The guardians are philosopher-kings, not to be confused with contemporary philosophers, who are more accurately called “lovers of sights and sounds.” These lovers of sights and sounds are drawn only to the appearance of things, whereas true philosophers have knowledge of the unchanging, eternal Forms that lie behind appearance. The world of sights and sounds consists of objects that both are and are not—for example, a beautiful woman is both beautiful and, in comparison to a goddess, not beautiful. Therefore, the things we see and hear are objects of opinion or belief; maybe they’re beautiful, maybe not. The world of Forms, such as the Form of Beauty, however, has being in an absolute sense, and these Forms are the objects of knowledge.

The highest knowledge to which the philosopher-kings aspire is knowledge of the Form of the Good. Socrates cannot articulate directly what this is but rather explains it by offering three analogies: the sun, the line, and the cave. Socrates invites us to imagine prisoners chained to a bench in a cave. All they can see are the shadows moving on the wall in front of them, which are cast by statues being moved above and behind, where the prisoners cannot see. Not knowing any better, these prisoners think of the shadows as real, like a person seduced by the imaginative world of stories, unable to recognize a higher reality. If the prisoners were released, they could turn around and see that the shadows they thought were real were only projections of the statues behind them. They would then think of these statues as real, like a person who thinks the world of sights and sounds is the most real thing there is. The prisoners might then wander out of the cave and into the outside world. At first, they would be blinded by the light, but they would eventually come to see all the objects of the world around them. They would think of these objects as real, like a person who can grasp by means of thought the Forms that underlie everyday existence. Finally, these prisoners might be able to look at the sun itself and recognize it as the source of all light and all life. The sun is like the Form of the Good: just as the sun is the source of everything in the visible world, the Form of the Good is the source of everything in the intelligible world.

Socrates invites his friends to imagine a line divided first in two and then in four. The lower part represents the visible realm and the upper part represents the intelligible realm. The visible realm is divided into imagination and belief, belief being better than imagination just as seeing the statues is better than seeing the shadows. The intelligible realm is divided into thought and understanding, where thought hypothesizes the existence of Forms based on the visible world and understanding grasps the Form of the Good as a first principle from which everything else follows. The divided line is diagrammed in the following figure. The corresponding stages in the prisoner’s escape from the cave are in parentheses.

The education of the philosopher-kings is like the progress of a prisoner from out of the cave. In youth, they study mathematics to give them an intimation of an abstract world behind the visible. After rigorous physical training, they study philosophy and then dialectics. At thirty-five, they spend the next fifteen years running affairs of state before finally achieving the rank of philosopher-king at fifty. These philosopher-kings are like the prisoners who can see the sun, and contemplation of the Form of the Good will be their highest aim. However, they must also take care of the republic and train the next generation, just as freed prisoners must return to the cave to help their comrades.

Because the guardians will inevitably make some errors in judgment, this ideal republic will gradually decline through four stages of progressively worse government: timocracy, oligarchy, democracy, and tyranny. Similarly, a just man can slide into four kinds of degeneracy, tyranny being the worst. People who take whatever they can to please themselves live like tyrants, so people who follow Thrasymachus’s “advantage of the stronger” are worst off of all people. Only philosophers live just lives because only they have the ability to recognize the true pleasure to be found in the love of truth. All other pleasures are really just the cessation of pain.

Poets are banished from Socrates’ republic because they portray falsehoods and appeal to our emotions and baser instincts in a way that corrupts us. Socrates regrets this necessity and invites others to persuade him not to banish poets.