Prior to the present century, the subject of mutuality in specific performance was an unsolved puzzle. The decisions of the courts were in the main wise, but the attempts both of courts and text writers to formulate a statement of the principle governing these decisions were far from happy. After two centuries of litigation and discussion, the current formula was that the remedy must be mutual, must be equally available to both parties. The worst fault of this formula was its plausibility. Equality is equity, and perfect equality between the parties to a contract is not attained unless remedies are equally available to both. The plausibility of the doctrine served to keep it alive, even to this day.
Durfee, Edgar N. "Mutuality in Specific Performance." Mich. L. Rev. 20 (1922): 289-314.