Also if you've ever pulled up at the traffic lights next to a car that's creeping forwards next to you, it can sometimes feel like your car is creeping backwards depending on where you're concentrating.

On the London Underground, if two trains are right next to each other and stationery, and the other train begins moving, it often feels like your train is the one actually moving. You usually figure it all out after a second or two when you realise you can't feel anything that you usually associate with movement, such as the feel of acceleration or the jolting of the carriage as it moves.

Usually, Opt-in systems have some sort of confirmation mechanism - if an unknown sender sends you mail, your whitelist system will send THEM an email asking them to confirm that they're a human. They'll then receive a token, keeping them in the whitelist for some predetermined amount of time.

That means you'll get no spam, but any human can mail you.

Seems like a pretty damn good way to go to me.