§3. To make a valid contract, _the parties must be capable of contracting_. They must be of sound mind. Hence idiots and lunatics are generally incompetent to make contracts. Contracts by lunatics and idiots are not necessarily void, but only _voidable_; the validity or invalidity depending upon facts to be proved. To avoid a contract on the ground of mental imbecility, it must be proved that the party contracting was at the time incompetent. But if a general derangement is once established or conceded, the person is presumed to be incompetent; and the party seeking to enforce the contract must prove the other to have been sane. The general rule in the case of idiots is, that if the party is incapable of acting in the ordinary affairs of life, or in the particular contract, his idiocy will annul the contract. §4. Drunkards also are incompetent to contract while in a state of intoxication, provided the drunkenness is such as to deprive them of reason for a time, and create impotence of mind. But for absolute necessaries, if the drunkard consumes them during his drunkenness, or keeps them after becoming sober, he is liable. Intoxication only renders a contract voidable, not void, as the party intoxicated may adopt it on recovering his understanding. §5. Another requisition to a valid contract, is the _mutual assent of the parties_. A mere offer by one party not assented to or accepted by the other, constitutes no contract. Assent must also be given freely. A contract entered into under duress, or compulsion, is not binding; as where assent is extorted by threats of personal injury. Assent must also be given with a knowledge of facts. A contract made under an injurious mistake, or ignorance of a material fact, may be avoided, even though the fact is not fraudulently concealed. But a mistake made through ignorance of the law, will not render a contract void. §6. A _valuable consideration_ also is necessary to a valid contract. A _consideration_ is what is given or done, or to be given or done, as the cause or reason for which a person enters into an agreement. Thus, the money given or offered, for which a man agrees to perform certain labor, is the consideration of the agreement. So the money or other thing for which a promissory note is given, is the consideration. A valuable consideration is any thing that is either a benefit to the party promising, or some trouble or injury to the party to whom the promise is made.