19.4.1 Thematic Roles

Consider how we represented the meaning of arguments in Ch. 17 for sentences like these:

A neo-Davidsonian event representation of these two sentences would be the following:

$$ \begin{array}{c}\exists e,x,y Isa(e,Breaking)\land Breaker(e,Sasha)\\\land BrokenThing(e,y)\land Isa(y,Window)\\\exists e,x,y Isa(e,Opening)\land Opener(e,Pat)\\\land OpenedThing(e,y)\land Isa(y,Door)\\\end{array} $$

In this representation, the roles of the subjects of the verbs break and open are Breaker and Opener respectively. These deep roles are specific to each possible kind of event; Breaking events have Breakers, Opening events have Openers, Eating events have Eaters, and so on.

If we are going to be able to answer questions, perform inferences, or do any further kinds of natural language understanding of these events, we'll need to know a little more about the semantics of these arguments. Breakers and Openers have something in common. They are both volitional actors, often animate, and they have direct causal responsibility for their events.

Thematic roles are one attempt to capture this semantic commonality between Breakers and Eaters. We say that the subjects of both these verbs are agents. Thus AGENT is the thematic role which represents an abstract idea such as volitional causation. Similarly, the direct objects of both these verbs, the BrokenThing and OpenedThing, are both prototypically inanimate objects which are affected in some way by the action. The thematic role for these participants is theme.

Thematic roles are one of the oldest linguistic models, proposed first by the Indian grammarian Panini sometime between the 7th and 4th centuries BCE. Their modern formulation is due to Fillmore (1968) and Gruber (1965). Although there is no universally agreed-upon set of thematic roles, Figures 19.5 and 19.6 present a list of some thematic roles which have been used in various computational papers, together with rough definitions and examples.