NICHOLAS D. SMITH AN ARGUMENT FOR THE DEFINITION OF JUSTICE IN PLATO'S Republic (433E6-434A1)* (Received 11 January, 1978) In Book IV of the Republic, Plato offers a definition of justice. Specifically, justice is there defined as a certain kind of doing one's own (% ~avrof~ 7rpdrreLu). As it is explicated in Book IV, it is characterized in the following two ways: (i) (social justice) -justice in the 7rdX~r is every citizen performing one social service in the state for which his nature is best adapt- ed (433A). (ii) (personal justice) - justice in the soul is each of the three parts of the soul performing its proper function (e.g. reason ruling, spirit and appetite being subject to reason's rule (441E)). Plato offers a series of arguments which purport to show that this characteri- zation of social justice correctly defines justice in the state. The first is fairly straightforward, relying on principles to which Socrates and Glaucon had agreed all along. Justice, they agreed, is that virtue of the state that allows the other three (soberness, courage, and wisdom) to come into being in the state, and that preserves them once they have come into being. What effects these things. is everyone doing his own. Therefore, justice is everyone doing his ow n (433B-E). The second argument for this immediately follows the first, but unfortunately is neither straightforward nor even transparently valid. After presenting the argument and explaining why it raises questions of interpretation, I will consider a view that purports to display those principles upon which the validity of the argument rests, and show why I find that inter- pretation problematic. I will then offer another interpretation that can both preserve the validity of the argument and avoid the difficulties I found in the interpretation I criticize. PhilosophicalStudies 35 (1979) 373-383. 0031-8116/79/0354-0373501.10 Copyright 1979 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.