Check out Simulacra and Simulation by Jean Baudrillard, a philosophical treatise that was a source of inspiration for the movie. The book appears as an Easter egg at the beginning.
Simulacra and Simulation is known for discussions of images and signs, and how they relate to our contemporary society, wherein we have replaced reality and meaning with symbols and signs; what we know as reality actually is a simulation of reality. The simulacra to which Baudrillard refers to are the signs of culture and media that create the reality we perceive: a world saturated with imagery, infused with communications media, sound, and commercial advertising. These simulacra of the real surpass the real world and thus become hyperreal, a world that is more-real-than-real; presupposing and preceding the real. In this world apathy and melancholy permeate human perception and begin eroding Nietzsche's feeling of ressentiment.