jurafsky&martin_3rdEd_17 (1).pdf

We say that the subjects of both these verbs are

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We say that the subjects of both these verbs are agents . Thus, AGENT is the agents thematic role that 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 that are affected in some way by the action. The semantic role for these participants is theme . 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 roles, Figs. 22.1 and 22.2 list some thematic roles that have been used in various computational papers, together with rough definitions and examples. Most thematic role sets have about a dozen roles, but we’ll see sets with smaller numbers of roles with even more abstract meanings, and sets with very large numbers of roles that are specific to situations. We’ll use the general term semantic roles for all sets of roles, whether small or large. semantic roles
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22.2 D IATHESIS A LTERNATIONS 379 Thematic Role Example AGENT The waiter spilled the soup. EXPERIENCER John has a headache. FORCE The wind blows debris from the mall into our yards. THEME Only after Benjamin Franklin broke the ice ... RESULT The city built a regulation-size baseball diamond ... CONTENT Mona asked “You met Mary Ann at a supermarket?” INSTRUMENT He poached catfish, stunning them with a shocking device ... BENEFICIARY Whenever Ann Callahan makes hotel reservations for her boss ... SOURCE I flew in from Boston . GOAL I drove to Portland . Figure 22.2 Some prototypical examples of various thematic roles. 22.2 Diathesis Alternations The main reason computational systems use semantic roles is to act as a shallow meaning representation that can let us make simple inferences that aren’t possible from the pure surface string of words, or even from the parse tree. To extend the earlier examples, if a document says that Company A acquired Company B , we’d like to know that this answers the query Was Company B acquired? despite the fact that the two sentences have very different surface syntax. Similarly, this shallow semantics might act as a useful intermediate language in machine translation. Semantic roles thus help generalize over different surface realizations of pred- icate arguments. For example, while the AGENT is often realized as the subject of the sentence, in other cases the THEME can be the subject. Consider these possible realizations of the thematic arguments of the verb break : (22.3) John AGENT broke the window. THEME (22.4) John AGENT broke the window THEME with a rock. INSTRUMENT (22.5) The rock INSTRUMENT broke the window.
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