In order to solve most nontrivial problems, it is necessary to combine some of the basic problem solving strategies. It is often also useful to divide the problem that must be solved into smaller pieces and to solve those pieces separately.
Planning refers to the process of computing several steps of a problem solving procedure before executing any of them. If solution steps in the real world can not ignored or undone, planning becomes extremely important. Although real world steps may be irrevocable, computer simulation of those steps is not. So we can circumvent the constraints of the real world by looking for a complete solution in a simulated world in which backtracking is allowed. After we find a solution, we can execute it in the real world.
2. How to plan