3.4 Meditations by Marcus Aurelius
4. Don’t waste the rest of your time here worrying about other people — unless it affects the common good. It will keep you from doing anything useful. You’ll be too preoccupied with what so-and-so is doing, and why, and what they’re saying, and what they’re thinking, and what they’re up to, and all the other things that throw you off and keep you from focusing on your own mind.
You need to avoid certain things in your train of thought: everything random, everything irrelevant. And certainly everything self-important or malicious. You need to get used to winnowing your thoughts, so that if someone says, “What are you thinking about?” you can respond at once (and truthfully) that you are thinking this or thinking that. And it would be obvious at once from your answer that your thoughts were straightforward ans considerate ones–the thoughts of an unselfish person, one unconcerned with pleasure and with sensual indulgence generally, with squabbling, with slander and envy, or anything else you’d be ashamed to be caught thinking.