It's that undefined something we're really afraid of-the flicker of movement we don't quite catch out of the corner of our eye, the bad dream we can't quite remember when we wake up, the sound of a door opening downstairs we thought we heard. And worst of all, the things we're not sure even happened, the things that we might just have imagined, that might mean we're going mad, all those nameless, nebulous things we can't quite put our finger on and can only guess at.