This is a shorter letter with not much to chew on, besides what has already been discussed.
It would seem Lucilius and Seneca are discussing two mutual friends; one young and one older. The younger one must focus on faults that need to be fixed with some minor correction, while the older one might take more effort to "crush out" stubborn habits.
A few concepts to take away from this letter:
- a reminder to limit your desires
- use a good, moral person to be your mental guardian; if you ask yourself 'what would you do if Cato saw you now?' that might persuade you to act more virtuously
- be careful of crowds and after some time and practice of 'guarding yourself' you can then trust yourself to go out into crowds and not be influenced by them
- but also be careful to be alone - 'an idle mind is the devil's workshop' sort of thing