Thematic relations capture many of the basic inferences which we can draw about participants in an event. On Jackendoff's account, thematic relations are not associated with syntactic positions but rather correspond to particular configurations in conceptual structure. Thematic relations are therefore essentially particular semantic relations which may hold among elements of verb meaning. They do not have independent status within the theoretical framework, but there are generalisations over certain semantic relations which can be made.
For example, Jackendoff (1983, pp. 206-207) shows that the understood subject of a complement infinitive can depend on the thematic relations among elements in the sentence. In example ji6, the relations between both the main verb and its arguments and the infinitive and the noun governing the infinitive, heavily influence the inferences drawn. Thus in ji6a,d the person understood to be leaving is Bill while in ji6b,c that person is John despite the syntactic parallelism between ji6a,c and ji6b,d.
John gave Bill orders to leave. John got from Bill orders to leave. John gave Bill a promise to leave. John got from Bill a promise to leave.
Furthermore, thematic relations allow the capture of different inferences from syntactically parallel sentences (from Jackendoff (1990:54):
Harry buttered the bread. Joe pocketed the money.
These convey opposite notions -- in ji9a, the butter goes onto the entity picked out by the direct object, the bread, while in ji9b, the entity picked out by the direct object, the money, goes into the pocket. These differing inferences must be derived from the meaning of the verbs and the roles which the (syntactic) direct objects play semantically. Since according to Jackendoff thematic relations are not associated with syntactic positions but rather with structural positions, thematic relations in his framework provide a mechanism for accounting for the differences between ji9a&b. Specifically, different thematic relations may correspond to the same syntactic position. The two direct objects in these sentences are associated with different structural positions in the representation of the meaning of these related verbs (both verbs essentially conveying that someone caused something to go somewhere, but differing on what goes where), thereby accounting for the differing interpretations we get of the sentences. Which thematic relation an entity fulfills depends on the underlying lexical semantics of the verb for which the entity is an argument. Representation of the relations between the verb and its arguments is therefore critical to accurate interpretation.