Res publica virum docet.
"Public affairs (or "affairs of state") teach a man."
~ Attributed to Plutarch, a Greek author, apparently translated into Latin by Erasmus of Rotterdam.
The term res publica literally means "public matters," sometimes translated as "Republic," though that is not the sense here. The idea that involvement in public affairs - social and governmental matters in the community - is part of the education of a young man is common in ancient Greek democracies such as Athens and in ancient Rome. Our English word "idiot" actually comes from the ancient Greek word for "private," meaning a person who lives a private life and does not take part in public life is an . . . idiot. A less sexist, modern version might be Res publica civem docet - "Public affairs teach the citizen."
With macrons: Rēs pūblica virum docet.
Rēs pūblica cīvem docet.