Cassandra proceeds as described in the previous section until either the plan is completed or an uncertainty is introduced. This section describes how uncertainties are introduced and how they are handled.
As an example of a plan involving an uncertainty, let us consider a version of Moore's classic ``bomb in the toilet'' problem [McDermott 1987], in which the goal is bomb is disarmed, and the initial conditions are bomb in package1 or bomb in package2. The uncertainty in this case lies in the initial conditions: depending on the outcome of the uncertainty, the start operator can either have the effect that the bomb is in package1 or the effect that the bomb is in package2.