“Greece would not have fallen had it obeyed Polybius in everything, and when Greece did meet disaster, its only help came from him” Pausanias, 8.37.2, Inscription on the Temple of Despoina near Arakesion.
In Book VI of his Histories, the ancient Greek historian Polybius described three basic forms of government, each categorized by the number of those in power. He listed monarchy (rule by the one); aristocracy (rule by the few); and democracy (rule by the many). Polybius described, over time, how each type of government would gradually decline into their various corrupted forms of tyranny, oligarchy and mob rule, respectively.
Polybius believed that Republican Rome had designed a new form of government that could help check this inevitable decline. Rome combined all three forms of government -- monarchy (its elected executives, called consuls); aristocracy (the Senate); and democracy (the popular assemblies). In this mixed form of goverment, each branch would check the corrupting ambitions and power of the others.
Polybius, Aristotle and Cicero all praised the construction of a "mixed constitution" and the requirement of a separation of powers within government.
The French nobleman and legal expert Charles-Louis de Secondat, the Baron de Montesquieu, studied the rise and fall of the Roman Republic. He believed that a properly designed government, in order to prevent tyranny, would require three branches of government. He wrote, "If it is to provide its citizens with the greatest possible liberty, a government must have certain features. First, since 'constant experience shows us that every man invested with power is apt to abuse it … it is necessary from the very nature of things that power should be a check to power' . This is achieved through the separation of the executive, legislative, and judicial powers of government... [to prevent any one] from acting tyrannically."
The British philosopher John Locke was also keenly interested in a design for government that would prevent it from descending into tyranny. In the late 17th century, Locke argued that monarchs had no "divine right" to rule; instead, he asserted that the source of power lay in the people. Furthermore, he stated that humans were born into this world with certain natural and "inalienable" rights including to "life, liberty and property". Locke believed that government could not grant these rights because they were God-given; therefore, no government could take them away or withhold them from the people...