What are the actions?

Quantify all of the primitive actions or events that are sufficient to describe all necessary changes in solving a task/goal. No uncertainty associated with what an action does to the world. That is, given an action (also called an operator or move) and a description of the current state of the world, the action completely specifies (1) if that action CAN be applied to the current world (i.e., is it applicable and legal), and (2) what the exact state of the world will be after the action is performed in the current world (i.e., we don't need any "history" information to be able to compute what the new world looks like).

Note also that actions can all be considered as discrete events that can be thought of as occurring at an instant of time. That is, the world is in one situation, then an action occurs and the world is now in a new situation. For example, if "Mary is in class" and then performs the action "go home," then in the next situation she is "at home." There is no representation of a point in time where she is neither in class nor at home (i.e., in the state of "going home").

The number of operators needed depends on the representation used in describing a state (see below). For example, in the 8-puzzle, we could specify 4 possible moves for each of the 8 tiles, resulting in a total of 4*8=32 operators. On the other hand, we could specify four moves for the "blank" square and there would need to be only 4 operators.