在《理想國》第二卷一開始（357d4-358a3）將正義歸為善的第二類—其自身為好，且會帶來好的結果—之後，對話者葛勞孔與阿蒂曼圖便要求柏拉圖就此分類進行更詳盡的說明。本篇論文的目的便是就柏拉圖對正義的定義進行探討，說明他之所以認為正義本身為好，且值得我們擁有的原因。我將試著首先證明柏拉圖藉由對正義定義的論述，的確說明了正義本身的價值；其次，論述柏拉圖對政義的觀點，適切地回應了第二卷中的對話者們的問題。與此同時，我也希望能夠指出佛斯特（M. B. Foster）教授在化的文章〈柏拉圖在《理想國篇》的一個錯譣〉（A mistake of Plato’s in the Republic）之中的不適當論證。

After classifying justice into the second category of “good”—it is welcomed not only for its own sake, but also for its consequences—at the beginning of the Republic bookⅡ (357d4-358a3), Glaucon and Adeimantus then ask Plato to explain this classification more explicitly. The purpose of this paper is to discuss the definition of justice in the Republic and to explain the reason why Plato thinks that justice is itself good and worth having. I would like first to show that Plato does give an account of the value of justice itself by presenting an argument for the definition of justice, and then to demonstrate that Plato’s view of justice properly answers the question proposed by his interlocutors in booksⅡ. In so doing, I hope also to point out the inadequate argument in Prof. M. B. Foster’s paper “A mistake of Plato’s in the Republic”.