Plato drew a single line to rank hallucinations, objects, and mathematical truths
In the Republic, Plato has Socrates describe a line cut into four unequal segments, each a different grade of knowledge: shadows and reflections at the bottom, physical objects above that, mathematical reasoning above that, and understanding of the Forms at the top. Each higher segment is supposed to be as much clearer and more real than the one below it as that one is than the segment below it — one image meant to rank not just what we know, but how real the things we know about actually are.