What is the theoretical basis for giving significance to mental reservations in marriage consent? Mental reservation is a kind of doubletalk where a party says one thing but actually means something else. For example, a spouse does not intend to be bound exclusively to the other spouse and instead is going to have a so-called open relationship. However, this condition is not pronounced during the wedding and externally the party promises exclusivity. How can a thought entertained solely in the mind render a promise invalid?
Usually promises are binding according to the external declaration, not according to some internal thought that is not manifested during the declaration. The Law of Obligations: Roman Foundations of the Civilian Tradition (Reinhard Zimmermann, 1996) states that mental reservation (reservatio mentalis) was a theoretical challenge for the lawyers in order to forumulate a consistent model for contracts but the practical conclusion was never in dispute: reservatio mentalis must be irrelevant. Otherwise, secret thoughts could undermine the trustworthiness of social interaction.
That book talks disparagingly about the canon law of the Catholic Church, in that it allows mental reservation as a ground for annulment. A quotation is cited inquiring whether anything could be more absurd than that. The defining case was, according to the book, a decision by Pope Innocent III, who allowed a marriage to be dissolved because a man had consented to marriage in order to seduce a woman, without intention of living in marriage.
I agree that the errors of the public rite are unproblematic as a ground for annulment, compared to mental reservations. For example, if the spouses were not present at the same time during the wedding, or the proper questions were not asked by a competent minister, it is easy to admit that the marriage can be declared invalid. I think it would be theoretically satisfying if mental reservations did not affect the validity of the marriage. Subsequent acting according to the mental reservations would then be living in violation of the vow, which would in itself stay valid.
I think would be mostly the same solution that is adopted with regards to the baptism, for example. Not everyone has consented to the baptism with right motives. They may have intended to continue living in sin regardless of the baptism. Nevertheless, the validity of baptism is rarely if ever challenged on these grounds. Living in sin is a violation of the baptismal vow, but the baptism itself is valid.
When I have talked about this issue in the past, the answers had mostly dealth with the consequences of the decision: it would be unfair to the other spouses, if they could not remarry after a deceitful promise. Thus it is reasonable to grant annulments because of mental reservations. I have not seen any actual statistics, but I think mental reservations are one of the most common grounds for annulments. If annulments were granted only because of the errors of the public rite, the numbers would plummet significantly. But I don’t think this is the real justification for the current rule.
What should Catholics answer if someone presents them the view of Zimmermann’s book and asks for the explanation of this practice?