 Those who believe that a quantum state completely describes the system to which it is assigned and that this state always evolves linearly (e.g., according to the Schrödinger equation) face the notorious quantum measurement problem: Application of quantum theory to the interaction between a quantum system and a quantum measuring device would almost always leave these in a state that describes the measurement as having no outcome, contrary to the direct experience of countless experimentalists (see entry on  philosophical issues in quantum theory,  §4).