36 Problems for Semantic Interpretation Gabriele Scheler Institut fur Informatik Technische Universitat Munchen 80290 Munchen, Germany e-mail:
[email protected] Abstract
This paper presents a collection of problems for natural language analysis derived mainly from theoretical linguistics. Most of these problems present major obstacles for computational systems of language interpretation. The set of given sentences can easily be scaled up by introducing more examples per problem. The construction of computational systems could bene t from such a collection, either using it directly for training and testing or as a set of benchmarks to qualify the performance of a NLP system.
1 Introduction The main part of this paper consists of a collection of problems for semantic analysis of natural language. The problems are arranged in the following way: example sentences concise description of the problem keyword for the type of problem The sources ( rst appearance in print) of the sentences have been left out, because they are sometimes hard to track and will usually not be of much use, as they indicate a starting-point of discussion only. The keywords however can be used in a bibliographical search, when the full range of discussion concerning a single problem is sought. Some example sentences will be recognized by every linguist, in some cases the example sentences to a well-known problem have been set up by the author. All examples are given in English. Language-speci ty has not been a topic in 1
this collection, some problems appearing in many or all languages, others being more con ned. A preliminary section lists the problems by keyword and introduces a grouping into 5 classes, which correspond to subject areas of theoretical linguistics. (Note that a class \logical semantics" is used, although it concerns a certain methodology as well as a pure subject area.)
2 Overview 1. Problems of syntactic structure
PP-attachment grammatical relations passivization topicalization explicative vs. referring relative clauses
2. Problems of coreference and anaphora
determiner, coherence anaphora, coreference temporal anaphora \paycheck" pronouns, anaphora narrative iconicity
3. Problems of lexical semantics
lexical disambiguation lexical reversibles neologisms idiomatic usage metaphor
4. Problems of syntactical and grammatical categories
abstract property nouns transformation of syntactic category verb-noun interaction temporal adverbial-aspectual verb meaning aspectual verb meaning, aktionsart: semantic implications progressive aspect
5. Problems of logical semantics
oating quanti ers
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collective predicates mass nouns \lazy" pronouns intensional reference intensional coreference non-compositionality of meaning generics, intensional coreference relative adjectives counterfactuals belief contexts time-negation-quanti cation presupposition-negation embedded question degree particles bare plurals, generics
3 The Problems
3.1 Problems of syntactic structure
1. I saw the Grand Canyon ying to New York. I saw the man on the hill with a telescope. Syntactic structure is underdetermined, gerunds and PP-phrases may be attached to the verb phrase or a noun phrase.
PP-attachment
2. John is easy to please. John is eager to please. Visiting relatives can be boring. Shooting lions may be dangerous. Embedded subject or object is equal to main sentence subject, active/passive is undistinguished in gerundival constructions: Identical syntactic structure with di erent semantic (grammatical) relations.
grammatical relations
3. Caesar was killed by Brutus. Caesar was killed. Fronting of constituents and/or suppression of active subject as syntactical variation.
passivization
4. Brutus killed Caesar. It was Brutus who killed Caesar. Di erent syntactic constructions with same denotational meaning, but pragmatic di erences, i.e. di erences in use and occurrence.
topicalization
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5. Cars which hinder trac will be towed. Cars, which greatly contribute to pollution, are still subsidized by the government. The trees in your garden which are not healthy have to be felled. A referring relative clause further delimits the referent of its noun phrase, an explicative relative clause functions like an conjuncted assertion noun phrase - predicate of rel. clause. The distinction is sometimes obscure.
explicative vs. referring relative clauses
3.2 Problems of coreference and anaphora
6. Mary took the picnic supplies out of the car. The beer was warm. The de nite determiner for beer is interpreted as an indication that beer is coreferred to by picnic supplies, i.e. that it is not a new entity and has been mentioned before.
determiner, coherence
7. The housekeeper opened the door, he let the dog out, and closed it again. Bill told the doctor that he was sick. Coreference of anaphorical pronouns (it, he) is often constrained by pragmatical inferences (or background knowledge).
anaphora, coreference
8. Bill told the doctor that Mary became sick two hours ago. Yesterday he said he would come tomorrow. Diculty in anaphorical (tense or pronoun) interpretation with indirect speech: Anchor ago to time of speech, or to time of told.
temporal anaphora
9. Sue, when she got a divorce, remarried within a few months. Mary didn't remarry until years later. The temporal reference in the second sentence involves a shift in the pronominal reference of the temporal adverbial of the rst sentence.
