The Rule of Law is a three-legged stool on which freedom sits. The first leg requires that all laws be enacted in advance of the behavior they seek to regulate and be crafted and promulgated in public by a legitimate authority. The goal of all laws must be the preservation of individual freedom. A law is not legitimate if it is written by an evil genius in secret or if it punishes behavior that was lawful when the behavior took place or if its goal is to solidify the strength of those in power. It also is not legitimate if it is written by the president instead of Congress.
The second leg is that no one is above the law and no one is beneath it. Thus, the law's restraints on force and fraud need to restrain everyone equally, and the law's protections against force and fraud must protect everyone equally. This leg removes from the discretion of those who enforce the law the ability to enforce it or to afford its protections selectively. This principle also requires that the law enforcers enforce the law against themselves. Of course, this was not always the case. In 1628, the British Parliament spent days debating the question "Is the king above the Rule of Law, or is the Rule of Law above the king?" Thankfully, the king lost -- but only by 10 votes out of several hundred cast.
The third leg of the Rule of Law requires that the structures that promulgate, enforce and interpret law be so fundamental -- Congress writes the laws, the president enforces the laws, the courts interpret the laws -- that they cannot be changed retroactively or overnight by the folks who administer them. Stated differently, this leg mandates that only a broad consensus can change the goals or values or structures used to implement the laws; they cannot be changed by atrophy or neglect or crisis…
We live in perilous times. The president acts above the Rule of Law and fights his own wars. Congress acts below the Rule of Law by letting the president do whatever he can get away with. And this summer, the Supreme Court rewrote the Rule of Law.