Imagine you are standing on the Grand Canyon Skywalk, a horseshoe-shaped bridge suspended 1,200 meters above the Colorado River. You are likely to get dizzy and freeze up at the thought of venturing out onto the 10-cm thick glass.

Nevertheless, you step forward and breathe a sigh of relief after realizing you are not tumbling toward the river.

But how can you be sure what you see is really there? And more fundamentally, how do we know that we live in the real world and not a virtual one?