the agreement between reality and the concepts we use to conceive it arises not because our mental concepts have come to passively mirror reality, but because reality must conform to the human mind's active concepts to be conceivable and at all possible for us to experience. Kant thus regarded the basic categories of the human mind as the transcendental "condition of possibility" for any experience.[6]

Anyone with normal color vision agrees that blood is roughly the same color as strawberries, cardinals and the planet Mars. That is, they're all red. But could it be that what you call "red" is someone else's "blue"?

Such prisoners would mistake appearance for reality. They would think the things they see on the wall (the shadows) were real; they would know nothing of the real causes of the shadows