Polybius was, whilst a Greek historian, a Roman historian, in that his work dealt with explaining how Rome came to be so great.
Little is known about Polybius' later life with much certainty. What we do know is that most of it must have been spent writing up his works (the Histories being the only surviving one) and that he must have died at some point after the construction of the Via Domitia in 118 BC, since this is chronologically the latest event mentioned in his Histories. This perhaps gives some credit to the Pseudo-Lucian's claim that Polybius died at the age of 82, having fallen from his horse.
In book 6 of his histories, the Greek author Polybius outlines his theory of the mixed constitution, describing the Roman government as an example. After first describing the three "simple" types of constitution, Polybius proceeds to outline how the mixed constitution combines these three types to create a more coherent and stable government than any of the "simple" constitutions.
Before describing the mixed constitution, Polybius outlines the three kinds of simple constitution, each of which has two forms: a noble form and a debased form. In any simple constitution, the governing authority can rest in one man, a group of men, or the majority of all the citizens. Rule by one man can be either a kingship (if the man rules justly and rationally) or a tyranny (if the man rules by brute force). If a group of men rule, the state is either an aristocracy (if the best and wisest men are in charge) or an oligarchy (if the rulers are chosen by any other criteria). Rule by popular majority can constitute a democracy (if civic order and rule of law prevail), or mob rule (if lawlessness prevails and civic institutions break down).
Polybius' main criticism of the three simple forms is that they are unstable. Using biological terms, he details a "natural" constitutional cycle in which simple constitutions are always engaged in a violent cycle of degeneration and revolution. His analysis rests on the idea that power inevitably corrupts: each kind of simple constitution degenerates from its noble to debased form when the ruling authority becomes corrupt, arrogant and overbearing.
Beginning with the origin of human culture, Polybius argues that political societies arise because humans have a natural instinct to live in groups. Among primitive groups the strongest man will inevitably rise to power. Since he rules by force, not by reason, Polybius describes this man as a tyrant. In time, however, society matures, and humans begin to understand justice: when they begin to choose their rulers based on who is the wisest and most just, rather than on who is the strongest, tyranny becomes kingship.
Kingship, according to Polybius, inevitably corrupts the royal line. The king's children, raised to power, become too corrupt and arrogant for the community to tolerate. At this point, the noblest men in the state, who are most disturbed by their kings' failings, stage a coup and take over the government. Since they rule justly and with concern for the people, Polybius describes them as an aristocracy. Their descendents, however, fall prey to the same flaws as the royalty: born into their position, they take their privilege for granted and abuse their power. The people, enraged, overthrow this group as well.
Afraid to set up another king or aristocracy, the people have no choice but to take over the government themselves. Democracy endures as long as the people still remember the evils of oligarchy and monarchy, but power soon corrupts the people as well. The rich begin to influence the state through bribes, civic cohesion breaks down, and violence and mob rule takes over the state. A tyrant arises from the mob, and the cycle begins again.
After this pessimistic forecast of the fate of simple constitutions, Polybius describes the wisdom of the Athenian lawmaker Lycurgus, who first invented the mixed constitution. The mixed constitution incorporates elements of monarchy, aristocracy, and democracy; thus, the three elements balance each other out and no one of the elements holds enough power to become corrupted and degenerate into its debased form.
Following his praise of Lycurgus, Polybius examines the Roman system as a model of the mixed constitution. At the head of the Roman Republic stand two elected officials, called consuls, who carry out the decisions of the Senate, lead the army, and generally hold the highest executive authority. Polybius describes the consuls as the monarchical element of the Roman government. The consuls' power, however, is tempered by the authority of the Senate, a small group of Roman political elites who form the aristocratic element in the Roman constitution.
The Senate controls the treasury and the public building programs, passes judgement in certain cases of the highest importance (particularly treason), and handles foreign diplomatic affairs. Both the consuls and the Senate are controlled by the people. Since the people elect all public officials, the magistrates are ultimately responsible to them. The people also have power to accept or reject any law, to decide whether or not to go to war, and to ratify or reject all alliances and treaties. On top of these powers, the people have special magistrates, called tribunes, elected to look after their interests.
The Roman government is thus a "mixed" constitution because it mixes elements of all three simple forms. This ensures the stability of the constitution because the three parts of the government keep each other in check, so that no one of them becomes arrogant and overbearing through abuse of power. The consuls cannot wage war unless the Senate votes them enough funds to pay for it. And after waging a war, the consuls still have to refer any treaties back to the people for ratification.
The Senate cannot infringe on the rights of the people because the tribunes, the magistrates elected to protect the people, have the right of veto over any Senatorial decision. The Senate is not completely dominated, however, because it controls the public works projects, which the majority of citizens have some kind of financial stake in. Additionally, the people must be cautious about antagonizing the Senate because Senators are the judges in civil trials.
The mixed constitution therefore combines monarchy, aristocracy, and democracy in such a way that each element keeps the others in line, and the government does not become abusive. Above all, Polybius praises this system for its stability: it avoids the endless, violent cycle of the simple constitutions.
Scholars disagree about whether Polybius gave a fair description of the Roman constitution (most modern scholars agree that the Senate had by far the lion's share of the power, and that the participation of the people was largely symbolic), and in places his analysis appears to be internally inconsistent. No one can deny, however, that the idea of the mixed constitution has influenced lawmakers ever since - especially the authors of the American Constitution.
Echoes of Polybius are apparent throughout the American government, and his theory of the mixed constitution, however flawed, continues to spark debate in part because of its continuing relevance to modern political institutions.