Thematic Roles

This web page summarizes the thematic roles or case relations used in the book Knowledge Representation by John F. Sowa. In predicate calculus, they can be represented by dyadic predicates. In conceptual graphs, they are represented by conceptual relations that link the concept of a verb to the concepts of the participants in the occurrent expressed by the verb. In frame systems, they are represented by slots in the frame for the corresponding verb. All those notations are equivalent ways of representing the links between a process and its participants.

In the KR ontology, the thematic roles are classified as subtypes of Participant, which is further subdivided by two pairs of distinctions: determinant or immanent and source or product. That subdivision generates the four basic types of participants shown in the graph of Figure 1.

Figure 1: Graph representation of the subtypes of Participant.

In Figure 1, the type Participant is subdivided by two pairs of distinctions to generate four subtypes at the bottom level of the graph. That same subdivision is represented by the matrix in Figure 2.

Source Product
Determinant Initiator Goal
Immanent Resource Essence

Figure 2: Matrix representation of the subtypes of Participant

Every participant is an entity that plays some role in a process. The four-way subdivision shown in Figures 1 and 2 distinguishes the participants according to the kind of role they play. In natural languages, those distinctions are expressed by grammatical markers such as prepositions and case markers, which link the verbs that express the processes to the nouns that express the participants. In logic, those distinctions are expressed by relations or predicates that link the symbols that identify the processes to the symbols that identify the participants.

As an example, consider the sentence Sue sent the gift to Bob by Federal Express. The gift and Federal Express are immanent participants, since the gift (essence) and Federal Express (resource) are present from beginning to end. Sue and Bob, however, are determinant participants, since they determine the course of the process from the initiator (Sue) to the goal (Bob). Unlike the immanent participants, the determinant participants are involved primarily at the endpoints. If Sue happened to write the wrong address, the intended recipient, Bob, might not get involved at all.

After analyzing and summarizing various systems of case relations or thematic roles, Harold Somers (1987) organized them in a matrix with four types of participants at the top and six categories of verbs along the side. In the 24 boxes of the matrix, Somers had some boxes with duplicate role names and some boxes with two roles that were distinguished by other properties: ±animate, ±physical, ±dynamic, or ±volitional. In using Somers's classification, Judith Dick (1991) applied the roles to conceptual graphs as a knowledge representation for legal arguments.

Stimulated by the work of Julius Moravcsik (1991) and James Pustejovsky (1995), Sowa (1996) related the four columns of the Somers-Dick matrix to Aristotle's four causes or aitia, as described in the Metaphysics:

The four terms intiator, resource, goal, and essence better describe the participants of an action than the traditional translations for Aristotle's four causes. Figure 3 is a version of the Somers-Dick matrix of thematic roles with the terminology adapted to the ontology presented in the KR book.

Initiator Resource Goal Essence
Action Agent,
Instrument Result,
Process Agent,
Matter Result,
Transfer Agent,
Spatial Origin Path Destination Location
Temporal Start Duration Completion PointInTime
Ambient Origin Instrument,
Result Theme

Figure 3: Thematic roles as subtypes the four types of participants

The options and duplications in the boxes of the table indicate that further distinctions can be made. The option of Agent or Effector as the initiator of an action is determined by the distinction of a voluntary intiator (Agent) or involuntary initiator (Effector). The duplication of the Agent role for actions, processes, and transfers indicates implicit interactions between the types of verbs and the types of participants. Figure 3 is an important start, but it should be extended with further analysis along the lines of Beth Levin's classification of verbs.

In case of ambiguity, the hierarchy shown in Figure 1 permits a more specialized type of participant at the bottom to be generalized to any supertype above it in the graph. In the sentence Tom baked the pie, the pie might be a result (determinant product) that is being created or a patient (immanent product) that is being warmed. Those two interpretations would be expressed by two different conceptual graphs:

[Person: Tom](Agnt)[Bake](Rslt)[Pie: #].

[Person: Tom](Agnt)[Bake](Ptnt)[Pie: #].
But according to the hierarchy of participants, Result < Goal < Product, and Patient < Essence < Product. Since Product is a common supertype, the initial interpretation could have the label Prod. The resulting representation would be a single conceptual graph that expressed exactly the same information as the original sentence without making any assumptions about the immanent or determinant status of the pie:
[Person: Tom](Agnt)[Bake](Prod)[Pie: #].
In the sentence The dog broke the window, the dog could be an agent that broke it deliberately, an effector that broke it accidentally, or an instrument that was pushed through the window by the actual agent. Each interpretation would be expressed by a different conceptual graph:
[Dog: #](Agnt)[Break](Ptnt)[Window: #].

[Dog: #](Efct)[Break](Ptnt)[Window: #].

[Dog: #](Inst)[Break](Ptnt)[Window: #].
But Agent < Initiator < Source, Effector < Initiator < Source, and Instrument < Resource < Source. Since all three of the more specialized types of participants are subtypes of Source, a single CG with the Srce relation would express the equivalent information in the original sentence.
[Dog: #](Srce)[Break](Ptnt)[Window: #].
When further information about the dog's role becomes available, the relation type Srce can be specialized to one of the three subtypes used in the preceding graphs.

The following list gives a brief decription and an example for each of the thematic roles that appear in Figure 3. The first term in each entry is the name of the role, such as Agent. Following the symbol < is the supertype, such as Intiator. Then comes an abbreviation, such as Agnt, followed by category restrictions on the concept type of the verb (Act) and the concept type of the participant (Animate). As discussed in the web page on roles and relations, each relation is defined in terms of the corresponding role and the primitive dyadic relation Has. Agnt, for example, is defined as HasAgnt, Benf is HasBeneficiary, and Cmpl is HasCompletion.

