One of the categories in the Code of Canon Law which describes natural (physical) person in the area of their authorities and obligations is the capacity of use of the reason. This article is attempt of analyze three canons contained in general norms of the Code mentioned above. The first of them determines how the lack of use of the reason affects on being submitted to law merely ecclesiastical (which means to this legislation, which is not a codification of the natural or the divine law). The second of them is legal presumption of having capacity to use the reason since a person is over 7 years old. The last one defines the canonical position of those who because of some kind of mental disorder habitually lack the use of reason. The subject of this article is not merely theoretical and speculative, but very practical, because it exerts influence on realization of rights and duties of persons in the ecclesiastical field.