DELUSION. In Medical Jurisprudence. A perversion of the judgment, obviously er roneous and persistent. A symptom of men tal disease, in which persons believe things to exist which exist only, or in the degree they are conceived of only, in their own imaginations, with a persuasion so fixed and firm that neither evidence nor argument can convince them to the contrary. A faulty be lief concerning a subject capable of physical demonstration, out of which the person cannot be reasoned by adequate means for the time being. 1 Wood, American Text Book of Med. See 114.1.IICINATION.
The individual is, of course, insane. For example, should a parent unjustly persist, without the least ground, in attributing to his daughter a coarse vice, and use her with uniform unkindness, there not being the slightest pretence or color of reason for the supposition, a just inference of insanity or delusion would arise in the minds of a jury ; because a supposition long entertained and persisted in, after argument to the con trary, and against the natural affections of a parent, suggest that he must labor under some morbid mental delusion; Whart. Cr. L. § 37; Whart. & S. Med. Jur.; 1 Redf. Wills; Ray, Med. Jur. § 20 ; Shelf. Lun. 296; 3 Add. Encl. 70, 90, 180; 1 Hagg. Eccl. 27. See Guiteau's Case, 10 Fed. 170; Mann, Med. Jur. of Insan. 58.
Where one "labors under a partial delu sion only, and is not in other respects in sane, we think he must be considered in the same situation as to responsibility as if the facts with respect to which the delusion ex ists were real. For 'example, if under the influence of his delusion he supposes another man to be in the act of attempting to take away his life, and be kills that man, as he supposes, in self-defence, he would be ex empt from punishment." This is the rule
as stated by the English judges, cited in 1 Whart. Cr. L. § 37. Shaw, C. J., in Com. v. Rogers, 7 Mete. (Mass.) 500, 41 Am. Dec. 458, says: "Monomania may operate as an excuse for a criminal act," when "the delu sion is such that the person under its in fluence has a real and firm belief of sota fact, not true in itself, but which, if it were true, would excuse his act ; as where the belief is that the party killed had an im mediate design upon his life, and under that belief the insane man kills in supposed self defence. A common instance is where he fully believes that the act he is doing is done by the immediate command of God, and he acts under the delusive but sincere belief that what he is doing is by the command of a superior power, which supersedes all hu man laws and the laws of nature." Where a testator was laboring under a delusion that his brother was exercising his muscle preparatory to killing him, that of itself would not justify a rejection of his will on the ground of unsound mind; In re Fricke, 64 Hun 639, 19 N. Y. Supp. 315. A person persistently believing supposed facts which have no real existence, against all evidence and probability, and conducting himself on the assumption of their exist ence, is, so far as such facts are concerned, under an insane delusion ; Haines v. Hayden, 95 Mich. 332, 54 N. W. 911, 35 Am. St. Rep. 566.
See PARANOIA.