Fon the past couple of minutes you have (let’s suppose) been riding on an escalator; with little else to do, you’ve been gazing absent—mindedly at the advertisements on the wall as they slowly and smoothly slide by. How would your visual experience change if the escalator were to come to a stop? It’s not diilicult to predict: the advertisements would stop sliding by and you would find yourself staring at a single motionless patch of wall. And it would not be long before you would find yourself getting more than a little bored.