The key question in constitutional law is ‘who decides’ and the genius of the U.S. Constitution is separation of powers. That's the greatest protection of liberty any government has known. And you can separate powers on horizontal lines and vertical lines, so horizontally, at the national level, you have separation of powers between the President, the Congress, and the Courts. But vertically, and this is really one of the great insights of the Framers, you can have vertical separation of powers between the national government and the states. And that in its essence is federalism. Vertical separation of powers between one national government and fifty state governments. I really do think the key innovation of the U.S. Constitution is federalism and I really think it probably is the only patentable idea in the document. Everything else in the U.S. Constitution really is borrowing from constitutions from other countries, other political thinkers. In the words of Justice Kennedy, the Framers split the atom of sovereignty. If ‘who decides’ is the key question, one of the real complications at the founding was how do you have distinct state governments and a distinct national government? They knew a confederation wouldn't work. They still wanted a national government, but they wanted to limit that national government's powers. Make them few, discreet, and enumerated, and give the general, law-making, police powers to the states. There are some powers, some ‘who decides’ questions, that are better answered at the state level and some that are better answered at the national level. A great example is national defense. We wouldn't want each of the states in charge of our national defense. That's something properly given to the President and Congress. Ah there are other things, criminal law, family law, that we really wouldn't want to entrust in general to the national government. These are things that can be handled quite adeptly at the local level by the states and by local government. The greatest anxiety of the founding Framers and the people in the eighteenth century was that the federal government would be answering too many of the ‘who decides’ questions. They would get too much power. They would look too much like the British Monarchy, and they really wanted the states to be the first responders, the first policy makers, the first bulwarks of protecting liberty.