Rule of/by law is distinguished from the rule of/by man.
Rule of/by man
This is where, for example, in a monarchy, a tyrannical or theocratic form of government, governance, and rules of conduct are set and altered at the discretion of a single person, or a select group of persons.
Rule of/by law
In a political system which adheres to the supremacy of the rule of law, the law always overrides and holds power over the acts of the government and its people.
Reference:
Rule of Law Definition. (n.d.). Retrieved from
http://www.duhaime.org/LegalDictionary/R/RuleofLaw.aspx