\paycheck" pronouns, anaphora
10. Hans took his boots o and went to bed. Succession of statements mirrors the succession of events.
narrative iconicity
3.3 Problems of lexical semantics
11. This is the baby's pen. The lawyer is working at the bar. The spy searched the room for bugs. Words with di erent unrelated lexical meanings may not be assignable to 4
a meaning within a sentential context.
lexical disambiguation
12. The star is above the line. The line is below the star. Summer is after spring. Spring is before summer. Some pairs of words refer to binary relations in a di erent order of the arguments. The resulting sentences are denotationally equivalent, though not in use, e.g. in answering questions.
lexical reversibles
13. The farmer was watching the grebe diving. Assuming that grebe is an idiosyncratic wordform, or otherwise unknown word, some interpretation may be attached to the word (such that it is a noun, a concrete object, probably an animal, maybe a water bird etc.)
neologisms
14. He has a bee in his bonnet. He kicked the bucket. The system is a white elephant. Words maybe used with non-literal meaning in an idiomatic phrase which is common linguistic knowledge.
idiomatic usage
15. Euromoney collapse hangs on knife edge. Road test shows gaps in plastic. (concerns credit-card insurances) Words maybe used with non-literal meaning metaphorically, i.e. in a novel way, which is intelligible to language users, at least in a sucient context.
metaphor
3.4 Problems of syntactical and grammatical categories
16. The beauty of the landscape is remarkable. The landscape is remarkably beautiful. Noun plus attributive adjective has the same denotational meaning as adverb plus adjective, where noun and adjective are derivationally related.
abstract property nouns
17. Mary watched the boys (') play. Mary listened to the playing of the violin. He xeroxed the paper. Some verbs may be used as nouns (with or without morphological marking), nouns may be used as verbs. Their content meaning stays the same in the pure case, but the syntactically di erent role also shapes the semantic representation.
transformation of syntactic category verb-noun 5
18. Mary puts the beer in the fridge for an hour. Hans keeps silent for an hour. The time span an hour refers either to the action of the main verb or to a resulting state.
interaction temporal adverbial-aspectual verb meaning
19. Hans drowned. Hans ate. Hans slept. Hans ate a melon. These verbs have di erent implications with respect to resulting states (drown/eat a melon: Hans is dead, the melon is gone,(cause-state)), and to the inner structure of the process (atelic: sleep, telic: eat).
aspectual verb meaning, aktionsart: semantic implications
20. John was crossing the street, when he was hit by a truck. The action cross may not be complete when the verb is used in the progressive.
progressive aspect
3.5 Problems of logical semantics
21. The students have all solved the problems correctly. The quanti er all is not placed with the noun phrase it logically belongs to.
oating quanti ers
22. The children resemble each other. These boxes look alike. Some verb phrases take only the individual referents of a plural noun phrase as logical arguments.
collective predicates
23. Gold is a substance. He has found gold/some gold/pieces of gold in his garden. Mass nouns may refer to pieces(instances) of a substance, or to the substance itself (allquanti ed over all instances?).
mass nouns
24. Every farmer who has a donkey beats it. The pronoun it refers to an all-quanti ed entity.
\lazy" pronouns
25. John seeks a unicorn. The government has changed. unicornis used non-referentially, seek being an intensional verb (non-referential in object position), government refers to two di erent entities, as indicated by the verb phrase.
intensional reference
26. The temperature is ninety and it is rising. The government has changed, it was time it went out of oce. 6
temperature/it and government/it are used with di erent reference in these sentences, although they syntactically corefer.
intensional coreference
27. The alleged murderer, the supposed driver. An alleged murderer is not necessarily a murderer, the supposed driver quali es the predication driver (temporary quality) for a person within a certain context.
non-compositionality of meaning
28. The grizzly becomes rarer the further north you go. Grizzly refers both to a species and to individual exemplars in this sentence.
generics, intensional coreference
29. A small elephant is bigger than a big mouse. small and big indicate measures relative to some standard, or the measure itself (bigger).
relative adjectives
30. If Napoloen had not lost at Waterloo, Europe would look di erent today. Hypothetical statements like this may be evaluated and discussed only with respect to di erent, alternative world models.
counterfactuals
31. Hans believes that Napoleon died on Elba and Detlef believes that Mary knows that he was killed at Waterloo. Nested belief contexts are hard to interpret: they require separate belief models for persons, and the presupposition of know might or might not be attributed to the speaker.
belief contexts
32. I did not turn o the stove. I called Mary, when John was not at home. Interpreting a negated temporal sentence requires a contextually constrained assignment of a time to that sentence.
time-negation-quanti cation
33. The Chancellor has not decided this question falsely/correctly, he has not decided this question at all. The implicit presupposition of a negation as a strong negation may be cancelled and may be stronger with some predicates than with others.
presupposition-negation
34. Hans does not know where we can buy a bicycle (8). Hans knows where we can buy a bicycle (9). Hans asked John who attended the party (8 or 9). The quanti cation of the embedded question seems to shift between existential and universal, possibly an embedded question contains an unspeci c 7
quanti er with respect to existence and number.
embedded question
35. Only the small box is green. Only Mary will also be leaving. Degree particles have scope, which may be contextually determined (small or small box) and is signaled by intonation. Also several degree particles may be interacting.
degree particles
36. Beavers build dams. Lions are mammals. Dams are built by beavers. Dogs are expensive nowadays. Bare plurals may correspond to a quanti cation \typically",\most" (beavers) or \all" (lions) or undetermined (dogs), passivization changes the interpretation.
bare plurals, generics
4 Conclusion This collection has been started during the work on a semantic interpretation system ([Scheler89]). It could certainly be improved by a broader collective e ort. If a substantial number of additions, revisions and further suggestions are received by the author, a second edition may be issued.
References [Scheler89] Scheler,G.: LISL-Konzeptuelle Repraesentation naturlichsprachlicher Information. Doctoral dissertation,LMU, Munchen 1989.
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