There is a world that exists—an uncountable number of differently-flavored quarks bouncing up against each other. There is a world that we perceive—a hallucination generated by about a pound and a half of electrified meat encased by our skulls. Connecting the two, or conveying accurately our own personal hallucination to someone else, is the central problem of being human. Everyone’s brain makes a little world out of sensory input, and everyone’s world is just a little bit different.