Never contract friendship with a man that is not better than thyself.

Company shapes you more than intention does. Spend enough time around people who are more honest, more disciplined or more generous than you and the standard quietly rises; spend it around the reverse and it quietly falls. That is the durable part, and it does not require ranking anybody, which is a cold way to think about friends and a poor basis for affection.

A softer version of the same test still works. Ask who you are when you are with each of the people you see most, and whether you like that person. You are allowed to love somebody who makes you no better, and it is worth knowing which friendships are which.