The Romans summed up their political existence in a single phrase: senatus populusque Romanus, “the Senate and the Roman people,” which they abbreviated SPQR. This sentiment reflects the republican ideal of shared government rather than power concentrated in a monarchy. It stands for the beliefs, customs, and laws of the republic.

In the early republic, social divisions determined the shape of politics. Political power was in the hands of a hereditary aristocracy—the patricians, whose privileged legal status was determined by their birth as members of certain families. The common people of Rome, the plebeians (plih-BEE-uhns), were free citizens with a voice in politics, but they had few of the patricians’ political and social advantages. While some plebeian merchants increased their wealth in the course of Roman expansion and came to rival the patricians economically, most plebeians were poor artisans, small farmers, and landless urban dwellers.

The Romans created several assemblies through which men elected high officials and passed legislation. The earliest was the Centuriate Assembly, in which citizens were organized into groups called centuries. Each citizen was assigned to a century depending on his status and amount of wealth, and the patricians possessed the majority of centuries. When an election was ordered, each century met separately and voted as a bloc, which meant that the patricians could easily outvote the plebeians. In 471 B.C.E., plebeian men won the right to meet in an assembly of their own, the concilium plebis, and to pass ordinances.

The highest officials of the republic were the two consuls, who were elected for one-year terms by the Centuriate Assembly. The consuls commanded the army in battle, administered state business, presided over the Senate and assemblies, and supervised financial affairs. In effect, they ran the state. The consuls appointed quaestors (KWEH-stuhrs) to assist them in their duties, and in 421 B.C.E. the quaestorship became an elective office open to plebeian men. In 366 B.C.E., the Romans created a new office, that of praetor (PREE-tuhr). When the consuls were away from Rome, the praetors could act in their place; they could also command armies, act as governors in the provinces, interpret law, and administer justice.

The most important institution of the republic was the Senate, a political assembly that most likely originated in the monarchical period as a council of the heads of powerful families who advised the king. During the republic the Senate grew to several hundred members, all of whom had previously been elected to one of the high positions, which automatically conferred Senate membership. Because the Senate sat year after year with the same members, the Senate provided stability and continuity. It passed formal decrees that were technically “advice” to the magistrates, who were not bound to obey them but usually did. The Senate directed the magistrates on the conduct of war and had the power over the expenditure of public money. In times of emergency, it could name a dictator.

Within the city of Rome itself, the Senate’s powers were limited by laws and traditions, but as Rome expanded, the Senate had greater authority in the outlying territories. The Romans divided the lands that they conquered into provinces, and the Senate named the governors, most of whom were former consuls or praetors, for each province. Another responsibility of the Senate was to handle relations between Rome and other powers.

A lasting achievement of the Romans was their development of law. Roman civil law, the ius civile, consisted of statutes, customs, and forms of procedure that regulated the lives of citizens. As the Romans came into more frequent contact with foreigners, the consuls and praetors applied a broader ius gentium, the “law of the peoples,” to matters such as peace treaties, the treatment of prisoners of war, and the exchange of diplomats. In the ius gentium, all sides were to be treated the same regardless of their nationality. By the late republic, Roman jurists had widened this principle still further into the concept of ius naturale, “natural law,” based in part on Stoic beliefs (see "Philosophy and the People" in Chapter 4). Natural law, according to these thinkers, is made up of rules that govern human behavior and that come from applying reason rather than customs or traditions, and so apply to all societies.