Returning to the VR definition above, the idea of ``fooling'' an organism might seem fluffy or meaningless; however, this can be made surprisingly concrete using research from neurobiology. When an animal explores its environment, neural structures composed of place cells are formed that encode spatial information about its surroundings [238,242]; see Figure 1.3(a). Each place cell is activated precisely when the organism returns to a particular location that is covered by it. Although less understood, grid cells even encode locations in a manner similar to Cartesian coordinates  (Figure 1.3(b)). It has been shown that these neural structures may form in an organism, even when having a VR experience [2,44,113]. In other words, our brains may form place cells for places that are not real! This is a clear indication that VR is fooling our brains, at least partially. At this point, you may wonder whether reading a novel that meticulously describes an environment that does not exist will cause place cells to be generated.
We also cannot help wondering whether we are always being fooled, and some greater reality has yet to reveal itself to us. This problem has intrigued the greatest philosophers over many centuries. One of the oldest instances is the Allegory of the Cave, presented by Plato in Republic. In this, Socrates describes the perspective of people who have spent their whole lives chained to a cave wall. They face a blank wall and only see shadows projected onto the walls as people pass by. He explains that the philosopher is like one of the cave people being finally freed from the cave to see the true nature of reality, rather than being only observed through projections. This idea has been repeated and popularized throughout history, and also connects deeply with spirituality and religion. In 1641, René Descartes hypothesized the idea of an evil demon who has directed his entire effort at deceiving humans with the illusion of the external physical world. In 1973, Gilbert Hartman introduced the idea of a brain in a vat (Figure 1.4), which is a thought experiment that suggests how such an evil demon might operate. This is the basis of the 1999 movie The Matrix. In that story, machines have fooled the entire human race by connecting to their brains to a convincing simulated world, while harvesting their real bodies. The lead character Neo must decide whether to face the new reality or take a memory-erasing pill that will allow him to comfortably live in the simulation without awareness of the ruse.
Steven M LaValle 2020-01-06