Plato's Republic is a seminal work of Philosophy in which Plato, the great Greek thinker, expounded his ideas on knowledge and society. He sets the work as a dialogue between Socrates and his associates.
In Plato's view the world we inhabit is less real than the world of ideas. He expressed this view in the simile of the cave. The cave is composed of people chained so that they face a wall. Behind them is a bonfire, and between it and the prisoners people and objects pass, throwing shadows onto the wall. This is the world of shadows that normal humans see. Eventually a man escapes, reaches the surface and sees real objects. As his eyes become accustomed to light he can look at the sun. This simile [parable] shows Plato's view that only the world of ideas, the upper world, is real. All else that we see in this world are shadows or images of people and things.
For Plato the objects of this world participate in the forms/ideas, which exist in the upper word. Thus I as a male participate in the form of maleness. A cat has the form of catness. It is better to know the form than the individual being.Thus knowing a person is less valuiable than knowing the basic forms in which humans participate.
This led to his theory of knowledge, which he expressed in his simile of the divided line. Below the line At the bottom level there was the knowledge of fictions [eidones], which have no basis in reality. Above that, but still below the line, was pistis, belief. This is the knowledge of the world gained by experience, but it is a lower form of knowledge. Above the line was dianoia, the knowledge of forms/ideas not linked to the form of the good, the ultimate form/idea. Dianoia includes mathematical knowledge. Above this is episteme or noesis, the knowledge of forms dependent on the form of the good. This includes philosophy and justice and its components, such as truth and mercy. The form of the good is the highest form, the idea of goodness that all worthwhile things have in common. Only philosophers can reach so far as the grasp this form of the good.
Plato believed that fitness to rule society should depend on philosophical skill. So he designed what was to him an ideal society graded on ability. At the bottom were the hoi polloi, the many, who had no philosophical skills. Above them there were the guardians. These were divided into two levels. The guards would control the many and defend society, but they were drawn from guardians with no grasp of philosophy. Above them there were the guardians proper, the philosophers. They were to be chosen early in life and taken away from their families for training in philosophy. The highest of them would be the philosopher-king, who would rule. Plato was quite advanced for his time in believing that there might be some women among the guardians. However, guardians would live in common and share the women between them, and no child would know its own father. No society has ever really been run on Plato's lines, though he has been influential, especially in the early mediaeval church, where the elite of celibate priests ruiled the many [or tried to.]
One of Plato's similes was the simile of the ship, in which a mutinous and argumentative crew each try to steer the ship, resulting in disaster. This is a metaphor for democracy, which Plato hated, because it gave power to non-philosophers. Only the true navigator, the philosopher-king, should be allowed to steer. Hel aso gave a simile of a wild beast, which is the mob, which he identified with the bulk of the people. The keepr keeps bribing the beast rather than controlling it. This, for Plato is how democratic politicians fail, because they appease rather than control the people.