Cicéron
True law is right reason in agreement with nature.
On the Republic essential
Dialogue in 6 books on the best constitution, set in 129 BC. Lost in the Middle Ages: about a third was found in 1819 under a commentary by Augustine (a Vatican palimpsest). Book VI, Scipio's Dream, survived separately, with Macrobius' commentary (c. 430). The definition of true law (III, 33) is known only from a quotation by Lactantius.
On the Laws essential
Dialogue between Cicero (Marcus), his brother Quintus and Atticus. Book I: right comes from nature. Unfinished (3 books preserved, at least 5 planned) and probably published after his death. Our corpus contains book I.
Tusculan Disputations essential
Five days of discussion in his villa at Tusculum, written after his daughter's death. Book I: death is not an evil; book V: virtue suffices for happiness. Cicero speaks ("M.") to an anonymous listener ("A."). Our corpus contains books I and V.
On the Nature of the Gods essential
Dialogue in 3 books: the Epicurean Velleius, the Stoic Balbus (existence of the gods, order of the world, Providence), the Academic Cotta (who refutes Balbus). The young Cicero listens. These are characters: at the end, Cicero only says that Balbus seems to him closer to probability. Our corpus contains books II and III.
On Duties (De officiis) essential
A letter-treatise in 3 books to his son Marcus, written in autumn 44, in the middle of the fight against Mark Antony. Book I: the honourable; II: the useful; III: apparent conflicts between them (Gyges' ring). He follows the Stoic Panaetius, but "not as a translator". He could not revise it before his death.
Laelius, On Friendship
Dialogue dedicated to Atticus. Laelius, friend of the recently dead Scipio, speaks about friendship; the preface says he voices Cicero's ideas.
On Ends; Academica
Happiness according to the Epicureans, Stoics and Peripatetics; knowledge according to the Academy. Not yet in our corpus.
On Divination; On Fate
A critique of divination and a defence of freedom against Stoic fate. Not yet in our corpus.
Letters (to Atticus, to his friends…)
More than 900 letters, published after his death: a unique record of the end of the Republic. Not yet in our corpus.