2. One is not obliged to perform illicit acts: "for," says Puffendorf, "it would be a contradiction, to be held by civil or moral law, to perform things which the civil or moral law interdicts. It is already doing wrong to promise illicit things, and it is doing wrong twice to perform them."[35] 3. One cannot promise what belongs to another: for I cannot promise what I cannot dispose of. =50. Contracts.=--A _contract_ is an agreement by which one or several persons engage to do or not to do a certain thing for one or several others. (Code Civ., Art. 1101.) _Conditions of the contract_ (Art. 1108).--Four conditions are necessary to constitute a valid and legitimate agreement: 1. The _consent_ of the parties. 2. The _capacity_ of the contractors. 3. A sure _object_ as a basis for the contract. 4. A licit _cause_ in the obligation. (1.) The _consent_.--The consent is the voluntary acceptance of the charges implied in the contract. It is _express_ or _implied_: express, when it is made manifest by words, writing, or any other kind of expressive signs. It is implied, when, without being expressed by outward signs, it may be deduced, as a manifest consequence of the very nature of the thing, and other circumstances. All consent presupposes, 1, _the use of reason_: the insane cannot contract any obligation; children neither;[36] 2, _necessary knowledge_. Therefore all real consent excludes error, at least "when it falls on the very substance of the thing which is its object."[37] It is, besides, for the jurists to define with precision what is to be understood by error in matter of contract; 3, the liberty of the contracting parties: whence it follows that consent extorted by constraint and violence is not valid. (2.) The _capacity to make a contract_ is deduced from the foregoing principles. All those who are not supposed to be able to give an intelligent and free consent, are incapable and cannot make contracts: for instance, persons under age, persons interdicted, insane or idiots, etc. (3.) The _matter of a contract_.--"All contract has for its object something that a certain party engages to give, or do or not do." It is evident that a contract without subject-matter and bearing on nothing, is void, and does not exist. (4.) The _cause_ of the contract must be real and legal. Contracts are subject here to the same rules as are promises. The preceding distinctions are all borrowed from the civil law; but they express no less principles of justice and equity which may be resolved into the following rules: 1. No one should take by surprise or extort a consent through artifice or violence. 2. No one should make a contract with one whom he knows to be incapable of understanding the value of the engagement he is called upon to make: for example, with one under age, incapable before the law, but of whom it is known that the parents will pay the debts; or with one feeble-minded, though not yet an interdicted person, etc.