The Republic and
". . . law in the proper sense is right reason in harmony with nature. It is spread through the whole human community, unchanging and eternal, calling people to their duty by its commands and deterring tham from wrong-doing by its prohibitions." (p 68)
Cicero wrote his dialogue, The Republic, just before the civil war that ended the Roman Republic. In it he discusses the history of Rome and its constitution. The Republic of Cicero is in one sense modeled after Plato's Republic, but it is different as well. Cicero presents a more realistic view of the state based on the Roman Republic that was in its last stages during Cicero's lifetime. He assimilates the philosophy of Plato, but also Aristotle's Politics and others.
In it he discusses the nature of different political organizations including Monarchy, Aristocracy, and Democracy, among others. His discussion of the best states and his comparison of monarchy, aristocracy, and democracy is thoughtful --- highlighting the differences and weighing the advantages and disadvantages of each; he concludes that the best regime may be one that is a blend of all three.
In the sequel essay entitled The Laws he promulgates a doctrine of Natural Law, which he then applies to all mankind. His code of law is developed for a reformed Roman Republic that, unfortunately, he never lived to see -- and after his death was preempted by the imposition of the Empire under the leadership of Augustus Caesar.
The following remarks give some indication of the best of his thinking: "The aim of a ship's captain is a successful voyage; a doctor's, health; a general's, victory. So the aim of our ideal statesman is the citizen's happy life---that is, a life secure in wealth, rich in resources, abundant in renown, and honorable in its moral character. That is the task which I wish him to accomplish---the greatest and best that any man can have."