Roger Penrose proposed that the Schrödinger–Newton equation mathematically describes the basis states involved in a gravitationally induced wavefunction collapse scheme. Penrose suggests that a superposition of two or more quantum states which have a significant amount of mass displacement ought to be unstable and reduce to one of the states within a finite time. He hypothesises that there exists a "preferred" set of states which could collapse no further, specifically the stationary states of the Schrödinger–Newton equation. A macroscopic system can therefore never be in a spatial superposition since the nonlinear gravitational self-interaction immediately leads to a collapse to a stationary state of the Schrödinger–Newton equation. According to Penrose's idea, when a quantum particle is measured, there is an interplay of this nonlinear collapse and environmental decoherence. The gravitational interaction leads to the reduction of the environment to one distinct state and decoherence leads to the localisation of the particle, e.g. as a dot on a screen.