limited by their nature, cannot continue always active, but require rest at stated intervals.
82. The cause of sleep is the exhaustion and fatigue of the sensitive nature by which communication through the nervous system is interrupted. -- After excessive or prolonged labor, and above all during the process of digestion, the senses must rest, that all the vital energy may be given to the digestive and recuperative action then going on; from this results the repose of the senses, or sleep. First the sense of sight ceases to act, then that of taste, of smell, of hearing, and last of all, the sense of touch. The senses also awake successively, but in a reverse order.
83. Somnambulism is an imperfect sleep in which all communication between the common sense and the external senses, is not interrupted. It happens in those persons in whom there is a superabundance of vital spirits. -- In this state, in consequence of an abnormal condition of the system, if the individual has an abundance of vital spirits, the images produced in his imagination can act as in a waking state. But as he is in a state of sleep, he is not conscious of his acts, and though he may recall the images that troubled him, he will forget the consequent motions, because they have left no trace in him.
84. Dreams and deliria have three causes: the activity of the imagination, the state of the body, and physical impressions received from without. -- Dreams and deliria are produced in the imagination, and hence the matter of dreams and deliria has always previously been in some way present to the external senses.
Dreams "are in some respects akin to states of reverie which occur during waking life. In dreaming the imagination assumes the part played in waking life by the external senses. During sleep the activity of these latter falls into abeyance; volitional control over the course of thought ceases; the power of reflexion and comparison is suspended; and the fancy of the dreamer moves along automatically under the guidance of association." The chief characteristics of the dream are "(a) its seeming reality, (b) its incoherence and extravagance, (c) its possession of a certain coherence amid this inconsistency, and (d) the exaggeration of actual impressions."{1}
85. Insanity and its different degrees, as hallucination, monomania, etc., are nothing but a derangement of the organ of imagination, in consequence of which it sees images that correspond to no reality. -- Insanity is a sort of waking dream. Its cause is to be sought either in the disorder of the brain functions, due to poisonous materials present in the blood, or in some organic disease of the brain.
86. Insanity affects the intellect indirectly through the imagination. -- Insanity injures only the sensitive faculties; yet, as these faculties supply the intellect with matter for its operations, insanity reaches the intellect indirectly. But just as the sight is not injured when we look through a colored glass, although it is deceived as to the color of the objects, so in cognizing and in reasoning from a false image, the intellect, although deceived, suffers no injury in itself.
{1} "(a) The apparent reality of the dream is, in great part, a consequence of the cessation of the action of the external senses. In sleep the images of the fancy which may arise within us are not subject to the correction which the presentations of the senses are ever furnishing during waking life. . . (b) The inconsistency of the dream seems to be due to its course being left entirely to the guidance of the fortuitous associations modified by the interference of accidental sensations at the moment. The absence of control over our thoughts disables us from reflecting upon the ideas which arise spontaneously, and prevents us from comparing them with past experience or with each other. . . (c) The coherence of the dream in so far as it occasionally exists, probably results in part from an orderly succession of previously associated ideas, in part from a faint power of selection exerted by a dominant tone of consciousness at the time, which may be able to reject striking eccentricities. -- (d) The exaggeration of occasional real impressions is accounted for by the fact that while the great majority of external sensations are excluded, those which do find entrance are thereby in a peculiarly favorable position. They are in novel isolation from their surroundings; their nature is vaguely apprehended; and they cannot be confronted with other experiences. . . . Another striking feature of dreams is the extraordinary rapidity with which trains of thought sometimes pass through the mind. " -- (Psychology, Stonyhurst Series, pp. 176-179.)