Note to Hamlet, 3.4.73-76: "Is apoplex'd; for madness would not err, / Nor sense to ecstasy was ne'er so thrall'd / But it reserved some quantity of choice, / To serve in such a difference"

Is apoplex'd; for madness would not err, / Nor sense to ecstasy was ne'er so thrall'd / But it reserved some quantity of choice, / To serve in such a difference: i.e., Is paralyzed; for madness itself could not go so far astray, nor were the senses ever so enslaved by lunacy that they did not retain the power to make the right choice, when the differences (i.e., between Hamlet's father and King Claudius) are so obvious.