discourse referents - Association for Computational Linguistics

Lauri K a r t t u n e n U n i v e r s l = y of Texas at A u s t i n Department of Linguistics Austin, Texas 78712 D I S C O U R S E REFEREFTS C o n s i d e r an interoretive device that in some m a n n e r keeps track of i n d i v i d u a l s that have been m e n t i o n e d iu a discourse and what has been said a b o u t them. One feature any such device must have is to he able to r e c o g n i z e wPen a novel individual aopears in so~e sentence. For examole, in D r o c e s s i n ~ sentence (1), it N~st recognize that the NP a car refers to some yet u n m e n t i o n e d object, which in the following sentence may be referred to a~ain by a n y of tke alternative ways in (2). (a) (1)

I have a car.

It is black.

(2) (b) .The car is black.

However, the same noun phrase does not refer to any car in (3)o C o n s e q u e n t l y , none of the a l t e r n a t i v e s in (4) is a c c e p t a b l e as a c o n t i n u a t i o n of (3). (a) (3)

I don't have a car.

(I~) (b)

* I t is black. u *The car is black.

The above examples show that sometimes the occurrence of an i n d e f i n i t e NP in the d i s c o u r s e e s t a b l i s h e s a 'discourse referent' s o m e t h i n ~ that may'be r e f e r r e d back to by a p r o n o u n or a definite noun p h r a s e - and sometimes it does not. T h e r e are of course many o t h e r f a c t o r s i n v o l v e d b e s l d e s negation. For example, (5) is a m b i g u o u s b e t w e e n two i n t e r p r e t a t i o n s and only in one sense w o u l d it be acceptable to continue the discourse with (6). -

(5)

M a r y wants to marry a Swede.

(6)

H_ee is tall.

S e v e r a l l i n g u i s t s (e.g. L e R c y Baker, Janet Dean) have recently s t u d i e d these c o n s t r a i n t s on coreferentlallty. T h e y have g e n e r a l l y i n v e s t i g a t e d the p r o b l e m in the f r a m e w o r k of N o a m Chomsky's A s p e c t s of the T h e o r y of S.?ntax. It has been a s s u m e d that noun phrases are a s s o c i a t e d with ' r e ~ r e n t i a l indices' and m a r k e d with respect to the feature [ ~ s D e c i f l c ] , ~ h a t is relevant e s p e c i a l l y in the case of (5). I n this p a p e r I will argue that Chomsky's indices and the notion of s o e c i f i c i t y are not a d e q u a t e for h a n ~ l l n g the p r o b l e m of discourse r e f e r e n t s and that we must a d o p t the more abstract analysis of noun p h r a s e s f i r s t put forth by Emmon Bach and James D. NcCawley. That is, n o u n p h r a s e s must be c o n s i d e r e d as h a v i n g an u n d e r l y i n g variable w h i c h is b o u n d by a q u a n t i f i e r s i m i l a r to those in o r d i n a r y ~ r e d l c a t e calculus. In this framework, it is a r e l a t i v e l y sinple m a t t e r to d e s c r i b e the c i r c u m s t a n c e s u n d e r w h i c h a n indefinite NF establishes a d i s c o u r s e referent. - This happens in case the sentence bound by the ~ u a n t l f l e r is a s s e r t e d or p r e s u p p o s e d to be true by the speaker. ~owever, there are some special cases that have to be recognized. These involve at least the following: (1) c o u n t e r f a c t u a l conditional, (ii) m o d a l s , and (iii) q u a n t i f l e r - l i k e adverbs, such as always, often, etc.