When it comes to how participants, or the arguments in a sentence, are given specific roles, there are differences between the theories. While Parsons represents the thematic roles as predicates, which relate an event or state to a constant, VerbNet instead uses the thematic roles themselves as arguments to predicates. For Jackendoff’s theory, the roles are not seen directly in the semantic structures, but are instead given to instances of semantic categories based on what kind of functions the they take part in.
Jackendoff’s framework makes it possible for one participant to have more than just a single role in an event. This was seen, for instance, in the transaction example (see structures (121)), where Keith had both the role of Source and Goal in the three events. At the same time, it seems the same role can be applied to multiple participants in the conceptual structure theory, as in the same example, both the car and the money acted as arguments to a GO function, making them both Themes. These features of the conceptual theory means that certain wanted entailments are captured, such as the fact that the participants in a transaction always occupy mostly the same roles (Keith and Joel are at least both the Source and the Goal in structures (119)).
In the underlying event theory, participants typically only occupy one single role, though Parsons does offer conjuncted role predicates, like Agent-Theme and Performer-Theme. This feature captures the fact that Jane is both the instigator and the thing moving, in the climb example (see
structures (116)). However, the fact that Keith is also the Goal, in addition to being the Agent, in structure (123a), is not captured, because Parsons does not provide a role like Agent-Goal, for instance. Unlike Jackendoff, Parsons does not allow one role to apply to multiple participants, so there will only be one Theme, for instance, in a given event.
VerbNet typically doesn’t allow roles to be applied to multiple parti- cipants, nor does it allow one participant to have more than one role. In that database, the available roles are included within the given class, and the individual frames can then only make use of these. However, even though participants are applied only to one role each, they can still act in any predicate. So, for instance, the fact that Tom also moves in one the senses of the sentence in (106a), is captured in VerbNet by including Tom in the MOTION predicate. So, even though a Theme is typically the thing that moves or is moved in an event of motion, the idea that the Agent is also known to be moving can be captured like this in the VerbNet data- base. Still, some role labels that are used capture the idea that more than one participant can have the same kind of role. Examples are roles such as Co-Agent and Co-Theme, which are present in the class exchange-13.6, for instance. In that class, the roles Agent and Theme are also present, so the possibility that two participants can both be the Agent (or Agent and Co-Agent), for example, is achieved. So, while VerbNet technically only allows role labels to each apply to one participant, the use of such Co-roles means that multiple participants (or at least two) occupying the same role is possible.
VerbNet seems to be the most consistent in its labeling of roles across multiple semantic structures, as participants typically occupy the same role within all the frames in a class. For instance, in the example involving hit, the roles never change between the three structures (see structures (125)). This consistency is not as prevalent in the other two theories, where different syntactic constructions often result in different role labels, even though this is not what we want. This can also be seen in the hit example, where the role that John occupies differ in some of the conceptual structures (see structures (87)), and where the role that the hammer plays differ in the underlying event formulas (see structures (88)). This has the effect of wanted entailments not being captured.
When we have a verb event where one optional participant is left out of the given sentence, VerbNet typically says that this participant is unknown, using the ? symbol. Jackendoff, similarly, leaves the argument unspecified, though still retains the fact that it exists. This can be seen in the eat example, where the thing being eaten is still part of the structures for the intransitive sentence (see structures (102) and (105)). For those sentences, Parsons simply leaves out the missing argument (see structure (104)), which means that we have an event of eating without the thing being eaten included at all. So, basically, Parsons can refer to an event of eating that only involves the eater, while Jackendoff and VerbNet requires that an eating event also involves a thing being eaten. When it comes to entailment, however, there really isn’t any difference here, as all three representations perfectly capture the wanted entailment in the eat example.