A single example of extravagance or greed does a lot of harm--an intimate who leads a pampered life gradually makes one soft and flabby; a wealthy neighbor provokes cravings in one; a companion with a malicious nature tends to rub off some of his rust even on someone of an innocent and open-hearted nature--what then do you imagine the effect on a person's character is when the assault comes from the world at large? You must inevitably either hate or imitate the world. But the right thing is to shun both courses: you should neither become like the bad because they are many, nor be an enemy of the many because they are unlike you. Retire into yourself as much as you can. Associate with people who are likely to improve you. Welcome those whom you are capable of improving. The process is a mutual one: men learn as they teach.

--Seneca, Letter VII