Canon 1095.1 -- The following are incapable of contracting marriage: those who lack sufficient use of reason.
Those persons are considered to lack sufficient use of reason who are affected by mental illness (irrespective of whether it is congenital or acquired, endogenous or exogenous, or whether it is of general relevance or relevant only in relation to marriage) and lack, at the moment of consent, the effective use of their intellectual and volitional faculties. The expression sufficient use of reason refers to the degree of effective possession of the faculties of comprehension and will which is normally acquired from the age of seven years and which is absolutely necessary for a person to be the subject of human acts and to be morally responsible (Caparros, E.; Thorn, Thériault, J., eds. Code of Canon Law Annotated. Montreal: Wilson and Lafleur Limitée, 1993).
Canon 1095.2 -- The following are incapable of contracting marriage: those who suffer from a grave lack of discretion of judgment concerning the essential matrimonial rights and obligations to be mutually given and accepted.
The object of matrimonial consent -- the very persons as man and woman give themselves to each other as an obligation and as man or as woman accept the other as a right to be claimed -- require on the part of the contracting parties a degree of maturity that is not only greater than simple use of reason, but also than one necessary for many affairs. There is a grave lack when it is proven that a contracting party lacks intellectual and volitional maturity necessary to discern, in view of binding oneself in an irrevocable manner the essential rights and duties of marriage which are the object of mutual surrender and acceptance. The discretion of judgment refers to that degree of maturity of comprehension and of will of the contracting parties which enables them to give and receive each other, through a juridical bond, in a unique community of life and love (Caparros, E.; Thorn, Thériault, J., eds. Code of Canon Law Annotated. Montreal: Wilson and Lafleur Limitée, 1993).
Canon 1095.3 -- The following are incapable of contracting marriage: those who, because of a psychological nature, are unable to assume the essential obligations of marriage.
The legislator acknowledges, as consensual incapacity and cause of nullity of marriage, a complex series of psychic anomalies (among which psychosexual disorders are prominent although these are not the only conditions) which affect the personality structure of the subjects, and may deprive them of sufficient use of reason, or impede directly and clearly their discretion of judgment or discernment of the object of consent, nevertheless produce in them a psychological incapacity to assume, in undertaking the task in a truly committed and responsible manner, the essential obligations of marriage. It is important to remember that these essential obligations must be mutual, permanent, continuous, exclusive, and irrevocable so that there would be incapacity of one of the contracting parties should be, due to psychological cause, incapable of assuming these obligations with these essential characteristics (Caparros, E.; Thorn, Thériault, J., eds. Code of Canon Law Annotated. Montreal: Wilson and Lafleur Limi.
Canon 1101 -- §1. The internal consent of the mind is presumed to conform to the words or signs used in the celebration of a marriage.
§ 2. If, however, either or both of the parties should by a positive act of will exclude marriage itself or any essential element of marriage or any essential property, such a party contracts invalidly.
A marriage comes into being when two juridically capable persons express, in a legally recognized fashion, their consent to be married (c. 1057, 1). By means of this consent, a man and a woman irrevocably give and receive each other (c. 1057, 2) so as to establish a relationship embracing the whole of their lives; a relationship that intrinsically tends towards the good of the spouses and the procreation and education of children (c. 1055, 1). Because marital consent is an internal act not subject to direct inspection, church law presumes that two people who undergo a ceremony of marriage elicit it (c. 1101, 1). This presumption does yield to contrary proof. “If, however, either or both of the parties by a positive act of the will exclude marriage itself, some essential element of marriage, or some essential property of marriage, the party contracts invalidly” (c. 1101, 2). Jurisprudence terms the act of excluding marriage itself total simulation and calls the exclusion of some essential element or property of marriage partial simulation (coram De Lanversin, 18 February 1984, Sacrae Romanae Decisiones, Dec. 76 (1984), 100). Total or partial simulation invalidates the marriage because either form of simulation describes the discrepancy between the words or signs used in the marriage ceremony and the internal attitude.
Canon 1098 -- A person contracts invalidly who enters marriage inveigled by deceit, perpetrated in order to secure consent, concerning some quality of the other party, which of its very nature can seriously disrupt the partnership of conjugal life.
Even though any person who is deceived errs, error and deceit are not to be confused. While in error the subject makes a false judgment concerning the object and is the author of the lack of correspondence between his or her idea and reality, in deceit it is a third person who, through fraud, fabricates a false reality which gives rise in the subject to an apparently "true" perception of an object that is in itself false. Consequently, there is, in deceit, an inappropriate manipulation by a hired person of the formulation of the act of comprehension in the subject who is its victim. This act of comprehension is absolutely necessary to consent which of its very nature must correspond to the self-determination of the contracting parties themselves. The attempt at controlling a person's process of comprehension as a prerequisite of the act of the will, and also deceitfulness incompatible with the dignity of marriage, in our opinion, are more than sufficient reasons for deceit to be grave in itself, and for the legislator to determine it as a cause of nullity.
Canon 1103 -- A marriage is invalid which is entered into by reason of force or of grave fear imposed from without, even if not purposely, from which the person has no escape other than by choosing marriage.
Grave fear proceeding from an external cause and inflicted unjustly to extort matrimonial consent renders marriage null and void, as indicated in Canon 1103, §1. The reasons for this norm of law are given by St. Thomas to be the following: the matrimonial bond is perpetual; wherefore, whatever is in opposition to this perpetuity does away with marriage. Unjustly inflicted fear of such a nature as to overcome a steady and firm character destroys the perpetuity of the contract, as restitutio in integrum can be invoked against this according to the principle that those things done by force and fear must be revoked as null and ought to be devoid of the binding force of validity. Furthermore, marriage signifies the union of Christ and His Church, which has been effected according to the liberty of love. Fear as described above is absolutely and unalterably opposed to such liberty. Therefore, the sacrament cannot proceed from such a source. In view of this, Pope Alexander stated that since consent could not be present where fear and constraint interfered, it is necessary that where one's consent is required the force of constraint be excluded. If such constraint were employed, the marriage would be null.
Canon 1099 -- Provided it does not determine the will, error concerning the unity or the indissolubility or the sacramental dignity of marriage does not vitiate matrimonial consent.
Provost describes error as “the false apprehension of a thing, or false judgment of the mind. Over and above ignorance, error adds a fallacious judgment about the object that is willed. People who are in error know something, and know that they know something; but what they know is not correct, objectively speaking (“Error as a Ground in Nullity Cases,” James H. Provost, CLSA Proceeding, 57 (1995) 306-324, p.308). Thomistic philosophy teaches us that the two faculties of intellect and will harmoniously cooperate to allow a person to function in a truly human way. Before a person makes a decision the intellect provided a practico-practico [sic] judgment which then moves the will to act. Such judgment are formed on the basis of the information provided to the intellect and if this information or knowledge should be erroneous and the will would be directed toward a mistaken end ("Deceit and Induced Error About a Personal Quality," Kenneth E. Boccafola, Monitor Ecclesiasticus, Vol. CXXIV, Series XXXIX (a. 114), Oct.-Dec. 1999, 692-710, p. 695). Error of law, then, is present when the false apprehension or judgment is so pernicious (pervicax) that the person cannot desire or act otherwise than how he or she thinks. Error of law regarding unity or indissolubility or sacramental dignity (c. 1099), is error about something which amounts to a condition sine qua non (c. 126). Other elements of marriage have also been identified as conditions sine qua non, for instance, the procreation and education of offspring, or the good of the spouses (c. 1055 §1).