(a) involuntary commutations, which are those in which reparation is made for the use against the will of another of the things, persons, or works that pertain to him. Thus, the property of another is used unlawfully by secret theft and by open robbery; the person of another is injured by murder and wounds; the honor of another by secret calumny and detraction, by open false testimony and contumely; the rights of another to persons are used unlawfully by adultery with his wife, by seduction of his servant, and the like;
(b) voluntary commutations, which are those in which compensation is made for a benefit that one derived with the owner’s consent from something that was his, or in which one gives or returns to another what is his. They include the various forms of contracts, or agreements between two parties in which the consent of both to the same proposal is externally manifested and obligation is produced to abide by the terms of agreement.
1749. Forms of Contract.—The chief forms of contract are the following:
(a) gratuitous contracts, which are those that confer advantage on only one of the contractants, or those in which no payment or compensation for his acts or goods is made to one party by the other party. They include unilateral contracts, which produce obligation on one side only (e.g., a promise, gift, testament), and bilateral contracts, which produce obligation on both sides. The bilateral contracts are also known as bailments, or understandings whereby a thing or business is transferred from one person to another in trust, on condition that a return will be made to the owner, They include the following contracts: loans, in which return must be made of the identical things borrowed (_commodatum_), or of a thing similar in kind (_mutuum_); deposit, in which a thing must be returned after safekeeping (_depositum_); an agency, in which one conducts the business of another with the obligation of making returns, either from express contract (_mandatum_) or from imputed agreement (_negotiorum gestio_). In _commodatum_ and _mutuum_ the advantage is had by the bailee, in the other three by the bailor;
(b) onerous contracts of certain event, which are those that confer an advantage on both parties, and in which the thing agreed on is certain and definite. They include contracts in which one party transfers ownership to the other (e.g., buying and selling, barter, loan at interest, contracts for annuities, stocks and bonds) or useful dominion (e.g., lease of property, contractor’s agreement, hire of labor), and contracts in which both parties transfer rights to a moral person of which they are the members (partnership);
(c) onerous contracts of uncertain event, which are those that confer advantage on both parties, but in which the thing agreed on is contingent and uncertain. Examples are insurance, wager, gaming contracts, lottery, and stock market speculation;
(d) subsidiary contracts, which are those that are made in order to give security to principal contracts to which they are annexed or for whose sake they are made. Such are guaranty and surety, pledge and pawn, and mortgage.
1750. The Equality Sought by Commutative Justice.—The equality in quantity sought by commutative justice means that in involuntary transactions the offender must suffer a punishment equal to the injury he offered or must pay a recompense equal to the damage he caused, and that in voluntary transactions one must give the equal of what one receives. But this can be understood in two ways.
(a) Thus, equality may be taken for identity in species, in the sense that the same kind of thing must be taken or returned (e.g., a life for a life, an eye for an eye, a tooth for a tooth). This kind of equality will do in some instances, as in cases of exchange of goods, but as a rule it would not be fair to both parties. Thus, if a subject strikes a ruler, he is not sufficiently punished if he receives the same kind of blow, for the injury to the ruler is greater on account of his office; when a man steals a cow or a sheep, he is not sufficiently punished if he restores what he took, for he would suffer no loss and the community whose peace he had offended would go without satisfaction (Exod., xxii. 1); if one gives one’s cow for another’s cow, or if a shoemaker trades his products for the clothes made by a tailor, the exchange may be unfair, since the thing given on one side may be better than that given on the other side.
(b) Equality may be understood as identity in value, in the sense that the thing taken or returned has the same quantity of goodness or excellence as the thing received, no matter how they differ in species. This kind of equality must be observed as a rule both in involuntary and voluntary transactions. Thus, for injury done to merchandise payment is made in money, or vice versa. If equality in value is not possible, because the good for which one owes is on a higher plane than the good which one is able to give, it seems that justice requires one to approximate equality as far as possible, and hence mayhem or defamation should be compensated for by the goods of fortune (see 1802 and 2090).