David Pym, Louise Pryor and David Murphy
The world is a dynamic place. Things change because of our actions and because of others’ actions. Moreover, we have no privileged status: others are able to act without our permission in unexpected ways, without explanation. We contend that it is both natural and necessary to give an account of planning in such a world. Classical theories of planning are essentially static | they assume that the world changes only as a result of the planner’s actions, that it has complete knowledge of the consequences of its actions and that the environment is predictable. Thus we contend that classical theories of planning are inadequate for domains with dynamic environments and evolving plans | essential features of problem-solving in the real world. We propose a solution based on the algebraic theory of processes that shifts the focus of attention in the account of planning from changes of state to the processes by which transitions occur. One can ask what actions are required to bring about a given change of state; one can also ask what changes have been brought about by a given action. What must Dr. Who  do in order to conquer the Daleks? What happens if the Doctor attempts to destroy the Daleks? The description of complex actions via the algebraic theory of processes allows one to see that the latter is superior to the former.