Our world is filled with things that are neither mysterious and ghostly nor simply constructed out of the building blocks of physics. Do you believe in voices? How about haircuts? Are there such things? What are they? What, in the language of a physicist, is a hole—not an exotic black hole, but just a hole in a piece of cheese, for instance? Is it a physical thing?… These things are not physical objects with mass, or a chemical composition, but they are not purely abstract objects either—objects like the number π, which is immutable and cannot be located in space and time. These things have birthplaces and histories. They can change, and things can happen to them.
Daniel Dennett, from the Introduction to The Mind’s I