Monarchy first changes into its vicious allied form, tyranny; and next, the abolishment of both gives birth to aristocracy. Aristocracy by its very nature degenerates into oligarchy; and when the commons inflamed by anger take vengeance on this government for its unjust rule, democracy comes into being; and in due course the license and lawlessness of this form of government produces mob rule to complete the series. ~ Polybius
Greek historian Polybius (200–118 BCE) was first taken to Rome as political prisoner. Once there he made influential friends, turned into an admirer of Roman governance, and so became an ally.
Polybius was the first to study polity from an institutional standpoint, and to relate foreign policy to domestic policies and practices. (The Histories was originally written by Polybius in 40 volumes, covering 264–146 bce.) Polybius’ analysis of the evolution of Roman institutions convinced him that Rome’s rise to supremacy was not the work of a few political leaders or military leaders, but instead a product of the less visible and less dramatic force of political standards and practices.
In his day, Aristotle considered Carthage to have been a 1st-rate city-state government. Polybius compared the standards in Carthage, where “nothing which results in profit is regarded as disgraceful” to those of Rome, which condemned “unscrupulous gain from forbidden sources.”
At Carthage, candidates for office practice open bribery, whereas at Rome death is the penalty for it. ~ Polybius
Polybius saw monarchy, aristocracy, and democracy as the 3 chief forms of government. Without checks and balances, each carried within itself the seed of its own demise.
Polybius considered the secret of Roman political health its mixed constitution in political authority. The counsels represented the principle of monarchy; the senate was aristocratic, and the popular assemblies a democratic form.
Many contemporaries of Polybius saw the tripartite system as having a divisive effect on political unity, and an impediment on the ability to act. Polybius thought otherwise.
Such being the power that each part has of hampering the others or cooperating with them, their union is adequate to all emergencies. None of the 3 is absolute. The purpose of the one can be counterworked and thwarted by the others. Any aggressive impulse is sure to be checked and from the outset each estate stands in dread of being interfered with by the others. ~ Polybius