Current concepts of delusion rely on the assumption that one single objective external reality exists as a benchmark for our internal experiences. With the advent of quantum theory which holds that reality within the atom is probabilistic and observer-dependent this assumption has become untenable. However, the deduction that reality in general is thus informational rather than material in nature seems also absurd and incompatible with our daily experience. Drawing a line between the subatomic world and the macroscopic is equally impossible. To do so would also be essentially reductionist in nature assuming that reality is built hierarchically. The many worlds / multiverse interpretation of quantum theory allows for an objective material reality but come at the price that many universes simultaneously exist in parallel. But the notion of an infinite number of objective realities is as disconcerting as the idea of a reality born out of observation only. The idea of parallel universes though may become more realistic with the advance of virtual reality. Virtual reality is a way to systematically alter the usual relationship between external reality and the user's experience of it. In future, psychiatrists may thus find it more difficult to diagnose psychosis based on positive symptoms alone. It will become necessary to rethink our currently accepted concept of psychosis by the time we master technologies to create sophisticated virtual reality environments individualizing human experience. In the meantime, there is no substitute for meticulous history taking and accurate clinical observation.