The Lexical Component of START

In order to understand an English sentence, the START system needs to have access to morphological, syntactic, and semantic information about the words in the sentence. All the words that the system is aware of, along with information about their part of speech, inflection, gender, number, etc. are stored in the Lexicon. Virtually every branch of START uses the Lexicon to accomplish its task. In this section we discuss the way in which the Lexicon extends the system's ability to deal with semantic-syntactic interdependencies. We show that the Lexicon provides a place where a verb's membership in a semantic class can be registered, allowing more general S-rules to be stated.

Note that formulating a special purpose S-rule which applies only to the verb surprise does not seem to be the best solution to the problem. Surprise is only one of many verbs which exhibit the so-called property-factoring alternation. This alternation holds of a large class consisting of over one hundred verbs, among them

(8) amuse, anger, annoy, disappoint, embarrass, frighten, impress, please, scare, stun, ...

These verbs also share a certain semantic property: they all denote emotional reactions. For this reason we identify a class of emotional-reaction verbs and say that the property of the verb surprise responsible for the alternation shown in (3) and (5) holds for all verbs that comprise the emotional-reaction class.

Once we have tied the ability to participate in the property-factoring alternation to a particular class of verbs, we no longer need to indicate this property in the lexical entry of each verb in the class or write verb-specific S-rules, such as the Surprise S-rule. Rather, we can associate the alternation with the emotional-reaction class and then simply indicate in the lexical entry of a verb whether it belongs to this class. That is, we augment a verb's lexical entry with an indication of its semantic class membership. For instance, we would register in the entry for surprise that it is a member of the emotional-reaction class. Now instead of writing a number of verb-specific S-rules, we can write a single general S-rule which triggers on any verb from the emotional-reaction class:

(9) Property-factoring S-rule

    If <<subject verb object1> with object2>

    Then <object2 verb object1>

    Provided verb emotional-reaction class

The revised S-rule contains a Provided clause which specifies the class of verbs to which the rule applies, ensuring that it applies to the emotional-reaction verbs. Provided clauses may impose restrictions on any of the S-rule variables.

S-rules are based on the observation that semantics intrudes into syntax in an interesting way, controlling the allowable lexical alternations. For example, consider the verbs "surprise" and "present;" the following pairs of sentences show allowable alternative lexical renditions for each of these verbs:

(10) Bill surprised Hillary with his answer.

    Bill's answer surprised Hillary.

    Hillary presented Bill with a gift.

    Hillary presented a gift to Bill.

However, as the following non-sentences indicate, neither of these verbs can support the same lexical alternatives as the other:

(11) * Hillary's gift presented Bill.

    * Bill surprised his answer to Hillary.

This is because these verbs are in different semantic categories. Any verb having to do with emotional reactions can support the same lexical alternations as `"urprise". Similarly any verb having to do with change of ownership can support the same alternations as "present." START has exploited this phenomenon to achieve a compact representation of its lexicon.

However, it is just as important to realize that such lexical information provides significant clues about the similarities and distinctions which must be made by HPKB's core ontologies and domain theories. We think that this is an additional value that our project with its focus on Natural Language can bring to the HPKB program.