The Republic contains Plato's Allegory of the cave with which he explains his concept of The Forms as an answer to the problem of universals.

The allegory of the cave primarily depicts Plato's distinction between the world of appearances and the 'real' world of the Forms, as well as helping to justify the philosopher's place in society as king. Plato imagines a group of people who have lived their entire lives as prisoners, chained to the wall of a cave in the subterranean so they are unable to see the outside world behind them. However a constant flame illuminates various moving objects outside, which are silhouetted on the wall of the cave visible to the prisoners. These prisoners, through having no other experience of reality, ascribe forms to these shadows such as either "dog" or "cat".

Plato then goes on to explain how the philosopher is akin to a prisoner who is freed from the cave. The prisoner is initially blinded by the light, but when he adjusts to the brightness he sees the fire and the statues and how they caused the images witnessed inside the cave. He would see that the fire and statues in the cave were just copies of the real objects; merely imitations. This is analogous to the Forms. What we see from day to day are merely appearances, reflections of the Forms. The philosopher, however, will not be deceived by the shadows and will hence be able to see the 'real' world, the world above that of appearances; the philosopher will gain knowledge of things in themselves. In this analogy the sun is representative of the Good. This is the main object of the philosopher's knowledge. The Good can be thought of as the form of Forms, or the structuring of the world as a whole.