which is itself a representation of Plato’s ontological theory.
The allegory centers on a fictional group of slaves held captive in a cave their entire lives. They are chained in a seated position so that all they can see is directly in front of them. Behind the slaves is platform with a wall upon it, and behind the wall people walk back and forth carrying objects that stick up above the wall, so that the objects act like shadow puppets. Because of the light source in the cave, the objects cast shadows on the wall of the cave in front of the slaves. Those carrying the objects speak to one another and their voices bounce off the wall before the slaves appearing to make the shadows speak. All the slaves have ever known are the shadows in front of them, and the sounds they believe the shadows make.
Suddenly, one of the slaves is unchained and forced to turn around facing the platform and the shadow puppets protruding above the wall. After his eyes adjust to the sudden glare of light, he slowly comes to realize that the shadows were not as real as he had formerly believed. They are in some sense caused by the objects that he had previously never seen.
Next the slave is forced up, past the platform, and out of the cave into the “real” world. Of course, at first his vision is unclear as he is not used to the brightness of the outside world; it takes some time for his vision to adjust. But, eventually, as his vision improves, the slave sees the objects of the real world and comes to realize that even the “real” objects from the cave (i.e., the shadow puppets) were themselves only replicas of the true reality he now sees clearly.
Feeling sorry for his companions who remain chained below ground, the slave returns to the cave to try and set his fellows free. But upon returning he finds his vision so unclear that his companions think he has gone mad by leaving his place and traveling to the other “world”. They mock him and refuse to leave the world of shadows that is their “reality”; they even threaten to put him to death if he should try to set anyone else free.
In the allegory we see four distinct stages that correspond to the four levels of the Divided Line: the slaves watching the shadows corresponds to the world of reflections and images. The platform with its “real” people and objects which cast shadows on the wall before the slaves represents the world of physical objects which we know through our senses. The world beyond the cave represents what is “really” real, or at least more real than either the shadow puppets and their shadows. This is the world of numbers on the Divided Line and is filled with Ideas, or universals which are, for Plato, more real than physical objects. Finally there is the sun which illuminates everything outside the cave and makes it knowable. This represents Plato’s idea of the Being itself, the source of all reality and that, in light of which, all things which can be known are knowable.
What we discover in the Divided Line and the Allegory of the Cave, is that Plato is a metaphysical dualist. He believes that reality is composed of two different kinds of substances: material substance, and non-material substance. Since time and space (whereness and whenness) are qualities of matter, any object made of matter (i.e., anything in the world of shadows or objects) will be limited by temporality and location. However, things made of the non-material substance will not be subject to these limitations because they are composed of a completely non-physical stuff. Therefore, these things (i.e., Ideas) will not be subject to spacio-temporal limitations. That is, they are not changeable, and therefore they must not be created or destroyed in the way things made of matter are. Because they are more permanent, they are more, they have more being, they are more real.
The epistemological implications of the ontology outlined above should become clear. Since what is knowable must be true, and if what is true is rooted in what is, if something has more is-ness, it will be more knowable. More simply put, for Plato there is a hierarchy of knowledge corresponding to the hierarchy of being. Things with the least amount of being (e.g., images, shadows, representation of all kinds) will be the least knowable kinds of things, while things with more being (e.g., Ideas, mathematics, etc.,) will be more knowable (precisely because they have more is-ness). Indeed, in these passages Plato distinguishes four different cognitive states (i.e., types of knowing) associated with each of the levels of the divided line (and presumably with the allegory): imagination (eikasia), belief (pistis), intellect (dianoia), and reason (noesis). Knowing should only be asserted of the latter two since these are not subject to error or change while the former two are more associated with what we would commonly call opinion which is changeable and often false).
Watch a classic animated interpretation of the Allegory of the Cave.
Read Republic 506-521 which contains both the Divided Line and Allegory of the Cave in context.