According to Cicero's De Officiis, the Stoic philosopher Panaetius discusses the notion of proper conduct in terms of the theory of four personae, i.e. rationality, personal characteristics, circumstance and chance, and choice. The purpose of this paper is twofold: firstly, the content and connotation of the theory will be explored; secondly, by virtue of the discussion of the theory, the contemporary content-free slogan ‘Be Yourself' could hopefully be provided with substance. This paper therefore consists mainly of three parts: The first part of this paper explores the four personae respectively, and discusses the problem: Whether Panaetius's theory is his own creation or influenced by some other philosophers? The second part deals with the issue: How can the four personae harmoniously work together? The third part focuses on the problem: What kind of life will one have, when he can make his four personae harmoniously work together?