There are two possible accounts of the difference between the legislative and judicial powers granted by the Constitution and each has surprising implications. According to one, the difference is purely between two different government functions, making legal rules and applying them. If that is correct, then the legislative power can accomplish any legal result the judicial power can, but not vice versa (putting aside constitutional limits on the legislative power that do not result from its separation from judicial power). According to the other, the two powers differ because only the judicial power may operate on certain legal interests. If that is correct, the structural difference between the two powers depends on differences among the legal rules being made or applied, not the functions of government institutions. That understanding of the distinction underlay 19th century vested rights doctrine and underlies the Supreme Court’s current doctrine that limits Congress’ power to undo final judgments. Although the wholly structural understanding of the two powers may seem to make their separate vesting in independent institutions pointless, it does not, and not only because constitutional restrictions limit American legislature’s ability to create any legal rules they wish. Even a legislature with that power would be substantially constrained by an independent judiciary, because it would have to exercise its power openly, through legal rules, and not covertly, by influencing the judge’ incentives.
Harrison, John, "Legislative Power and Judicial Power" (2016). Constitutional Commentary. 22.