A fundamental role in our study of actions is played by the situational fluent
Here, p is a person, is an action or more generally a strategy, and s is a situation. The value of is the situation that results when p carries out , starting in the situation s. If the action or strategy does not terminate, is considered undefined.
With the aid of result we can express certain laws of ability. For example:
This formula is to be regarded as an axiom schema asserting that if in a situation s a person p has a key k that fits the safe sf, then in the situation resulting from his performing the action opens(sf,k), that is, opening the safe sf with the key k, the safe is open. The assertion fits(k,sf) carries the information that k is a key and sf a safe. Later we shall be concerned with combination safes that require p to know the combination.