A planner simply constructs a plan and may be a machine or a human. If the planner is a machine, it will generally be considered as a planning algorithm. In many circumstances it is an algorithm in the strict Turing sense; however, this is not necessary. In some cases, humans become planners by developing a plan that works in all situations. For example, it is perfectly acceptable for a human to design a state machine that is connected to the environment (see Section 12.3.1). There are no additional inputs in this case because the human fulfills the role of the algorithm. The planning model is given as input to the human, and the human ``computes'' a plan.