The first thing to know is that the mind isn't a mirror, or even a passive observer of reality. Much of what we think of as being out there actually comes from in here, and is a byproduct of how the brain processes sensation.

In recent years scientists have come up with a number of simple tricks that expose the artifice of our senses, so that we end up perceiving what we know isn't real - tweaking the cortex to produce something uncannily like hallucinations. Perhaps we hear the voice of someone who is no longer alive, or feel as if our nose is suddenly 3 feet long.