We don't attend to everything we see. As Andy Clark has written in Natural Born Cyborgs, our brains don't bother to create rich inner models of the world. We can't create such models because there is simply too much information out there for us to process it all. Further, the world is generally available to revisit periodically, so why bother? In other words, we use the world as its own best model--we "cheat" (because we must cheat). But this cheating can be exposed through dramatic experiments. I've previously posted on some of those experiments here (by Quirkology). Here are some additional experiments demonstrating our need to "cheat." In this experiment, done by Dan Simons and Dan Levin, only 50% of the subjects noticed that the person to whom they were talking had changed. Here are additional demonstrations by Simons, along with a bit of explanation. Here's a more elaborate write-up on the "door" experiment. And see here. When I use the word "cheat," I'm being facetious. Our strategy of using the world as an adequate model is one of the many cognitive heuristics we must use in order to survive.