Letter 7 is one of Seneca's most quotable — and most easily misread. Lucilius has apparently been attending the games, and Seneca writes back with something between a warning and a confession.
"I come home more avaricious, more ambitious, more voluptuous, and even more cruel and inhuman, because I have been among human beings."
He's not being a snob. He's being a realist. The point isn't that other people are bad — it's that we are porous. Seneca watched the crowd at the midday interval, when the serious gladiatorial bouts had paused and what remained was pure slaughter: convicted men sent in with no armor, no skill, just to be killed for the crowd's entertainment. And what he noticed wasn't the brutality of the spectacle — it was what the spectacle did to the spectators. Men who arrived decent left coarser. The cruelty was contagious.
The deeper principle he draws from this is about attention and influence. We become, slowly and without noticing, like what we spend time around. "Show me your friends and I'll show you who you are" is a folk saying; Seneca makes it a philosophy. He doesn't mean you should surround yourself only with sages — he knows that's impossible, and he'd find it insufferable anyway. He means you should be deliberate. "Withdraw into yourself as much as possible; spend time with those who will make you better."
Elsewhere in the letters
Letter 25 returns to this theme, more gently — choose your companions the way you choose your books, for what they might make of you. Letter 10 opens with "First, withdraw into yourself as much as possible" — same impulse, different context. And in Letter 16, Seneca talks about philosophy as a kind of armor you build precisely for the moments when you can't avoid the crowd.
Modern echo
Bill Watterson. The creator of Calvin and Hobbes is one of the stranger success stories of the 20th century: at the height of the strip's fame — 18 million readers a day — he refused to license his characters for merchandise, sabbatical'd twice when his contract didn't require it, and then simply stopped, at 37, and disappeared. He didn't hate his readers. He was protecting the work from what fame does to work. In a rare 2013 essay he wrote: "We're not really taught how to recreate constructively... I've had to unlearn the assumption that busier is better." The crowd was offering him everything. He declined on behalf of something quieter. His 1990 commencement address at Kenyon College is essentially Letter 7 in disguise.
There's a version of solitude that's about hiding, and a version that's about returning to yourself so you actually have something to bring back. Seneca wasn't advocating isolation — he was advocating against dissolution. The question isn't how much time you spend with people. It's whether you come home still recognizably yourself.