“Suppose that in a mountain glen there was a lake that was transparent, clear, and unclouded. A person with clear eyes standing on the bank would see the clams and mussels, pebbles and gravel, and schools of fish swimming about or staying still. They’d think: ‘This lake is transparent, clear, and unclouded. And here are the clams and mussels, pebbles and gravel, and schools of fish swimming about or staying still.’ ”