Socrates : Therefore such people {these prisoners in the cave} would consider nothing else to be the unconcealed than the shadows of fabricated things.
Glaucon: Absolutely!
The first section depicts the condition of human beings in the underground cave, which has its way out above, toward the daylight that nevertheless does not shine in. In the cave there are human beings chained by the legs and neck; they are forced to look straight ahead at the wall of the cave that faces them. Behind them burns a fire that casts a light. Between the fire and the prisoners there is a passageway behind a little wall; objects—implements and equipment—are carried back and forth along this passageway. Sometimes the carriers keep silent, sometimes they talk.
If there were an echo in the cave, then the prisoners would attribute the sounds of the words to the human beings they saw on the wall. This is the question: how does the presentation of this first stage end? With an explicit indication that what is at stake here is ἀλήθεια in the sense of the unconcealed. Socrates says that these prisoners would take nothing other than shadows of things to be the unconcealed. So the question is how these human beings relate and behave toward the ἀληθές, the unconcealed.
As strange as the condition of these human beings is, and as odd as the setting is, these human beings are nevertheless related to τὸ ἀληθές, to the unconcealed itself: human beings from childhood on, by their nature, are set forth into the unconcealed, no matter how strange their condition may be. Human beings are set forth in advance into the unconcealed, that is, into a connection to the things πρὸς τὸ πρόσθεν [facing what is in front of them]. To be human means to stand in the unconcealed and relate to it.
But precisely because of this, the question will arise: what is unconcealed to human beings in this condition? It is simply what they immediately encounter, what faces them. These are the shadows that the people behind them cast against the wall in the glow of the fire.
This presentation is ambiguous and calls for more precision. The prisoners see the shadows, to be sure, but they do not see them as shadows. What they see, we call mere shadows. They themselves are not in a position to call what shows up on the wall in front of them shadows. For this, they would have to know about the fire and about the light that it casts. Yet the prisoners cannot know anything about all this.