Honor
It would be impossible to mention themes of On Duties without mentioning honor. After all, that is what the book is all about. Cicero explains the difference between honor and usefulness, the other theme of the work. Honor is something that combines wisdom, justice, and moderation - therefore, it is often difficult to conjure up. Only the bravest of men can be honorable, Cicero explains, because they do not only do what is best for themselves, they do what is best for others first.
Usefulness
In a way, usefulness could be seen as the antagonist of the letter, or at least some personified version of an adjective that Cicero is always looking down on. In his own words, Cicero says that usefulness is not honorable because it is doing what is best for oneself. His views on the topic were influenced, likely, by the assassination of Julius Caesar in Rome, who himself often did what was useful. Usefulness may be good in the short term, as it gets you what you want, but, as seen by the case of Julius Caesar, it is not good for the long term.
