Sherman agreed to sell Mansfield a certain lot in Maine, which Sherman believed did not contain a valuable building site. By mistake, Sherman named a lot, which had a building site. Mansfield knew the price was very low for lots containing a building site. When the mistake was discovered, Sherman refused to convey the lots as per the contract. Mansfield (P) sued in specific performance. D claims that he made a mistake as to a material fact and not merely an error in judgment.