Marcus Tullius Cicero

In De Natura Deorum, On the Nature of the Gods, Cicero stages a debate rather than delivering a lecture. He gives a Stoic, an Epicurean and an Academic Skeptic each a book to argue their position on whether the gods exist, what nature they have, and whether they concern themselves with human affairs, and he closes the dialogue without declaring a winner, letting the reader judge which case is more persuasive. The form itself is an argument: Cicero, trained in the Academic school's method of arguing both sides of a question, treats theology as a subject on which reasoned people can disagree honestly, a stance distinct from both the confident materialism of the Epicureans, who held the gods existed but took no interest in human affairs, and the providential cosmos of the Stoics, who identified the divine with the rational order pervading nature itself.

His companion work, De Divinatione, On Divination, examines augury, oracles and prophetic dreams with the same dialogic method, again dividing the argument between a defender and a skeptic, this time cast as Cicero's own brother Quintus and Cicero himself. The pairing is not incidental: Cicero held the priestly office of augur at Rome for the last years of his life, so the dialogue sets his private philosophical doubt about the reliability of divination against a public religious role he continued to perform, a tension he never fully resolves any more than he resolves the theological debate in the companion work.

In De Legibus, On the Laws, Cicero offers a single formulation that later thinkers return to more often than anything else he wrote on religion or ethics: true law is right reason in agreement with nature, spread through all people, constant and eternal, and no senate or people can release us from it. The claim grounds legal and moral obligation in a rational order built into nature itself rather than in the decree of any particular state, an idea Cicero inherited from Stoic philosophy and restated in Latin so influentially that it became the standard citation for the whole later natural law tradition, the position that a moral order exists independently of human law and is knowable by reason.

Thomas Aquinas's treatment of natural law in the Summa Theologiae, the most influential statement of the idea in Western thought, builds directly on this Ciceronian and Stoic foundation. The line runs further still: Hugo Grotius invoked a version of it to argue that natural law would hold even if one granted, for the sake of argument, that there is no God, and Martin Luther King's Letter from Birmingham Jail cites Aquinas directly on the distinction between a just and an unjust law, a distinction that traces back through Aquinas to Cicero's own sentence. Comparable positions exist outside the Western tradition entirely, in the Mu'tazili claim that good and evil are knowable by reason and in the Confucian appeal to the way of Heaven, but it is Cicero's Latin formulation specifically that became the citation Western natural law argument keeps returning to.