 Some have followed Dirac (1930) and von Neumann (1932) in assuming that a measurement is a physical process in which a quantum state almost never evolves linearly but rather changes discontinuously and stochastically into one of a variety of possible states, each of which may describe its outcome. But attempts to state precisely when such a process occurs and to verify its occurrence experimentally have been unsuccessful, and many understand quantum theory as excluding its occurrence.