Personality, as used in philosophy, signifies the distinctive attribute or attributes which distinguish a person from an animal or a thing. A thing we ordinarily consider to be unconscious, an animal to be conscious, a person to be self-conscious. That is to say, we suppose the animal to have intelligent experience of a kind, without being able to reflect upon that experience, and so to be conscious of itself as the unitary subject whose the experience is. The last is the essential mark of personality in the intellectual sphere. 'A person,' says Locke, 'stands for a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking being in different times and places' (Essay, ii. 27). In the moral sphere personality means self-determination or reason-directed will, and may be said to be the foundation of moral responsibility. Hence the central position which it occupies in the ethics of Kant and Hegel. The consciously realised unity and identity of the individual thus constitutes what is most distinctive of personality as such. But under the name of Double Personality or Double Consciousness the records of medical science contain many cases of mental disorder, in which the sense of personal identity is curiously interfered with. Cases are, of course, of constant occurrence in which the patient mentally affected conceives himself to be some one else (e.g. Napoleon or a Scripture characters). Others conceive that parts or properties of their frame belong to another person, or that they are inhabited and ruled by a spirit or entity acting in opposition to their will and interests. Others, again, are possessed by the idea that they are two persons at once, or rather that their body is the seat of two beings who are often in strife with one another, one being generally identified more strictly with the self, and the other being regarded as a hostile power and a mauvais sujet who prompts the better self to evil courses. The struggle between the two persons of this duality often takes bodily shape, and the patient maltreats his own body under the impression that he is castigating the vicious 'other one' who haunts him. This alienation or extrusion of part of the individual's experience from the inner circle of the personality may be due, it has been suggested, to a morbid alteration in the cœnesthesis or organic sensations which represent in consciousness the state of the body as a whole. Any part of the body in which this common sensibility is wanting or disturbed is regarded by the patient as no longer a part of himself, and even as belonging to some hostile being. It even happens in extreme cases of such somatic insensibility that the individual doubts or denies his own existence, as in the case of a patient cited by Ribot, who declared that he had been dead two years, though (according to his own account) he still continued to exist in a mechanical fashion in which he was not consciously interested.
These manifestations, however, are not what is meant by double consciousness in the strict sense of the term. Double consciousness does not necessarily imply the presence of any insane delusion as to the patient's present existence and surroundings, but consists in the fact that a certain portion of his past life is temporarily withdrawn altogether from his conscious memory, to reappear, however, at a later period, when he will have as completely forgotten his present experiences and the whole section of his life connected with them. In the normal human being the memory is unitary, and consequently the life-experiences of the individual are felt and recalled as parts of one whole. In these morbid cases, on the contrary, the conscious life seems, as it were, to be cut into sections or lengths which are entirely dissevered, and retained, so to speak, in separate memories. These mutually exclusive sections are remembered by the individual intermittently in successive periods, generally separated from one another by a swoon, a fit, or some violent nervous crisis. Now, as it is our memory of past experiences that may be said to form the anchor of personal identity, it follows that in such cases we shall have, in greater or less completeness, the extraordinary phenomenon of two separate and independent trains of thought—consequently two separate personalities—in the same physical individual.
Perhaps the most clearly defined and complete instance on record is that of the young American woman reported by Macnish in his Philosophy of Sleep. She fell without forewarning into a profound sleep lasting several hours beyond the usual term. Before her sleep she was well informed and possessed an excellent memory. 'On waking she was discovered to have lost every trace of acquired knowledge. It was found necessary for her to learn everything again. She even acquired by new efforts the art of spelling, reading, writing, and calculating, and gradually became acquainted with the persons and objects around, like a being for the first time brought into the world. In these exercises she made considerable proficiency. But after a few months another fit of somnolency invaded her. On rousing from it she found herself restored to the state she was in before the first paroxysm, but was wholly ignorant of every event and circumstance that had befallen her afterward. She is as unconscious of her double character as two persons are of their respective natures. For example, in her old state she possesses all the original knowledge, in her new state only what she acquired since. In the old state she possesses fine powers of penmanship, while in the new she writes a poor, awkward hand, having not had time or means to become an expert.' A similar experience is observable in the case of somnambulists, who are totally ignorant, in the waking state, of their somnambulistic experience, but when again in the somnambulistic state recall what happened in the previous crisis. Lost objects have been recovered, and even crimes brought to light by taking advantage of this peculiarity. The same phenomenon is also said to have been observed in cases of intoxication, what is done in one fit of drunkenness being remembered in the next, but forgotten in the sober interval. Instances of double consciousness, however, are not always of the precise type mentioned by Macnish. Thus, in one of the most interesting of recent cases (that of Féliba X., reported by Dr Azam), the woman was conscious during the second state of her whole life-experience, but during the first or original state knew nothing of anything that had happened in the second. The alternations began in this case in 1856, and continued for upwards of thirty years, and it is remarkable that the second state, which at first appeared only in short dream-like periods, has gradually supplanted the first state, which now recurs only at long intervals, and for a few hours. The second state is physically and mentally superior to the first, and the patient herself speaks of the first as état bête. A still more extraordinary case, reported from Paris, is that of Louis V., a young man of epileptic and hysterical temperament and criminal tendencies (born 1863), where the medical record signalises not two, but six states which are mutually exclusive, but which, taken together, embrace his whole past life. These and other cases are commented upon by Ribot in his Diseases of Memory and his Maladies de la Personnalité, and by Mr F. W. Myers in an article on 'Multiplex Personality' (Nineteenth Century, November 1886). The phenomena of double consciousness have also been aptly described as periodic amnesia. They evidently depend upon morbid action of the brain—it has been suggested, upon an abnormal severance and consequent independent action of the two hemispheres—but the physiological conditions are still full of obscurity. An ingenious literary use of the notion of double personality, on completely different planes of morality, is seen in Mr R. L. Stevenson's creation of Dr Jekyll and Mr Hyde.