Prisoners who have only ever seen shadows mistake them for the whole of reality
Plato describes prisoners chained since birth facing a cave wall, watching only the shadows cast by objects passed behind them, and mistaking the shadows for real things because they've never seen anything else. If one prisoner is freed and dragged into the sunlight, he'd be blinded and disoriented before finally grasping that the shadows were only ever imitations, Plato's picture of what philosophical education does to an unquestioned mind.