Hallucinations—“psycho-sensorial disturbances characterized by sensations perceived when the exercise of the sense has not been determined by any external excitation”—are characteristic of many conditions of disturbed health besides insanity; and the same is true of illusions—erroneous interpretations of sensations actually perceived. In both cases the existence of insanity is determined by the fact whether or not the erroneous impressions are corrected by the judgment. An important point is to consider most carefully every unnatural, strange, or unexplained action, whether deliberate or from impulse, particularly in the large class of eccentric, ill-balanced, or weak-minded persons on the border-line between sanity and insanity. There are people who at one time seem to belong to the sane and at another to the insane class. Baillarger states that the essential element of insanity is loss of free will. Ball of Paris describes an insane man as one who, in consequence of a profound disturbance of the intellectual faculties, has lost more or less completely his free will (liberté morale), and has ceased thereby to be responsible to society for his actions. Bucknill describes insanity, in his Sugden prize essay, as “a condition of the mind in which a false action of conception or judgment, a {104} defective power of the will, or an uncontrollable violence of the emotions and instincts has been separately or conjointly produced by disease.” Maudsley's definition is, “Insanity is, in fact, disorder of brain producing disorder of mind; or, to define its nature in greater detail, it is a disorder of the supreme nerve-centres of the brain—the special organs of mind—producing derangement of thought, feeling, and action, together or separately, of such degree or kind as to incapacitate the individual for the relations of life.... Mind may be defined physiologically as a general term denoting the sum-total of those functions of the brain which are known as thought, feeling, and will. By disorder of the mind is meant disorder of those functions.” Bucknill considers insanity a disease of the brain affecting the integrity of the mind. Maudsley calls it a disorder of the mind of such a degree as to incapacitate one for the ordinary relations of life, implying that there may be certain deviations from the condition of sound mind which do not constitute insanity. Tuke's definition is that “insanity consists in morbid conditions of the brain, the result of defective formation or altered nutrition of its substance, induced by local or general morbid processes, and characterized especially by non-development, obliteration, impairment, or perversion of one or more of its psychical functions.” Instead of itself being a disease, insanity, properly speaking, is a symptom of diseases which under varying manifestations probably affect different functions of the brain—at least they affect the brain in different ways.