Meditations by Marcus Aurelius translated by George W. Chrystal (~180) Book 11 Part 5 (19-39)
Bonus:
Summary: Listen to smart people from the past.
Commentary:
21. He whose aim in life is not always one and the same cannot himself be one and the same through his whole life. But singleness of aim is not sufficient, unless you consider also what that aim ought to be. For, as there is not agreement of opinion regarding all those things which are reckoned good by the majority, but only as regards some of them such as are of public utility; so your aim should be social and political. For he alone who directs all his personal aims to such an end can reach a uniform course of conduct, and thus be ever the same man.