While Montesquieu (1689–1755) is often regarded as the thinker who discovered the importance of fundamental principles such as the rule of law and the separation of powers, systematic research of his theory of the state is surprisingly limited. In this article, I argue that his masterpiece, The Spirit of the Laws (1748), points to a theory of the non-arbitrary state. Montesquieu’s comparative study of various governments demonstrates that modern liberty depends on the rule of law. Since many states have laws (...) but few of them guarantee liberty, he stressed that the rule of law requires the separation of the judicial from the legislative and executive powers. The constitution of an independent judicial branch separates the sphere of justice from the political sphere, which creates the basic structure of the non-arbitrary state. Depending on the distribution of power in either of the two spheres, non-arbitrary states can guarantee their citizens more or less liberty. The concept of the non-arbitrary state thus illuminates Montesquieu’s significance for the variety of political projects that emerged in the age of revolutions. (shrink)