Federalism, the division of power between the national government and the states, is not just a constitutional theory, it was a demand. It was a demand by the states at the time of the ratification of the original Constitution. And it has to do with the fact that we could create a national government, a new powerful national government. But one that wouldn't overwhelm the independent existence of the people in the states. The theory behind this and the reason for the demand actually goes all the way back to the Declaration of Independence, in which the colonies announced that they were and of right ought to be free and independent states. The worry was with the drafting, and the proposal, and the ultimate ratification of the new Constitution that this free and independent peoples who had come together, and who and worked together, and had cooperated with each other, both to win the Revolution and to coordinate their efforts under the Articles of Confederation were not just creating a better government, but possibly creating a government that would actually erase these independent peoples who had come to think of themselves as actual cultures, independent cultures. At the same time there was pressure in the other direction, an idea that there was a newly emergent American people, that we didn't have just common ideas, but we were a common people. These two ideas competed with each other, pressed against each other, and ultimately out of compromise there emerged the federal Constitution that actually maintained both ideas. According to Madison the Constitution we got was neither wholly national nor wholly federal. It had aspects of the creation of a brand new American people with a brand new national government that would oversee some of the most important responsibilities of national governments to protect this country and to protect the individual states in a very dangerous world. On the other hand it also preserved the independent exist and of the states, the people of Virginia, the people of Massachusetts, the people of New York, who had their own cultures, their own ideas, their own common religious beliefs, their own geography with it's own particular problems and particular regulatory needs. These independent local concerns would be preserved within the power of the independent peoples of the several states through the mechanism of limited delegated powers into the hands of the national government. It's powers would be few and they would be defined, whereas the reserved and retained powers of the people in the several states would be numerous and indefinite.