You know,' the Doctor said, resuming his pacing, 'how sometimes you only appreciate something when it is taken away from you.' 'You mean my freedom?' 'I mean more like the hum of the central heating or the air-conditioning. You only notice it was there when it stops. While it's constant, part of the nature of the things, it's unremarkable. Just the way things are. Your brain doesn't even brother to tell you about it, unless there is a change that might be important.