The truth of this doctrine cannot be contested. However, it is necessary to understand this exception in the strictest sense, and not to seek in the pretended interest of the person one obliges, a pretext to change one's mind. For example, if you have promised any one a post which he accepts and desires, you cannot be allowed to relieve yourself of it, by supposing that the post will in reality be a disadvantage to him, and that you will give him a better one another time.
Some other exceptions are pointed out by the moralists and jurists; for example:
1. Necessity relieves of all promise. If, for example, I have promised to go to a meeting and am kept in bed by a serious illness, it is impossible for me to go, and hence I am relieved of my promise.
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:
3. A sure _object_ as a basis for the contract.
(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.