Mоnty Miller wаs lаnd-rich by inheritаnce but mоney-pоor, having suffered severe losses on bad investments, but still owned several thousand acres of unencumbered timberland. He had a large family, and his normal, fixed personal expenses were high. Pressed for cash, he advertised a proposed sale of standing timber on a 2,000-acre tract. The only response was an offer by Tommy Norris, the owner of a large, integrated construction enterprise, after inspection of the advertised tract. Tommy Norris offered to buy, sever, and remove the standing timber from the advertised tract at a cash price 70% lower than the regionally prevailing price for comparable timber rights. Monty Miller, by then in desperate financial straits and knowing little about timber values, signed and delivered to Tommy Norris a letter accepting the offer. If, before Tommy Norris commences performance, Monty Miller’s investment fortunes suddenly improve and he wishes to get out of the timber deal with Tommy Norris, which of the following legal concepts affords his best prospect of effective cancellation?