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.
3. No one should contract a fictitious engagement bearing on matters non-existing, or such as have only an imaginary or illegal cause.
Interpretation of contracts.—Jurists give the following rules regarding the interpretation of obscure clauses in contracts. The rules which are to guide the judge in regard to the law are the same as those which are to enlighten the consciences of the interested parties:
“1. One should, in agreements, find out the mutual intention of the contracting parties, rather than stop at the literal sense of the words.” (Art. 1156.)