Sec.2.  Contracts are also distinguished as specialties and simple contracts.  A specialty is a contract under seal; as a deed, or a bond.  But we shall here consider chiefly that common class of contracts called simple contracts, or contracts by parol. Parol signifies by word of mouth.  Applied to contracts, however, it not only means verbal contracts, but includes written contracts not under seal.  Both are simple contracts; the distinction between them is in the mode of proof.  The mutual understanding of the parties to a verbal contract may be proved by parol evidence.  But as the real intention of parties is more likely to be expressed in a written contract, the rule of law is, that parol evidence may not be admitted to contradict or vary the terms of a written instrument.  It may however be admitted to explain what is doubtful, or to supply some deficiency.

Sec.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.

Sec.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.

Sec.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.