Rule of law can be seen as a compromise between positive law and natural law; it places limits on what law can be, as it denies that unclear, secret, or retroactive laws are actually laws. This allows legal positivists to separate morality from law while still maintaining certain standards for what they consider law, and it helps natural law theorists define their basic moral expectations for law. However, rule of law’s very status as a compromise robs it of much of its validity.
Rule of law is both too strong and not strong enough. It cannot fit in with legal positivism because it places moral restrictions on law, and legal positivism calls for a strict separation between morality and law. However, rule of law also does not fully encompass the moral requirements that natural law theorists place on law. Rule of law gives legal systems far too much leeway. For instance, a harsh totalitarian regime could follow rule of law to the letter while still being what many would consider unjust and taking away personal freedoms. This does not mesh well with natural law theory, which requires law to meet moral standards in all aspects.
Most natural law theorists ask more of law that simple rule of law can accomplish. Aquinas, for instance, believed that law not only had to meet moral standards, but had to serve a moral purpose—to serve the common good, yes, but also to educate and guide the people as to proper moral conduct. Any law that did not serve these purposes was not considered a law. Similarly, St. Augustine held that “an unjust law is no law at all,” as quoted in Martin Luther King’s “Letter from Birmingham Jail.”
While I do believe that rule of law can certainly be complimentary to natural law theory, and is a good way to judge our legal systems and laws as they are when searching for injustice, rule of law is not a comprehensive natural law theory on its own—it is simply too weak to be used as a fully developed idea of natural law, as it still allows some injustices to be done in the name of law. Rule of law is actually better used by those whose leanings lay somewhere between natural law theory and legal positivism, as it doesn’t hold law accountable to a strict moral standard but does still require a measure of justice and transparency from a system of law.