6 ideas
When the useful and the honourable appear to conflict, the conflict is an illusion: a gain secured by injustice or betrayal damages the character and trust that make any gain valuable, so it was never a real gain. The test is not to weigh profit against morality but to recognise that the dishonourable option has already been miscounted as profitable.
Cicero uses Plato's shepherd who finds a ring that makes him invisible, and then uses it to seduce a queen, kill a king and seize power. The good man, Cicero argues, would act no differently with the ring than without it, because honourable conduct is chosen for what it is rather than out of fear of being seen.
All moral duty flows from four virtues: wisdom (perceiving truth), justice and beneficence (keeping human society together and giving each their due), greatness of spirit (courage and indifference to external fortune), and seemliness or temperance (order and moderation in word and deed). Any proposed action can be tested against each source, and where they compete, duties sustaining human fellowship generally outrank solitary contemplation.
Every person wears four roles at once: the universal rational nature shared by all humans, an individual temperament and talent, the circumstances fortune assigns (birth, wealth, office), and the career they choose. Right conduct means acting within universal morality while fitting one's own nature, rather than straining to imitate another's gifts, since consistency with oneself is itself a component of the honourable.
There are two kinds of injustice: inflicting harm, and failing to defend others from harm when one could. People who withdraw into study, private business or fear of trouble to keep their own hands clean still commit the second kind, because justice demands intervention and not merely abstention.
Captured by Carthage and sent to Rome on parole to negotiate a prisoner exchange, Marcus Atilius Regulus told the Senate to reject the deal, then kept his oath and returned to Carthage, where he was tortured to death. Cicero presents this as the useful and the honourable converging: breaking a sworn word, even to an enemy, would have been the real loss.