" For each visual input, it takes a tiny but perceptible amount of time—about two hundred milliseconds, one-fifth of a second—for the information to travel along the optic nerves and into the brain to be processed and interpreted. One-fifth of a second is not a trivial span of time when a rapid response is required—to step back from an oncoming car, say, or to avoid a blow to the head. To help us deal better with this fractional lag, the brain does a truly extraordinary thing: it continuously forecasts what the world will be like a fifth of a second from now, and that is what it gives us as the present. That means that we never see the world as it is at this very instant, but rather as it will be a fraction of a moment in the future. We spend our whole lives, in other words, living in a world that doesn’t quite exist yet. "

― Bill Bryson , The Body: A Guide for Occupants