The U.S. Constitution’s separation of powers has its origins in the British idea of the desirability of a Mixed Regime where the King, the Lords, and the Commons all checked and balanced one another as the three great estates of the realm. Aristotle, Polybius, Cicero, St. Thomas Aquinas, and Machiavelli all argued that Mixed Regimes of the One, the Few, and the Many were the best forms of regimes in practice because they led to a system of checks and balances. The Enlightenment killed off the Mixed Regime idea forever because hereditary office-holding by Kings and Lords became anathema. The result was the birth of a functional separation of legislative, executive, and judicial power as an alternative system of checks and balances to the Mixed Regime. For better or worse, however, in the United States, Congress laid claim to powers that the House of Lords and the House of Commons historically had in Britain, the President laid claim to powers the King historically had in Britain, and the Supreme Court has functioned in much the same way as did the Privy Council, the Court of Star Chamber, and the House of Lords. We think these deviations from a pure functional separation of powers are constitutionally problematic in light of the Vesting Clauses of Articles I, II, and III, which confer on Congress, the President, and the courts only the legislative, executive, and judicial power. The United States badly needs a rebirth of the functional separation of powers idea.