Well, originalism is a claim that the words of a legal document ought to be understand by reference to what it meant at the time of enactment. That's the basic framework of originalism. I think that sort of framework is necessary if you really want to have a separation of powers, because otherwise it's quite possible to read these phrases in the Constitution in ways that would be inconsistent with the idea of separation of powers. For instance, the President's given the executive power in article two, that at the founding meant the authority over law execution and authority over foreign affairs. But if you had a living constitutional approach to what executive power meant, that could cover the legislative power, and that could cover the judicial power and if you were to so construe it you wouldn't have a separation of powers because all three powers would be vested with the president. Once you have the idea that the meaning of words and phrases can change over time whether they're applications to existing sets of facts can change over time. You make it possible that there won't be a separation of powers precisely because any one branch can claim it doesn't really matter what legislative power meant in the 18th century, what matters is what we take it to mean today. Of course, we can take it to mean anything we want to. Without fixed meanings of these clauses you run the risk of not having a separation of powers.