  Socrates’ strategy depends on an analogy between a city and a person. There must be some intelligible relation between what makes a city successful and what makes a person successful.  But to answer the Republic’s question, Socrates does not need any particular account of why the analogy holds, nor does he need the analogy to hold broadly (that is, for a wide range of characteristics). It works even if it only introduces an account of personal justice and happiness that we might not have otherwise entertained.