In Book VI of his Histories, the Roman-era Greek historian Polybius (circa 200-118 B.C.) discusses the concept of the "mixed constitution" which, he believed, was the characterization best describing the form of Roman government under which he lived. It is a treatise more along the lines of philosophical discussion than history per se, and Polybius anticipated criticism from various Greek schools of philosophy, notably that of Plato (circa 428-347 B.C.) and his followers. Polybius was correct in this sense: while both he and Plato consider justice an essential part of any government, the older philosopher approaches the concept of state from an ethical perspective, while Polybius concentrates upon the stability engendered by a well-designed, "mixed constitutional" government. Curiously, both cite the same source as a model - the Spartan system of government - and both consider the evolutions of government in similar fashion. Yet, as noted already, Polybius and Plato use their examples to different ends. It will be the purpose of this paper to identify some of their points in common and, equally important, the points of departure that lead these two Greeks to different considerations of the state and its essential nature.
In describing the superiority of a mixed constitution, Polybius puts forth the argument - based on historical examples and analogies with stories about human nature - that the Roman system is the best possible form of government since it provides a structure of checks-and-balances that minimizes the defects of other alternatives. In so making this argument, however, Polybius takes pains to forestall attacks from other Greek schools of thought, particularly the Platonists. In fact, he devotes a critical paragraph to attacking and dismissing Plato's Republic (Polybius VI, 47). This attack is reasonable from the perspective of one whose focus is on the forms of a working state. Polybius' mixed-constituti...