Freedom of men under government is, to have a standing rule to live by, common to every one of that society, and made by the legislative power erected in it; a liberty to follow my own will in all things, where the rule prescribes not; and not to be subject to the inconstant, uncertain, unknown, arbitrary will of another man.
—John Locke. Two Treatises of Government, 1689.
The essence of the rule of law is that the written law, rather than the arbitrary will of a person, should be the only way in which individuals are coerced by government.