One of the learned editors of Judge Story's work on Equity Jurisprudence, argues that "the idea of there existing in this class of cases (e. g., cases of mistake in the application of the law to a particular group of facts) a mistake of fact, as well as of law, might perhaps apply to all cases of mistake of law. . . . The mistake of one's title, when that depends upon a pure question of law, is a mistake of law, and nothing else." But what litigated case is there as to which we can say in advance that it depends upon a pure question of law ? After the facts are settled, and the testimony in the case closed, this may be said in cases where the facts are not proved in ambiguous terms ; but before the settling of the facts and the closing of the case there are always contingencies possible that may take a case out of one category of law and place it in another. Even in the case already cited, where a supposed grandson compromised a litigation with an uncle, on the supposition that the uncle, a younger brother of the grandson's father, was entitled to take as heir-at-law of a third brother deceased, the question was not a pure question of law. Who could tell, especially under marriage laws so complicated as those of England, that there might not be charged against the particular marriage under which the plaintiff claimed, some flaw that might raise a question of fact? "Who can tell Lansdown v. Lansdown, so far as concerns the reason given by the court, is disapproved by Lord Cottenham, in Stewart v. Stewart, 6 Cl. & F. 966. In Crosier v. Acer, 7 Paige, 143, Chancellor Walworth, in language repeated with apparent approval by Judge Story (Eq. Jur. 12th ed. sec 126, note), says, after examining the cases: "If this court can relieve against a mistake in law, in any case where the defendant has been guilty of no fraud or unfair practices, which is at least very doubtful, it must be in a case in which the defendant has in reality got nothing whatever by the mistake, and where the parties can be restored substantially to the same situation in which they were at the time the mistake happened." "This," is the comment of Judge Story, "indicates a reluctance to declare that all cases of injustice, produced by the mere mistake of law, are remediless in a court of equity." And Judge Story afterwards says (Eq. Jur. 12th ed. sec 130) : " There may be a solid ground for a distinction between cases where a party acts or agrees in ignorance of any title in him, or upon the supposition of a clear title in another, and cases where there is a doubt or controversy or litigation between parties as to their respective rights. In the former cases (as has been already suggested) the party seems to labor in some sort under a mistake of fact as well as of law." whether there might not be a conveyance from the plaintiff which, by its own force, might raise at least a shadow of a title in some other person?1 Who can say in reference to any particular litigated case, no matter how clearly it may appear to fall under some established principle, that some extraordinary casualty may not occur which will bring the case out of the range of such principle? And if so, a mistake as to whether a particular case falls within a particular rule is a mistake, which, if common to the parties, will justify the intervention of a court of equity decreeing rectification.2 Mr. Pollock declares it to be "the true rule, affirmed for the Roman law by Savigny, and in a slightly different form for English law by Lord Westbury,"3 "that ignorance of law, means only ignorance of a general rule of law, not ignorance of a right depending on questions of mixed law and fact, or on the true construction of a particular instrument."4 Mr. Pollock further says :5 "A. and B. make an agreement, and instruct C. to put it into legal form. C. does this so as not to express the real intention, either by misapprehension of the instructions or by ignorance of law. It is obvious that relief should be given in either case. In neither is there any reason for holding the parties to a contract they did not really make." But in place of terms the parties selected as the expression of their views, other terms giving a different sense cannot.be substituted. In other words, it may be shown that the document is not one the parties intended to execute, and the meaning of ambiguous terms may be cleared ; but unambiguous terms cannot be stricken out and others substituted by parol.1-According to Mr. Leake,2 "where the parties are under a common mistake of law as to the application of their contract, it can be applied only according to their intention, and not otherwise."3 Hence we may hold that a promise to pay, based on a mistaken belief that certain facts fall under an acknowledged legal rule, such mistake going to the essence of the contract, is not binding.4-As a rule, it may be asserted that ignorance of a right which is a mixed question of law and fact, is to be treated as ignorance of a fact.5-The Prussian code follows the distinctions in the text. It declares that ignorance of a published law shall be no defence; but it leaves the question open as to the subsumption of particular facts under a law ignorance of which is not set up.1 It declares that error in corpore prevents that consent which is necessary to a contract;2 but it holds that this does not apply to other cases of error, e. g., errors of motive.-The French code, while apparently holding that all error avoids contracts,3 virtually limits this to error in corpore.4 Error in motive, which goes to the worth and usefulness of an article bought or hired, falls under the general head of lesion, and is not, to a person capax negotii, a defence, though when induced by fraud is ground for rescinding or for damages. Error of law is in all cases put on the same footing as error of fact.-In conclusion, we must remember that if there can be no relief for mistakes of fact involving error of law, there can be no mistake of fact for which relief can be granted, since there is no mistake of fact in which some mistake of law is not involved. A mistake as to identity of a person, for instance, involves a mistake of law as to his legal relations; a mistake as to the substance of a thing would be of no moment did it not involve a mistake as to the thing's legal incidents.5 The term "law," in the rule that mistake of law is no excuse, is to be restricted to juridical law as a rule of action, and is not to be extended to law as a compound of law and fact. There are therefore two extremes, in this vexed issue, to be avoided. On the one side, when we say that mistake of law does not give ground for relief, we must restrict ourselves to such mistake of law as does not involve a mistake of fact. On the other side, when we say that mistake of fact gives ground for relief, we must remember that such mistake must go to some past or existing thing, and not relate to mere opinion of the law. When it does go to a past or existing thing, it does not cease to be ground for relief because it involves a mistake of law.