Book 1 reads in many respects as an earlier work of Plato which has been re-used to serve as an introduction to the main body of the Republic.
It has the form of a Socratic dialogue typical of Plato’s earlier works, but from which he has moved away by the time of the main body of the Republic
(a) It contains genuine dialogue, with people other than Socrates expressing their own ideas and quite fully developed as characters. Except for the presentation by Glaucon and Adeimantus at the beginning of Book 2, the rest reads as an interrupted monologue by Socrates, with the rest having no ideas or distinctive character of their own
(b) It shows up a so-called expert as no knowing what he thought he did, and it ends inconclusively with Socrates admitting that he still does not know the answer to the main question (354b). By contrast, Socrates is set up near the beginning of Book 2 (258a and 368a-d) as the expert who can answer the questions, though he is initially reluctant to admit it, and he proceeds to expound his ideas for the rest of the work.
(c) The subject matter is the definition of a virtue. The rest of the Republic, apart from giving the definition (which never happens in the early works), extends into provisions for the ideally just state, a theory of knowledge and being, a theory of the soul, a classification of political constitutions etc)
The rest of the Republic is presented as Socrates’ response to the challenge raised by Glaucon and Adeimantus in Book 2, and therefore, strictly speaking, Book 1 is irrelevant to the argument. The attempt to make Book 2 follow naturally from Book 1 seems artificial: Socrates is presented as having refuted Thrasymachus’ argument in Book 1 while still not knowing at the end what justice is. Glaucon makes it clear (357b) that he is not convinced by Socrates’ arguments against Thrasymachus. (Is Plato perhaps signaling that he is no longer satisfied with the arguments of Book 1 himself?) Both Glaucon (358b-c) and Adeimantus (367a) are made to present their case as an extension of Thrasymachus’ arguments, yet while their general standpoint is very similar to his, they use arguments that are very different. Glaucon, for instance, presents justice as a particular kind of good, whereas Thrasymachus did not regard it as good at all. They also present their respective cases at great length as though the issues had not been thoroughly aired before.
Book 1 may well have started out independently, but it does in fact introduce some of the issues of Books 2 – 10, possibly even adapted by Plato for the purpose.
It has been suggested that Plato was sensitive about his refusal to follow a political career in Athens. Instead he constructs an ideal political world, but is at pains to present it as the answer to the shortcomings of the world he actually lives in. So he has thought through and rejected the conventional ideas of Cephalus and Polemarchus, and the radical sophistic ideas of Thrasymachus and those who have influenced Glaucon and Adeimantus, as well as the Athenian political systems too (Book 8).
Cephalus is someone for whom the amassing of wealth has taken precedence over thinking about moral issues or active citizenship. Thrasymachus poses as an expert on moral and political philosophy, and charges a heavy fee for his supposed expertise. As the Republic develops we find that the people who are to be involved in political life are the true experts in moral and political philosophy, and they are neither engaged in any wealth creating activity nor even allowed to own private property.
Cephalus also raises the question of old age. Now that he is old – too old for sexual passion – he claims he feels disposed to take part in philosophical discussion. He makes a quick exit though when the discussion gets difficult. Plato also reserves philosophy until the end of a very long education, and his philosopher rulers are older people; however, they are limited to those who are by nature always able to restrain their appetites.
Cephalus regards justice as relatively unimportant – one can wait till old age before thinking much about it. Polemarchus sees it as a skill aiming at a trivial end. Already in Book 1 (335c) Plato describes justice as the human excellence, and this view is developed in the account of the individual justice in Book 4. A shortcoming of Polemarchus’ view is that justice could be a skill for doing harm – i.e. goodness is not essential to his concept of justice. Thrasymachus does not regard justice as good at all. Plato’s undertaking as the main item on the Republic’s agenda is to show that justice is good both in itself and in its consequence. Of course Glaucon also questions the goodness of justice in Book 2; perhaps Plato wants to use Book 1 to emphasise that people in general only pay lip service to justice as an important human virtue, and so the argument of the Republic is very necessary.
While both Glaucon and Thrasymachus admire the tyrant as the supremely happy man, it is Thrasymachus who has more to say about him specifically as a ruler. In some respects he gives us a distorted anticipation of Plato’s own later theory. The ruler alone is in a position to define the nature of justice by making it up to suit himself. He is like a shepherd who is concerned about his flock – to fatten them up for his own peofit. In Socrates’ reply to this (347) Plato anticipates his later demand that the guardians will have to sacrifice their own preferences and be compelled to rule. In 342 d-e Plato introduces his favourite analogies for the role of ruler, namely those of the doctor and the ship’s capatain.
A fundamental feature of Thrasymachus’ position is his assumption that society is a battleground of conflicting individual interests. Plato is constantly at pains to emphasise the importance of unity in the ideal state, so it has to be a fundamental feature of his theory that there is no conflict between people’s real interests at all.