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Handbook of Knowledge Representation

Edited by F. van Harmelen, V. Lifschitz and B. Porter

DOI: 10.1016/S1574-6526(07)03018-0

709

Chapter 18

Temporal Action Logics

Patrick Doherty and Jonas Kvarnström

18.1 Introduction

The study of frameworks and formalisms for reasoning about action and change [67,

58, 61, 65, 70, 3, 57] has been central to the knowledge representation ﬁeld almost

from the inception of Artiﬁcial Intelligence as a general ﬁeld of research [52, 56].

The phrase “Temporal Action Logics” represents a class of logics for reasoning

about action and change that evolved from Sandewall’s book on Features and Fluents

[61] and owes much to this ambitious project. There are essentially three major parts

to Sandewall’s work. He ﬁrst developed a narrative-based logical framework for speci-

fying agent behavior in terms of action scenarios. The logical framework is state-based

and uses explicit time structures. He then developed a formal framework for assessing

the correctness (soundness and completeness) of logics for reasoning about action and

change relative to a set of well-deﬁned intended conclusions, where reasoning prob-

lems were classiﬁed according to their ontological or epistemological characteristics.

Finally, he proposed a number of logics deﬁned semantically in terms of deﬁnitions

of preferential entailment

and assessed their correctness using his assessment frame-

work.

Several of these logics were intended to correspond directly to existing logics of

action and change proposed by others at the time, while the rest were new and were

intended to characterize broad classes of reasoning problems which subsumed some of

the existing approaches. Each of these deﬁnitions of preferential entailment were then

analyzed using the assessment framework, giving upper and lower bounds in terms

of the classes of reasoning problems for which they produced exactly the intended

conclusions. Much insight was gained both in terms of advantages and limitations

of previously proposed logics of action and change and in how one might go about

proposing new logics of action and change in a principled manner with formal assess-

ments included.

Preferential entailment reduces the set of classical models of a theory by only retaining those models

that are minimal according to a given preference relation, a strict partial order over logical interpretations

[66].

710 18. Temporal Action Logics

The starting point for Temporal Action Logics was one of the deﬁnitions of pref-

erential entailment in Sandewall’s book called PMON (Pointwise Minimization of

Occlusion with Nochange premises). It was one of the few preferential entailment

methods that were assessed correct for the K–IA class of action scenario descrip-

tions, where K is an epistemological characteristic stating approximately that explicit,

correct and accurate knowledgeis provided(with no requirements on complete knowl-

edge in the initial state and no restrictions on knowledge about other states), and IA

is an ontological characteristic stating approximately that discrete integer time is used

together with plain inertia (without ramiﬁcation constraints, delayed effects, or other

complicating factors).

Thus, PMON solved the frame problem relative to an explicit statement of assump-

tions (K–IA) under which it could be assessed correct. The nature of the deﬁnition of

preferential entailment was somewhat related to explanation closure [64, 19], although

a partitioning of action scenario theories was used where only parts of the theory were

minimized and other parts used as ﬁlters on the preferred model set for the theory.

Though ramiﬁcations and qualiﬁcations to actions were not allowed in K–IA, the class

is in fact quite broad, permitting the use of conditional effects, non-deterministic ef-

fects, incomplete speciﬁcation of states andthe timing of actions, actionswith duration

and speciﬁcation of dynamics within action durations.

18.1.1 PMON and TAL

While the original PMON was characterized semantically in terms of a preferential

entailment method, Doherty later developed an equivalent syntactic characterization

in classical 2nd-order logic (also called PMON), using a circumscription axiom to for-

malize the PMON deﬁnition of preferential entailment [7, 14]. In these papers, he also

showed that the 2nd-order circumscription axiom was equivalent to a 1st-order point-

wise circumscription axiom, enabling the use of standard ﬁrst-order theorem proving

techniques to reason about PMON action narratives. In extended versions of PMON

which led to TAL, it has also been shown that quantiﬁer elimination techniques or

predicate completion techniques (Deﬁnition 7.3.6 in Chapter 7 of this Handbook) can

be used to reduce TAL circumscribed theories to logically equivalent 1st-order theo-

ries under certain assumptions.

