The scant use in jurisprudence of canon 1095, 1° leads to the questioning of its effective normative content. After an examination of the occurrences in the current Code of the term usus rationis and of the sources indicated for canon 1095, 1°, some summary indications of a historical nature are given. The analysis continues with a look at some annotated editions of the Code as well as noted reference works, but it concentrates above all on the presentations of the subject offered in various textbooks, ordered according to the three prevalent lines of interpretation that emerge from them. The analysis of rotal jurisprudence after the promulgation of the 1983 CIC confirms how miniscule in number are the decisions wherein can. 1095, 1° is not only referred to obiter, in the motivations in iure, but above all applied to concrete cases in the motivations in facto. Some relevant considerations on the relationship between can. 1095, 1° and the processus brevior as well as some conclusions conclude this re. By the latter, indications are given as to the content of the lack of sufficient use of reason in relation to matrimonial consent and as to the definition of an adequate proof of the same.