The “rule of law” is a very familiar concept but it's one that is surprisingly difficult to articulate. A very familiar way of putting it is that we are a government of laws and not of men. But well, you might ask, what does that mean since it's men and women, of course, who make the laws that we are governed by. To speak of the rule of law means that the people who make the laws are constrained by rules themselves. And that constraint inhibits and hopefully prohibits altogether the exercise by government officials of arbitrary power and the exercise of personal whim as they bring the force of government down on the citizens. The purpose of the Constitution is to ordain and establish the government and to describe the government's powers. The Constitution establishes the three branches of the federal government and both gives those three branches power and limits their power. It prescribes a system of federalism that constrains national power vis-à-vis the states, and keeps the states as independent entities. So, basically, its purpose is to establish how the government is going to operate and who has what power to do what with respect to governing the citizens. Of course, the Constitution has to do a lot of things at once. Two competing goals in particular are to preserve order and to secure freedom. It's not an easy task. That's why our Constitution is such a brilliant document. The rule of law implies that not only are citizens required to obey the law, but so are those officials who wield the immense power of the government. It also implies that the rules that the government actors are constrained by are known, they are stable, they are public. So the fact that they are known and stable and public means that the citizens to whom these rules maybe are applied can hold their government officials to account by measuring their conduct against the rules by which the government officials are supposedly constrained.