Ita fac, mi Lucili...
Do thus, my Lucilius...
Outside of the few details gleaned from Seneca's Moral Epistles themselves, we have no other source of information about who Lucilius was. Because the epistolary genre was so common and these epistles are so finely crafted, some scholars question whether Lucilius was a real person at all, speculating that these Moral Epistles were not authentic correspondences and that Lucilius was a mere literary device. It is more likely, however, that Lucilius was a real person, as evidenced by references to his personal life. The Moral Epistles were almost certainly real communications, but Seneca surely composed them carefully with a potential broader authorship in mind.
Seneca was an informal teacher to Lucilius in these correspondences. As his philosophical guide, Seneca would correct and encourage, facilitate in his pupil's self-reflection, advise him in specific circumstances, and at times be an exemplar of Stoic principles for Lucilius. The relationship between teacher and student in Roman Stoicism was one of mutual benefit. Although the teacher, Seneca saw himself as a fellow learner, progressing alongside his student. The famous Latin saying, docendo discimus ("we learn by teaching") may ultimately derive from Epistle 7 where he writes "homines dum docent discunt" ("men learn while they teach").
More than a teacher, is is clear that Seneca and Lucilius were friends. Given the Stoic belief that virtue was the only good and all things else beyond one's personal control are 'things indifferent' (Greek: ἀδιάφορα; Latin: indifferentia), a person might assume that the Stoics did not place much value on friendship. Among the "things indifferent," there were "preferred indifferents" - and friendship was one of these. The orthodox Stoic would form friendships not out of strict necessity, but out of a natural inclination to share virtue; not as a prerequisite for happiness, but as a rational choice to enhance virtuous action (see Epistle 9).
While we say "do thus" to a student or friend in anything moral, we are exhorting ourselves as much as them.