Cicero: True Law Is Right Reason

The Roman statesman Marcus Tullius Cicero (106–43 BCE) wrote the most quoted ancient definition of natural law. In his dialogue On the Republic, written around 54–51 BCE, he declared that 'true law is right reason in agreement with nature', universal, unchanging and eternal. Such a law, he said, cannot be repealed by the Senate or the people, and 'there will not be one law at Rome and another at Athens'.

In On the Laws he argued that a decree contrary to justice no more deserves the name of law than a harmful prescription deserves to be called medicine. Cicero was translating Stoic ideas into Roman political language, and he wrote during the crisis that destroyed the Republic, which gives his appeal to unchangeable law an urgent tone. He was killed in 43 BCE on the orders of Mark Antony. Through Christian writers who preserved his words, Cicero carried natural law into the Middle Ages and the Renaissance.