918. A contract to be valid must have a subject and a lawful cause or consideration. The parties to it must be legally capable and their consent legally given.
919. All persons are capable of contracting, except those whose incapacity is declared by law.
920.
(1) Those legally incapable of contracting are:—
(a) minors except in the special cases described in this Code;
(b) persons of unsound mind to whom curators have been appointed;
(c) those who, by special provisions of law, are prohibited from contracting by reason of their relation to each other or of the object of the contract.
(2) The contractual capacity of persons of unsound mind other than those to whom curators have been appointed, of persons of weak understanding or temporarily deranged whether from disease, accident, drunkenness or other cause is determined according to the law of England. (Substituted by Act 34 of 1956)
921. The incapacity of a minor and of an interdicted person may, when the contract is not by this Code declared to be void, be pleaded by or for him or her but not against him or her.
922. Consent is either express or implied. It is invalidated by the causes specified in the Second Section of this Chapter.
923. A contract without a consideration, or with an unlawful consideration is void; but may be valid though the consideration be not expressed or be incorrectly expressed in the writing which is evidence of the contract.
924. The consideration is unlawful when it is prohibited by law, or is contrary to good morals or public order.