Doherty’s PMON logic used two languages for representing and reasoning about

narratives. The surface language L(SD), Language for Scenario Descriptions, pro-

vided a convenient high-level notation for describing narratives, and could be de-

scribed as a set of macros easily translated into a base language L(FL), which was

initially a many-sorted ﬁrst-order language and was later altered to be an order-sorted

ﬁrst-order language. The L(SD) language was later renamed to L(ND), Language for

Narrative Descriptions.

The logic was further extended and generalized in several steps in order to deal

with such issues as the ramiﬁcation and qualiﬁcation problems, use of concurrent

actions, use of structured object-oriented action theories, and use as a speciﬁcation

formalism for TALplanner. Each extension generally implied adding new macros to

L(ND), adding additional predicates to L(FL), extending the translation deﬁnition to

Essentially, an order-sorted language allows the use of sub-sorts; for example, CAR and BICYCLE may

be sub-sorts of the

VEHICLE sort.

P. Doherty, J. Kvarnström 711

L(FL) and providing slight modiﬁcations to the circumscription policies used. It is

important to observe that all extensions proposed have been made in a manner which

preserves the property of reducibility of the 2nd-order circumscription theory to a 1st-

order theory. This is essential for practical reasons.

A number of the main extensions to PMON which led to the TAL family of logics

include:

• PMON-RC [25], which provides a solution to the ramiﬁcation problem for a

broad, but as yet unassessed class of action scenarios. The main idea is the addi-

tion of a new statement type for causal constraints, where changes taking place

in the world can automatically trigger new changes at the same time or at a

speciﬁed delay from the original change. The solution is very ﬁne-grained in

the sense that one can easily encode dependencies between individual objects in

the domain, work with both boolean and non-boolean ﬂuents and represent both

Markovian and non-Markovian dependencies [21]. PMON-RC also correctly

handles chains of side effects.

• TAL 1.0 (PMON

) [8], which is an extended version of the original PMON

logic incorporating the changes made in PMON-RC together with other useful

extensions. This logic was originally called PMON

, but was later renamed

TAL 1.0 and provided the ﬁrst stable kernel for the TAL family of logics.

• TAL-C [35], which uses ﬂuent dependency constraints (an extended form of

causal constraints) as a basis for representing concurrent actions. A number of

phenomena related to action concurrency such as interference between one ac-

tion’s effects and another’s execution, bounds on concurrency, and conﬂicting,

synergistic, and cumulative effects of concurrent actions are supported.

• TAL 2.0 [10], which provides a basic stable kernel of TAL. It is essentially

TAL-C with some useful extensions and includes a tutorial on TAL and how it

is used.

• TAL-Q [11, 42], which introduces the idea of combining an encoding of de-

fault values for features using persistence statements together with dependency

constraints for representing qualiﬁcations to actions.

TAL 2.0 (TAL-C), extended with additions from TAL-Q, has been used as the basis

for much of the recent work with Temporal Action Logics and will be described in

some detail in this chapter. In the remainder of the chapter, we will use “TAL” as a

term to denote the latest stable kernel of this family of logics.

18.1.2 Previous Work

There has been a great deal of previous work in the development of the material de-

scribed in this chapter. We brieﬂy summarize this work chronologically.

The root node from which TAL originated is the Features and Fluents (F&F) mono-

graph [61]. Later developments with F&F are summarized in Sandewall [63].Do-

herty [7] provides a syntactic characterization of PMON using pointwise circumscrip-

tion and shows how a particular class of narratives can be characterized as ﬁrst-order

theories. Doherty [6] contains a detailed account of PMON circumscription theories