Agent < Initiator; Agnt(Act,Animate).
An active animate entity that voluntarily initiates an action.

Example: Eve bit an apple.

[Person: Eve](Agnt)[Bite](Ptnt)[Apple].

Beneficiary < Recipient; Benf(Act,Animate).
A recipient that derives a benefit from the successful completion of the event.

Example: Diamonds were given to Ruby.

[Diamond: {*}](Thme)[Give](Benf)[Person: Ruby].

Completion < Goal; Cmpl(TemporalProcess,Physical).
A goal of a temporal process.

Example: Mary waited until noon.

[Person: Mary](Thme)[Wait](Cmpl)[Noon].

Destination < Goal; Dest(SpatialProcess,Physical).
A goal of a spatial process.

Example: Bob went to Danbury.

[Person: Bob](Agnt)[Go](Dest)[City: Danbury].

Duration < Resource; Dur(State,Interval).
A resource of a temporal process.

Example: The truck was serviced for 5 hours.

[Truck: #](Thme)[Service](Dur)[Interval: @5hrs].

Effector < Initiator; Efct(Entity,Entity).
An active determinant source, either animate or inanimate, that initiates an action, but without voluntary intention.

Example: The tree produced new leaves.

[Tree: #](Efct)[Produce](Rslt)[Leaf: {*}](Attr)[New].

Experiencer < Goal; Expr(State,Animate).
An active animate goal of an experience.

Example: Yojo sees the fish.

[Cat: Yojo](Expr)[See](Thme)[Fish: #].

Instrument < Resource; Inst(Act,Entity).
A resource that is not changed by an event.

Example: The key opened the door.

[Key: #](Inst)[Open](Thme)[Door: #].

Location < Essence; Loc(Physical,Physical).
An essential participant of a spatial nexus.

Example: Vehicles arrive at a station.

[Vehicle: {*}](Thme)[Arrive](Loc)[Station].

Matter < Resource; Matr(Act,Substance).
A resource that is changed by the event.

Example: The gun was carved out of soap.


Medium < Resource; Med(Transfer,Physical).
A physical resource for transmitting information, such as the sound of speech or the electromagnetic signals that transmit data.

Example: Bill told Boris by phone.

[Person: Bill](Agnt)[Tell]-
   (Expr)[Person: Boris]

Origin < Initiator; Orgn(Process,Physical).
A passive determinant source of a spatial or ambient nexus.

Example: The chapter begins on page 20.

[Chapter: #](Thme)[Begin](Orgn)[Page: 20].

Path < Resource; Path(Process,Place).
A resource of a spatial nexus.

Example: The pizza was shipped via Albany and Buffalo.

[Pizza: #](Thme)[ShipAct](Path)[City: {Albany, Buffalo}].

Patient < Essence; Ptnt(Process,Physical).
An essential participant that undergoes some structural change as a result of the event.

Example: The cat swallowed the canary.

[Cat: #](Agnt)[Swallow](Ptnt)[Canary: #].

PointInTime < Essence; PTim(Physical,Time).
An essential participant of a temporal nexus.

Example: At 5:25 PM, Erin left.

[Time: 5:25pm](PTim)[Situation: [Person: Erin](Agnt)[Leave]].

Recipient < Goal; Rcpt(Act,Animate).
An animate goal of an act.

Example: Sue sent the gift to Bob.

[Person: Sue](Agnt)[Send]-
     (Thme)[Gift: #]
     (Rcpt)[Person: Bob].

Result < Goal; Rslt(Process,Entity).
An inanimate goal of an act.

Example: Eric built a house.

[Person: Eric](Agnt)[Build](Rslt)[House].

Start < Initiator; Strt(Entity,Time).
A determinant source of a temporal nexus.

Example: Bill waited from noon to three.

[Person: Bill](Thme)[Wait]-
     (Cmpl)[Time: 3pm].

Theme < Essence; Thme(Situation,Entity).
An essential participant that may be moved, said, or experienced, but is not structurally changed.

Example: Billy likes the Beer.

[Person: Billy](Expr)[Like](Thme)[Beer: #].

As subtypes of Participant, the thematic roles occupy an intermediate level in the ontology. Figure 4 shows a path through the hierarchy from the top levels of the ontology to the subtypes of Participant represented in Figure 1. Each of the thematic roles in Figure 3 could then be arranged under the four subtypes of Participant: Initiator, Resource, Goal, and Essence. The incomplete lines in Figure 4 suggest other branches of the ontology that have been omitted in order to keep the diagram from being cluttered.

Figure 4: Placement of the thematic roles in the ontology

At the bottom of Figure 4 are sample branches of the ontology under Agent and Theme. Doer, for example, has a subtype Driver, which has more specific subtypes like BusDriver, TruckDriver, and TaxiDriver. In principle, any of the thematic roles could be subdivided further to show distinctions that might be significant in some culture or domain of interest. Other thematic roles listed in Section B.4 could also be subdivided further to represent the participants of specific concept types: Sayer < Agent; Senser < Agent; Addressee < Recipient; Experienced < Theme; Moved < Theme; Said < Theme.

Although the thematic roles represent a linguistically important class of ontological categories, their common supertype Participant is several levels beneath the more general category Role. Therefore, Role would include many types that are not directly associated with verbs. As an example, the role Driver in Figure 4 represents only a person who is actively driving a vehicle; that role would be incompatible with the role Pedestrian. The category LicensedDriver, however, includes people who are legally authorized to drive, whether or not they are, at the moment, driving. In New York City, licensed drivers probably spend more time as pedestrians than as actual drivers. As another example, a person might have a continuous period of employment as a chauffeur, but would not be an active driver continuously. Therefore, the type Chauffeur would be a subtype of Employee and LicensedDriver, but not a subtype of Driver.

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