"There is no handy rule of thumb", a Judge observed in a Sentence that came before me recently, "for determining which type of lack of discretion or of incapacity invalidates marital consent". He was of course referring to c. 1095, but I felt he seemed unduly at sea about its interpretation. Canon 1095 itself gives a plain rule: such disabilities can invalidate only if they relate to the essential rights/obligations of marriage. In fact, according to n. 2 of the canon, contractual or consensual incapacity is to be imputed to those "who suffer from grave lack of discretion of judgment concerning essential matrimonial rights and duties (officia)"; and, according to n.
Incapacity for giving valid matrimonial consent, deriving from some notable defect of the person's psychic faculties, is dealt with in c. 1095 of the 1983 Code: "They are incapable of contracting marriage: 1º who lack sufficient use of reason; 2º who suffer from a grave lack of discretion of judgment concerning the essential matrimonial rights and duties which are to be mutually given and accepted; 3º who are not capable of assuming the essential obligations of matrimony due to causes of a psychic nature".
If one considers marriage as one of the most natural of institutions, then consensual incapacity, for an adult endowed with sufficient use of reason, is a most unnatural phenomenon. An inability to appreciate the essential obligations of marriage, just as much as an inability to fulfil them, is an extraordinary disablement ("incapacitas quid extraordinarium haberi debet" Pompedda, M.: "Incapacitas Adsumendi Obligationes Matrimonii Essentiales" (Periodica LXXV (1986) 138-140), which can only occur in the case of a gravely handicapped person who, as a result, is deprived of the natural right to marry (c.
The obvious answer to the question "what is the object of matrimonial consent?", is, "marriage itself". Just as obviously, however, this is not very enlightening from the juridic viewpoint which is interested in pinning down the specific, and above all the essential, rights and obligations that consent gives rise to.