Towards a Combined Representation for Spacial and Temporal Reference

be shown as in F i g u r e I e x c e p t w i t h BEING-AT r e p l a c i n g S l e e p i n g . Note t h a t o n t h i s p o i n t ou r a n a l y s e s a r e i s o m o r p h i c t o t hose o f Schube r t ( 1 9 7 6 ) . The above does n o t a l l o w f o r one i m p o r t a n t c o n n e c t i o n be tween e v e n t o r s t a t e l o c a t i o n s and o b j e c t l o c a t i o n . For example f r o m "The boy s l e p t i n t h e c a r . " , t h a t t h e boy was i n t h e c a r i s e v i d e n t . T o f i t t h i s i n w i t h our a n a l y s i s , w e p r o pose t h e use o f i n f e r e n c e . A s i s u s u a l w i t h seman t i c n e t s , what we have been u s i n g i s an a b b r e v i a t i o n o f a c o m p l e t e n o t a t i o n . I m p o r t a n t l y the d i s t i n c t i o n between c o n c e p t s and i n s t a n c e s , o r t y p e s and t o k e n s , i s m i s s i n g h e r e , see B r a c h man ( 1 9 7 6 ) . Our examples o n l y show i n s t a n c e s . W i t h t h e c o m p l e t e s y s t e m , a c o n c e p t u a l l e v e l i s a v a i l a b l e t o s u p p l y i n t e r p r e t a t i v e s t r u c t u r e i n c l u d i n g a p p r o p r i a t e i n f e r e n c e r u l e s . What w e p ropose i s t h a t t hese i n f e r e n c e r u l e s s u p p l y t h e c o n n e c t i o n be tween e v e n t and s t a t e l o c a t i o n and o b j e c t l o c a t i o n . For example i t w o u l d b e a p p r o p r i a t e t o use a r u l e l i k e " i f some th i ng i s s l e e p i n g some p l a c e t h e n i n f e r t h a t t h a t t h i n g i s a t t h a t p l a c e " . A n o t h e r a s p e c t o f t h e s p a t i a l r e f e r e n c e p r o b lem i s t h a t l i k e t e m p o r a l r e f e r e n c e a m u l t i p l e e v e n t a n d s t a t e a n a l y s i s i s somet imes c a l l e d f o r . For example i n "He h e l d he r o n h i s l a p i n the t u n n e l . " , o n l y she must b e shown o n h i s l a p w i t h b o t h o f them i n t h e t u n n e l . A t y p i c a l m u l t i p l e e v e n t a n d s t a t e r e p r e s e n t a t i o n wou ld a l l o w f o r t h i s b y h a v i n g one e n t i t y showing h im h o l d i n g her c a u s i n g a n o t h e r e n t i t y t o b e t r u e , n a m e l y , t h a t she rema ined somewhere. The "one h i s l a p " can p r e d i c a t e t h i s l a s t s t a t e . The " i n t h e t u n n e l " can a p p l y t o t h e h i g h e s t f o r m , w i t h i n f e r e n c e Knowledge R e p r . 5 : SonHh*?m<»r 281 ru les assoc ia t ing the p red ica t ion w i t h the p a r t i c ipan ts . Sentences about motion are among those that requ i re mu l t i p le -even t -and-s ta te ana l ys i s . However the p red ica t ion of the l o c a t i o n of motion presents new d i f f i c u l t i e s . I t i s poss ib le to p re d ica te po in ts in mot ion, as in "He walked i n t o the house." . I t i s a lso poss ib le to pred ica te segments of mot ion, as in "He walked across the lawn", The sense of "across" here p a r r e l l e l l n g that in " the path across the lawn is made of b a r k . " . Further in mot ional events, m u l t i p l e pred icat ions can show order ing as in "He walked across the lawn up the s t a i r s i n t o the house. " . To a l low for these phenomena our proposal takes the l oca t i on of the movement as a h i s t o r y of the moving objects and uses two func t ions to produce from that h i s to ry segments of the journey. Figure II gives an example. The func t i on "SEGMENT" has the movement as i t s parameter, P-arc, and a segment, shown by " Q] ", as i t s va lue , V-arc . These segments are predicated by s p a t i a l re ferences. The funct ion "UNIT" produces another type of segment, shown by " [T) ". When a [] is predicated the asser t ion is that the r e l a t i o n held at the end but not bef o r e . The segments themselves are compared by rel i n k s to show o rder ing . This is the g i s t of our s p a t i a l reference proposal . But as w i l l be seen there are problems. Rela t ing Spat ia l and Temporal Reference At t h i s po in t two separate analyses have been re la ted to events and s ta tes of a f f a i r s . I t is claimed that they can and should be combined in to one. There are good reasons to do t h i s con jo in ing . These can begin to be seen through a problem w i th Figure I I in i n t e r p r e t i n g the l e s s than comparisons. These must have a scale fo r comparison. However being between segments of a journey, the only s p a t i a l scale ava i l ab le is the ad hoc one based on the journey i t s e l f . The way out of the dilemma is to associate the time of the journey w i t h the segments and to compare these t imes. The l oca t i on of the motion can be thought of as e x i s t i n g in four-d imensional space, we can c a l l i t a "space/t ime zone" ( f o l l ow ing a suggest i o n of Lemmon (1967). The UNIT and SEGMENT funct i o n can produce par ts of zones on the basis of parameters showing f i n a l t ime and i n t i a l and f i n a l t ime, r espec t i ve l y . The times appearing there can then be used fo r o rder ing . But now that time is a part of mot ion, at leas t here a separate temporal e n t i t y i s redundant. This is only ha l f the b a t t l e . What of nonmot ional events and states of a f f a i r s ? Apparently a space/time zone analys is is not out of order . F i r s t , i n apparent ly a l l du ra t i ona l events and s ta tes , i t i s poss ib le to make motional r e f e r ences. So w i t h "He sat on the books from New York to Bos ton . " , the mot ional problems reappear. Second, times must be found in order to locate objects used in ground ro les in s p a t i a l r e f e r ences, e . g . , the car in Figure I . Tf the e n t i t y in the f i g u r e ro le is a space/time zone, then the appropr ia te time is immediately a v a i l a b l e . From t h i s we conclude that space/time zones can be and should be used fo r both s p a t i a l and temporal re ference. We propose a s t y l e of ana lys i s exempl i f ied by Figure I I I . There would be a Space/Time-arc i d e n t i f y i n g a zone, " gj " . This zone could then be predicated by both s p a t i a l and temporal re ferences. Hence we have what we set out to f i n d a simultaneous analys is of s p a t i a l and temporal re ference.