Karl Freidrich says, constitutionalism is both a theory and a practice about politics according to rule of the game. Rules of the game are needed to restraint the government. It is a system of restraint of government. Constitutionalism is based on the belief in a limited government and in the use of constitution to impose these limitations.
Constitutionalism in the proper sense of the term is still in the process of evolution. According to Karl Loe Wenstein, the ancient great empires- Egypt, Babylonia, Persia had no constitutional process, instead evolved what he called transcendental constitutionalism where the ruler and the ruled were all subject to divine law pronounced by prophets.
However, the true origin of this concept is found in ancient Greeks thinkers who challenged mythological legitimization of power.
Subsequently Roman constitutionalism also came into being and after the decline of the Roman Empire, medieval constitutionalism sought to invoke natural law, sometimes identified it with divine law.
But all these were partly sedimentary forms of constitutionalism. It was only towards the end of middle ages that significant advance was made in the direction of modern constitutionalism.