The Supreme Court first addressed the Non-Delegation Doctrine back in 1825, in an opinion by Chief Justice John Marshall. It was addressed in passing, because they didn't need to deal with the issue in the case, but Marshall made some observations about that doctrine that still have a lot of relevance today. He noted the big problem is figuring out where the lines are drawn among the Legislative, Executive, and Judicial powers. In that particular case, Congress had given the Federal Courts power to promulgate their own rules of procedure, for how to execute judgements and things like that. The question was whether that was something that needed to be specified by Congress in legislation, or whether Congress could just say to the courts, "Oh, promulgate whatever rules of procedure you'd like." And Chief Justice Marshall recognized that the issue was whether or not the power given to the courts was something they could exercise as part of The Judicial Power. If it was, no problem. If it wasn't, if the only constitutional box that you could put that power in was The Legislative Power, you would have a sub-delegation problem. And in 1825, the Supreme Court took it as given that once the boxes are drawn and you've got something in the box, no, you can't sub-delegate legislative power out of the legislative power box. In that case, Chief Justice Marshall just didn't think that the power in question was exclusively in the legislative power box. What we got in 1825 was the first clear articulation of an attempt to figure out how to draw those boxes. What makes something Legislative Power rather than Executive or Judicial? And what Chief Justice John Marshall said 200 years ago, was that if it was an important matter, Congress has to resolve it in the legislation. It could leave to Executive or Judicial implication matters of, there are his words, not mine, "less interest." So distinguishing important matters, those are the Legislative matters. Matters of “less interest,” those can be given to other actors, because filling them in is Executive or Judicial. Over the ensuing 200 years, nobody; no court, no scholar, not I, has been able to improve on Chief Justice Marshall's formulation.