La folie, dans les archives de la justice royale aux XIVe et XVe siècles, s’accompagne de deux effets de droit : l’incapacité dans les affaires civiles et l’irresponsabilité dans la sphère pénale. La démence (furor) est définie, de manière sommaire, comme une maladie relevant des lois de la nature, qui prive la personne de sa capacité à posséder une intention valable. Sur la base de ce canevas juridique, les descriptions de comportements fous sont assez diverses, car elles s’adaptent aux termes de chaque litige. L’argument de folie sert, en particulier, à excuser un crime, à faire annuler un contrat ou un testament, ou encore à empêcher un proche parent de dissiper les biens du lignage, en obtenant son interdiction et/ou sa mise en curatelle. Le pouvoir qui s’exerce sur l’individu déclaré fou est d’abord celui de la parentèle, qui l’empêche d’accéder au statut normal de l’adulte en raison de son désordre mental et qui, s’il est dangereux, le garde lié à domicile. Des usages coutumiers règlementent ces situations, mais le recours aux sentences des tribunaux royaux et aux règles du droit savant se développe au cours de la période. Le roi ne légifère pas sur ces affaires familiales, laissant certains acteurs intermédiaires, notamment urbains, réclamer la garde de ces sujets vulnérables. Ses gens de justice veillent néanmoins à rendre incontournable le recours à sa juridiction souveraine. / In the archives of the royal justice system of the 14th and 15th centuries, madness was distinguished by two distinct judicial attributes: full incapacity in civil proceedings and the exception from penal responsibility in judicial matters. Dementia (furor) was summarily defined as an illness, stemming from the laws of nature, which deprived the subject of his ability to express any valid intent. Within this legal framework, whether or not conduct was deemed mad depended in large part on the specific circumstances of each law suit. The insanity plea could be used, for example, to acquit a crime, to nullify a contract or a testament as well as to prevent a relative from squandering the possessions of the family line by either having him barred and/or placed under guardianship. Those who were regarded as insane found themselves placed, primarily, under the authority of their relatives who thus deprived them of the ordinary privileges associated to adulthood and, should they prove dangerous, kept them at home. If customary law was generally used to arbitrate these situations, more and more appeals to the royal courts and to the opinions of legal scholars were made during this period. Even if the king did not pass judgment on such family matters, he did deputize certain mid-level actors, such as the burghers, to take these vulnerable subjects in their custody. In turn, these lawmen remained particularly attentive to appeal systematically to his sovereign authority.