This paper identifies major inconsistencies in the threefold argument that Glaucon presents in defence of Thrasymachus in the second book of Plato’s Republic. Specifically, the paper argues for three claims. Firstly, it argues that in his account of the origin of justice Glaucon treats the consequences of justice as necessary, while in the test case he merely emphasizes incidental consequences. Secondly, the paper argues that in setting up the test case of the perfectly unjust man and the perfectly just man Glaucon—despite claiming to be restating Thrasymachus’ claims, and also to be presenting the ordinary view of justice — uses some Socratic assumptions about the nature of justice. That is, although Gyges’ example follows directly from the first part of the argument and, in particular, from the claim that justice is grounded in convention, the unshakably just man of the test example is supposed to be motivated by justice itself. Finally, it is argued that the test case and Gyges’ example are at variance with respect to public recognition: in the test case Glaucon suggests that most people would respect and admire a man who seemed to be perfectly just, while in his discussion of Gyges he intimates that tyrannical power strengthens itself not by the false appearance of perfect justice, but by appealing to a secret and yet widespread admiration for unpunished injustice.