Discrete State

Discrete State

Most applications of process ontologies are used to represent dynamic behaviour in the world so that intelligent agents may make predictions about the future and explanations about the past. In particular, these predictions and explanations are often concerned with the state of the world and how that state changes. The Discrete State core theory is intended to capture the basic intuitions about states and their relationship to activities.

The definitional extensions of this theory use different constraints on possible activity occurrences as a way of classifying activities with respect to preconditions and effects.

The basic ontological commitments of the Discrete State Theory are based on the following intuitions:

Intuition 1:

State is changed by the occurrence of activities.

Intuitively, a change in state is captured by a state that is either achieved or falsified by an activity occurrence. We therefore need a relation that specifies the state that is intuitively true prior to an activity occurrence and also a relation that specifies the state that is intuitively true after an activity occurrence.

Intuition 2:

State can only be changed by the occurrence of activities.

Thus, if some state holds after an activity occurrence, but after an activity occurrence later along the branch it is false, then an activity must occur at some point between that changes the state. This also leads to the requirement that the state holding after an activity occurrence will be the same state holding prior to any immediately succeeding occurrence, since there cannot be an activity occurring between the two by definition.

Intuition 3:

State does not change during the occurrence of an activity in the occurrence tree.

The Discrete State Theory cannot represent phenomena in which some feature of the world is changing as some continuous function of time (hence the name "Discrete State" for the extension). State can change during an activity occurrence only if the activity is complex.


Informal Semantics for Occurrence Trees

(state ?f) is TRUE in an interpretation of the Discrete State Theory if and only if ?f is a member of the set of states in the universe of discourse of the interpretation. States are a subcategory of object. They intuitively represent properties and relationships in the domain that can change as the result of the occurrence of activities.

(holds ?f ?occ) is TRUE in an interpretation of the Discrete State Theory if and only if the state ?f is true after the activity occurrence ?occ.

(prior ?f ?occ) is TRUE in an interpretation of the Discrete State Theory if and only if the state ?f is true prior to the activity occurrence ?occ.


Axioms of Discrete State


Last Updated: Wednesday, 15-December-2003 11:42:40

Return to the PSL homepage