De Legibus (On Laws)

The dialogue's core claim is that true law is 'right reason in accordance with nature' (recta ratio naturae congruens) — universal, unchanging, and valid for all peoples at all times . Cicero explicitly says the discussion should not be grounded in the Twelve Tables or Roman custom alone but in 'deepest philosophy' (Leg. 1.17) . This means a magistrate's decree or a people's vote that contravenes natural law is not law at all: 'no ruler and no people can make right what is wrong' . The Roman constitution is to be judged against this standard, not merely described.

De Re Publica, set in the age of Scipio Africanus, asks what the best commonwealth is and locates its answer in Rome's mixed constitution, with the ideal statesman (rector) as its guardian. De Legibus, set in Cicero's own lifetime with himself, his brother Quintus, and Atticus as speakers, abandons reliance on individual statesmanship and turns to law and structure . As one commentator puts it, 'the statesman, the rector or princeps, is gone' — what remains is the hope that fundamental law, grounded in right reason, can guide a commonwealth through moral and political chaos .

Cicero deliberately imitates Plato's pairing of Republic and Laws, writing his own Republic and then a sequel on laws . But the Loeb editor notes that Cicero appears to have overlooked that Plato's Laws was not actually a sequel to his Republic — his own plan made that relation necessary . Cicero follows Plato in setting the Laws later than the Republic, in making himself the chief speaker rather than Socrates or Scipio, and in the dialogue's peripatetic character. The content, however, is largely Roman: the second and third books draw on Roman law and jurists for most of what is not original .

Cicero began the work around 52 BC, shortly after completing De Re Publica, and returned to it in 46 BC, but he never completed or published it . It was originally six books, of which only three survive, with gaps even in the preserved text . Internal evidence and later citations suggest the work had at least five books, but the remaining three cover natural law, religious legislation, and the magistracies . The unfinished state may reflect Cicero's growing absorption in systematic philosophy and the political disruptions of the late Republic .

The phrase 'salus populi suprema lex esto' — 'the welfare of the people shall be the supreme law' — comes from De Legibus Book III . It has been invoked far beyond its original context: John Locke used it as the epigraph to his Second Treatise of Government, Hobbes endorsed it in Leviathan, Spinoza quoted it, and it became the state motto of Missouri . The maxim encapsulates Cicero's view that even the highest legal authority is subject to the fundamental principle that government exists for the good of the governed.

Cicero's formulation of natural law as right reason, universal and binding, passed into the medieval tradition through Augustine and Thomas Aquinas . In the early modern period, his influence is visible in the natural-law theories of Spanish jurists, British deists, and figures like Locke and Montesquieu . One study argues that Cicero's assertion that natural law requires no interpreter other than the individual, and his description of the magistrate as 'the law speaking,' contributed indirectly to the doctrine of judicial review in American constitutional thought . His conception of human equality based on shared reason marked a break from Aristotle that Carlyle called the dividing line between ancient and modern political theory .