Structure Of Sentence And Inferencing In Question Answering

I n t h e p r e s e n t p a p e r we c h a r a c t e r i z e i n more d e t a i l some o f t h e a s p e c t s o f a q u e s t i o n a n s w e r i n g s y s t e m u s i n g a s i t s s t a r t i n g p o i n t t h e u n d e r l y i n g s t r u c t u r e o f sentences (which with some approaches can be identified with the level of meaning or of logical form). First of all, the criteria are described that are used to identify the elementary units of under~ ing structure and the operations conoining them into complex units (Sect.l), t h e n t h e main t y p e s o f ~n~ t s and o p e r a t i o n s resulting from an empirical investigation on t h e b a s i s o f t h e c r i t e r i a a r e r e g i s t e r ed ( S e c t . 2 ) , and f i n a l l y the r u l e s o f i n f e r e n c e , a c c o u n t i n g f o r t h e r e l e v a n t aspects of the relationship between linguistic and cognitive structures are illustrated ~Secto3). I. A system of natural language understanding may gain an advantage from using the underlying structure of sentences (which with some approaches can be identified with the level of meaning or of logical form) as one of its starting p o i n t s , i n s t e a d o f w o r k i n g w i t h word specific roles. Ar~menta for such a standpoint, which were presented in Haji~ov~ and S~all (1980), include the following two maln points: (a) natural language is universal, i.e. its structure makes it possible to express an unlimited n~-.ber of assertions, questions, etc° t by finite means} once its underlying (tectogrammatical) struct= ure is known, it is possible to use it ai an output language of natural language analysis in man-machine communication and thus, without any intellectual effort on t h e s i d e o f t h e u s e r , t o e n s u r e t h e f u n c t i o n i n g o f a u t o m a t i c q u e s t i o n a n s w e r i n g s y s t e m s ( o r o f s y s t e m s o f d i a l o g u e s w i t h robots, etc.)} even if many simplifications have been included into such a system, it is then known what has been simplified and it is possible to remove the simplifications whenever necessary (e.g. if the system is to be used for ano t h e r s e t o f t a s k s , i n c l u d i n g t h e a n a l y s i s o f a b r o a d e r s e t o f input t e x t s , q u e s t i o n s , e t c . ) ; (b) linguistic meaning is ~ystematic, so that the configurations of "deep cases" (valency), tenses~ modalito ias, number, etc. make it possible to find full~ reliable information; on the other hand, such systems as those baaed on scenarios or scripts work in most cases with rules that are valid for the unmarked cases (in a marked case e.g. lunch in a restaurant can be taken by an employee of the restaurant, who does not reserve a table, order the meals and P~7 for them ***)° To find out which of the semantic and pragmatic distinctions are reflected in the system of language ~or, in other words, to find out in what respects the underlying structure of sentences differ from their surface patterns) testable operational criteria are needed~ these criteria should help to distinguishl (i) whether two given surface -_nits a r e s t r i c t l y synonymous ( i ° e . s h a r e a t l e a s t one o f t h e i r m e a n i n g s ) , o r no t~ (ii) whether a single surface unit has more t h a n one mean ing ( i s a m b i g u o u s ) , or whether a sibgle meaning is concerned s which is vague or indistinct (cf. Zwicky and Sadock, 1975; Kasher and Gabbay, |976} Keenan, 1978); (iii) whether a given distributional r e s t r i c t i o n b e l o n g s t o t h e t e c t o g r a . atical level, or whether it is given onl~ by the cognitive content itself, i.e. by extralinguistic conditions; (iv) between a case of deletion (of a tectogra~saatical unit by surface rules) and t h e a b s e n c e o f t h e g i v e n u n i t i n t h e u n d e r l y i n g s t r u c t u r e ; (v ) be tween d i f f e r e n t k i n d s o f tectogrammatical units (e.g. inner participants of cases, and free or adverbial modifications); (vi) which tectogrammatical unit has been deleted, in case more of them can occupy the deleted position (el.