The first part of his analogy of the cave considered society, and what society 'knew' as reality. For in his cave were shackled prisoners, forced to face a cave wall for the entirity of their lives, from birth to death. The only thing visible to these prisoners were the shadows cast by the fire far behind them. This perfectly symbolises society and how our senses can never be trusted as the public can only ever know what we are told is going on, and are forced to only witness the false reality if that is what is told to them.
The fire behind these prisoners cast the shadow of objects that pass it, upon the wall in front of the prisoners. However the fire is so high that the only shadows it can cast is of the inanimate objects that the traders that pass through the cave carry upon their heads. This is symbolism for the false reality that is told to our public. For we, as readers, see the larger picture of what is really taking place but as a prisoner that cannot possibly know any better, these inanimate objects are what the world is, and so, what reality is. This is what they see and this is why our senses cannot be trusted, for their senses tell these prisoners that, what are really lies, are facts. Therefore when, in the analogy, one of the prisoners escape their bonds and escape out of the cave