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One very widespread way of interpreting the Republic is to see Plato as concerned mainly or exclusively with the task of refuting the moral sceptic. On this view, what dominated Plato was the fragmentation of accepted moral consensus, and the erosion of confidence in familiar moral values, that he could see around him. The job he saw as primary was that of showing, against the sceptic, that there are objective moral truths, and that it is worth while accepting constraints on one’s self-aggrandizement because the moral order can in fact be relied upon. Traditional confidence in traditional values is to be re-established .

Now there is something to this interpretation. The bulk of the book is put forward as an attempt to answer Thrasymachus, who claims that the life of injustice is more worthwhile than the life of justice ; and he is one of the sophists whom Plato regards as producing scepticism about the value of justice. Thrasymachus derides conventional moral standards ; nobody with any intelligence, he maintains, would pay any attention to them if he could get away with violating them. Justice, the behaviour commonly regarded as right, is a mug’s game, because it can only benefit others ; it is not rational to act against your own interests, and so no-one with any sense would  take morality seriously.

This is the point of view reformulated by Glaucon and Adeimantus as the challenge Socrates has to meet, and most of the rest of the book is taken up by Plato’s attempt to show that it is wrong. And he takes care to claim that anyone j ust by his account would also be just according to the common moral understanding of the term . So there is indeed some temptation to see Plato, as he is on this interpretation, as a conservative, who thought that what was wrong in existing society was the corrosive scepticism produced by Thrasymachus and his like, and who offered the remedy of a social ordering that would reweld the shattered moral structure of society and make people unable and unwilling to question the truth of objective and accepted moral judgements.

Julia Annas, An Introduction to Plato’s Republic, pages 8 & 9