In this paper, I wish to explore a certain tension I find in Plato's Republic between two competing conceptions of human nature. One of these is set forth explicitly; the emphasis Plato gives it strongly indicates that he conceives of it as his “official” theory. The other is merely hinted at, or presupposed, by certain things he says about pre-social man. Since this is so, it may be more prudent for me to speak at this stage about two different accounts of human nature which occur in the Republic, leaving it open and thus as subject to proof whether the accounts do embody disparate conceptions. Accordingly, I will set myself two tasks here: first, to establish that Plato does espouse two such conceptions of human nature, and then to show how they differ from one another; and second, to explore some of the implications of this analysis for the political theory he constructs in the Republic. I will attempt to show in the light of these efforts that the plausibility of his political theory depends upon a subtle vacillation between these conceptions.