Philosophy
The Philosophy of Perception: Direct Realism and Representationalism
Quick fact
The debate between direct realism and representationalism was famously challenged by Plato's cave allegory, but the modern form of representationalism was cemented by John Locke's 'veil of ideas'—the view that we only ever perceive our own mental ideas, never external objects directly.
Why this is interesting
You look at a bright red apple, and instantly know it's red and round. But are you seeing the apple itself, or just a picture of it inside your mind? This question splits philosophers in a debate that's been running for centuries.