3. The obligation in justice may be on both sides (bilateral) or only on one side (unilateral), but consent must be on both sides.
The elements of a contract are made up of essentials and accidentals. (a) The essentials include the subject-matter, the parties contractant, their agreement, and the external form that manifests the agreement. (b) The accidentals include bonds, oaths, conditions and modes.
1878. The subject-matter of a contract—that is, the thing or action or forbearance with which the agreement is concerned—must have the following qualities:
(a) it must be something possible, for one may not undertake what one cannot perform. Thus, one cannot bind oneself by an accessory contract (such as suretyship), if the principal contract itself is _ipso facto_ invalid. But if the impossibility is only moral (i.e., great difficulty), one who knowingly undertakes the arduous is bound to fulfill his promise; if it is only partial, one is held to the part that is possible; if it is culpable, one is bound to repair damage caused the other party through non-fulfillment;
(b) it must be something disposable, for one may not transfer that over which one has no right of control or transfer. Thus, one may not contract to sell public property that is _extra commercium_, or property of which one has only the possession, or goods over which others have a claim (e.g., a debtor may not bestow gifts to the detriment of creditors’ rights), or goods not transferable for pay (e.g., payment for a favorable decision by a judge, or property owed to a third party) or for money (e.g., academic degrees, public offices, Sacraments, indulgences);
(c) it must be something existent and determinable, for no one wishes to contract for a right that is valueless and illusory. Thus, one may not sell shares of a stock company that has no assets, or an indefinite house or lot or chattel;
(d) it must be something good and lawful, for one may not bind oneself to iniquity. If it is sinful (e.g., a contract to sell a house in order to spite a third party), the agreement is _per se_ valid. But if the substance is evil (e.g., a contract for fornication made with a prostitute), the agreement is null before the performance of the promised sin; but it seems to many that after performance of the sin the promisor is obliged to pay the money promised, unless the law makes the contract void (see 1886 c). If the law merely denies protection to a sinful engagement, or forbids it under penalty, it would seem that after performance of the sin one may follow, as far as strict justice is concerned, the rule that right is with the possessor. In the United States immoral and illegal contracts and those that are opposed to public policy are generally regarded as null, but in some cases the law declares immoral conditions _de futuro_ non-existent and considers the agreement to which they are added as valid (e.g., wills and gifts _inter vivos_ in some codes).
1879. Sinful Contracts.—There is no form of contract that may not be made sinful as to its substance on account of the wicked offer or consideration (e.g., sale may deal with immoral objects, labor may be given to criminal projects), but there are certain forms of contract that are particularly open to abuse and hence are frequently associated with evil circumstances or results. Some contracts are often illicit according to natural law.
(a) Thus, a gift is sinful, on the part of the donor, when it is made by an employer for the purpose of seducing a servant, and on the part of the servant, when it is accepted for the purpose of encouraging the unlawful attentions of the employer; but if the gift is unconditional, there is no obligation in justice to return it.
(b) Borrowing is sinful, when the lender is in greater need, or when one becomes unduly obligated to the lender; lending is sinful when the lender cannot afford to part with the thing loaned, or when the borrower is encouraged in thriftlessness, or when he will make evil use of the thing borrowed, etc.