It is a fundamental principle of law that no man shall avail himself, either to establish or resist a claim, of his mistake or ignorance of law. So also in criminal law it is an ancient maxim: Ignorantia leg is nemi-nem excusat. The reason sometimes assigned, that the law supposes every one to be acquainted with it, is.nothing more than a repetition of the rule in other words. The true reason is, the extreme danger of permitting any person to shelter himself under his ignorance of the law, or to found a right upon it. For this would be, in the words of the kims bench in England, "to hold out a premium for ignorance," and ignorance of that which it is of the utmost importance that all men should know. Hence the law distinguishes most carefully between a mistake of law and a mistake of fact; for the latter, as a general rule, is rectified, and all mischievous consequences prevented, as far as possible; and a mistake as to the law of a foreign state or country is regarded only as a mistake of fact, because no one is under any obligation to become acquainted with a foreignlaw. - To this general rule there are some important qualifications; the principal one being, that no mere acknowledgment, or waiver of defence or right, made under a mistake of law, is binding.

Thus, if one has a good legal defence against a promissory note, but, through ignorance or mistake of the law, supposes himself bound to pay it, and on this supposition gives a promise to pay it, the promise will not, in general, be binding upon him. In many cases also much relief is to be obtained by the construction of a contract; but this is always governed and limited by certain definite rules. It is often said by ethical writers, that a party to a contract is bound to execute the contract in the sense which he knew the other party to put upon it. This may be true always in a moral sense; but it certainly is not true in a legal sense, although courts have sometimes seemed to think it was a good rule of law. The true rule and the reason of it are easily seen. If A contracts with B in writing to sell him 100 mules, and receives the money, and B at the time, being a foreigner perhaps or for some other reason, understood that he was buying horses, all which A knew, nevertheless B could not claim horses under the contract, He could, by proving his mistake and A's knowledge of it, make out a case of fraud, and this would annul the contract, and then he could recover his money.

But the reason why he could do no more is, that the law will not, under pretence of construing a contract, make a new contract for the parties. Hence, it is another way of expressing the same rule, that the actual intention of the parties to a contract shall be carried into effect, so far as it is possible to arrive at that intention by a rational construction of the words they have actually used, but no further; for it is one of the most reasonable, safe, and well established rules, that no evidence from without a written contract shall be permitted to control or vary it. While parties are negotiating they may change their minds and vary their demands and concessions, and generally do this to some extent. But when they have finally put their terms in writing, the law supposes that these are what they have concluded upon, and that they have chosen and used the very words which express their meaning; and that whatever is not therein stated, although it may have previously passed between them, has been purposely omitted because it was not finally agreed to.

It would therefore be manifestly unjust to permit evidence of any of these things to come forward and vary the written contract; and hence the rule, which is concisely expressed in the Scotch law thus: "Writing cannot be cut down or taken away by the testimony of witnesses." But while evidence must not vary, it may explain, the contract. Thus, in the most solemn deed, it may be necessary to explain the terms of the instrument, in order to show who the parties are, what the boundaries of land mean, or where it is situated. But it is a very different thing when one of the parties says that the deed contains a mistake; that the house or the field it conveys is not the house or field which it was intended to convey and on this ground demands to hold the house or field which, as he alleges, should have been given to him. And it may be regarded as the established rule concerning mistakes, that any mistake in an instrument may be corrected by construction, if the instrument itself affords the means of correction; but not, if it can be done only by going outside of the instrument.

Courts of equity, however, have large powers to reform conveyances and contracts where by mistake in drafting them they are made to express a different intent from the one agreed upon. (See Chancery, and Equity).