What does this Ralph Waldo Emerson aphorism mean: "Nothing can bring you peace but yourself. Nothing can bring you peace but the triumph of principles."?
He's encouraging self-sufficiency and individualism.
"Nothing can bring you peace but yourself" means that only you can make yourself happy- i.e., you can't- or shouldn't- count on anyone or anything else to bring you content.
"Nothing can bring you peace but the triumph of principles" means that you won't be happy if you do something that goes against your morals. Emerson is advocating for people to follow their own consciences instead of allowing themselves to be pressured by other people.