  John Wheeler devised a gedanken experiment in which a piece of apparatus can
be altered just before the arrival of particle, and this ``delayed choice''
can, seemingly, alter the quantum state of the particle at a much earlier time,
long before the choice is made.
  A slightly different gedanken experiment, which exhibits the same conceptual
difficulty, is analyzed using the techniques of consistent history quantum
theory. The idea that the future influences the past disappears when proper
account is taken of the diversity of possible quantum descriptions of the
world, and their mutual compatibility or incompatibility.
