"Nothing distinguishes more clearly conditions in a free government from those in a country under arbitrary government than the observance in the former of the great principles known as the Rule of Law. Stripped of all technicalities, this means that government in all its actions is bound by rules fixed and announced before-hand--rules which make it possible to foresee with fair certainty how the authority will use its coercive powers in given circumstances and to plan one's individual affairs on the basis of this knowledge." - F.A. Hayek, The Road to Serfdom
Update: I was feeling lazy this morning and neglected to type out the excerpt which follows.
"The Rule of Law was consciously evolved only during the liberal age and is one of its greatest achievements, not only as a safeguard but as the only legal embodiment of freedom. As Immanuel Kant put it (and Voltaire expressed it before him in very much the same terms), "Man is free if he needs to obey no person but solely the laws." As a vague ideal it has, however, existed at least since Roman times, and during the last few centuries it has never been so seriously threatened as it is today. The idea that there is no limit to the powers of the legislator is in part a result of popular sovereignty and democratic government. It has been strengthened by the belief that, so long as all actions of the state are duly authorized by legislation, the Rule of Law will be preserved. But this is completely to misconceive the meaning of the Rule of Law. This rule has little to do with the question whether all actions of government are legal in the juridical sense. They may well be and yet not conform to the Rule of Law. The fact that someone has full legal authority to act in the way he does gives no answer to the question whether the law gives him power to act arbitrarily or whether the law prescribes unequivocally how he has to act. It may well be that Hitler has obtained his unlimited powers in a strictly constitutional manner and that whatever he does is therefore legal in the juridical sense. But who would suggest for that reason that the Rule of Law still prevails in Germany?" - F.A. Hayek, The Road to Serfdom (1944)