Mutual agreement upon sufficient consideration concerning the transfer of a right. A contract is an agreement because there must be consent of at least two wills to the same object. It is mutual because the consent on one side must be given in view of the consent on the other side; it cannot be a mere coincidence that two people happen to want the same thing. The contracting parties transfer a right and so bind themselves in commutative justice, as person to person. By natural law a tangible consideration or compensation is not necessary in all contracts, because there can be gratuitous contracts such as a gift or promise. Even here, however, some intangible consideration in the form of affection or gratitude is normally expected. Finally the obligation in justice may be on both sides or only on one side, so that contracts can be bilateral (as in marriage) or unilateral (as in the promise of a donation). In either case the consent must always be on both sides.