19.4.2 Diathesis Alternations

The main reason computational systems use thematic roles, and semantic roles in general, is to act as a shallow semantic language that can let us make simple inferences that aren't possible from the pure surface string of words, or even the parse tree. For example, 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.

Thus thematic roles are used in helping us generalize over different surface realizations of predicate 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:

(19.21) John broke the window.

AGENT THEME

(19.22) John broke the window with a rock.

AGENT THEME INSTRUMENT

(19.23) The rock broke the door. INSTRUMENT THEME

(19.24) The window broke.

(19.25) The window was broken by John.

THEME AGENT

The examples above suggest that \textit{break} has (at least) the possible arguments AGENT, THEME, and INSTRUMENT. The set of thematic role arguments taken by a verb is often called the \textit{thematic grid}, $ \theta $-grid, or \textit{case frame}. We can also notice that there are (among others) the following possibilities for the realization of these arguments of \textit{break}:

AGENT:Subject, THEME:Object

AGENT:Subject, THEME:Object, INSTRUMENT:PP with

INSTRUMENT:Subject, THEME:Object

THEME:Subject

(19.26)

It turns out that many verbs allow their thematic roles to be realized in various syntactic positions. For example, verbs like give can realize the THEME and GOAL arguments in two different ways:

a. Doris gave the book to Cary.

AGENT THEME GOAL

b. Doris gave Cary the book.

AGENT GOAL THEME

These multiple argument structure realizations (the fact that break can take AGENT, INSTRUMENT, or THEME as subject, and give can realize its THEME and GOAL in either order) are called verb alternations or diathesis alternations. The alternation we showed above give, the dative alternation, seems to occur with particular semantic classes of verbs, including “verbs of future having” (advance, allocate, offer, owe), “send verbs” (forward, hand, mail), “verbs of throwing” (kick, pass, throw), and so on. Levin (1993) is a reference book which lists for a large set of English verbs the semantic classes they belong to and the various alternations that they participate in. These lists of verb classes have been incorporated into the online resource VerbNet (Kipper et al., 2000).