On the Laws by Marcus Tullius Cicero (106 - 43 BCE)

On the Laws (Latin: De Legibus) was written shortly after Cicero's "On the Commonwealth" during the last years of the Roman Republic. The three surviving books (out of an original six), in order, expound on Cicero's beliefs in Natural Law, recast the religious laws of Rome (in reality a rollback to the religious laws under the king Numa Pompilius) and finally talk of his proposed reforms to the Roman Constitution. (Summary Adapted from Wikipedia)