"What we see are only shadows of the truth"
In 'The Republic', Plato describes prisoners chained in a cave since birth, facing a blank wall. Behind them is a fire, and objects passing by cast shadows on the wall. To the prisoners, these shadows constitute reality. They name them and study them, unaware of the true objects casting them.
Plato argues that our physical world is like the shadows—an imperfect, changing copy of the perfect, eternal World of Forms (Ideas). True knowledge is not checking the shadows (sensory perception) but grasping the Forms (intellectual insight).
One prisoner escapes and sees the Sun (The Form of the Good). He is blinded at first but eventually sees reality. When he returns to the cave to free others, they mock him and try to kill him. This represents the plight of the philosopher (like Socrates) who tries to enlighten a society content with illusions.