Mistakes of fact may consist either of mistakes by the parties to the contract as to some matter which goes to the essence of the contract, or it may be the mistake of a third person who reduces the contract to writing.

A mistake of the parties to the contract, to be relievable, must be mutual. An exception is found in cases where there is mistake on one side and fraud on the other. This exception, perhaps, would rather come under the head of fraud.

Story's Equity Jurisprudence.

2 Eaton on Equity Jurisprudence.

The necessity for the mutuality of the mistake was asserted by the Court in the case of Dinian vs. The Providence W. & B. R. R. Co.,3 as follows:

"A court of equity has no power to alter or reform an agreement made between parties, since this would be in truth a power to contract for them; but merely to correct the writing executed as evidence of the agreement, so as to make it express what the parties actually agreed to. It follows that the mistake which it may correct in such a writing must be, as it is usually expressed, the mistake of both parties to it; that is, such a mistake in the draughting of the writing as makes it convey the intent or meaning of neither party to the contract. If the court were to reform the writing to make it accord with the intent of one party only to the agreement, who averred and proved that he signed it, as it was written, by mistake, when it exactly expressed the agreement as understood by the other party, the writing, when so altered, would be just as far from expressing the agreement of the parties as it was before; and the court would have been engaged in the singular office, for a court of equity, of doing right to one party at the expense of a precisely equal wrong to the other."