Phrase Structure Grammars and Natural Languages

D u r i n g most o f t he l a s t two decades , c o m p u t a t i o n a l l i n g u i s t s and A I r e s e a r c h e r s w o r k i n g o n n a t u r a l language have assumed t h a t phrase s t r u c t u r e grammars, d e s p i t e t h e i r c o m p u t a t i o n a l t r a c t a b i l i t y , were u n s a t i s f a c t o r y d e v i c e s f o r e x p r e s s i n g the s y n t a x o f n a t u r a l l a n g u a g e s , however , d u r i n g t he same p e r i o d , t h e y have come to r e a l i z e t h a t t r a n s f o r m a t i o n a l grammars, wha teve r t h e i r l i n g u i s t i c m e r i t s , a r e c o m p u t a t i o n a l l y i n t r a c t a b l e as t hey s t a n d . The a s s u m p t i o n , u n c h a l l e n g e d f o r many y e a r s , t h a t PSG's were i n a d e q u a t e f o r n a t u r a l l anguages i s based o n arguments o r i g i n a l l y advanced b y t r a n s f o r m a t i o n a l l i n g u i s t s i n t he l a t e 1950»s and e a r l y 1 9 6 0 ' s . b u t r e c e n t work has shown t h a t none o f t hose arguments were v a l i d . The p r e s e n t paper draws on t h a t work t o argue t h a t ( i ) t h e r e i s n o r e a s o n , a t t h e p r e s e n t t i m e , t o t h i n k t h a t n a t u r a l l anguages a r e no t c o n t e x t f r e e l a n g u a g e s , ( i i ) t h e r e a re good reasons t o t h i n k t h a t t he n o t a t i o n s needed t o c a p t u r e s i g n i f i c a n t s y n t a c t i c g e n e r a l i z a t i o n s w i l l c h a r a c t e r i z e phrase s t r u c t u r e grammars o r some m ino r g e n e r a l i z a t i o n o f them, and ( i i i ) t h e r e a re good reasons f o r b e l i e v i n g t h a t such grammars, and t h e m o n o s t r a t a l r e p r e s e n t a t i o n s t h e y i n d u c e , p r o v i d e t h e necessa ry b a s i s f o r the seman t i c i n t e r p r e t a t i o n o f n a t u r a l l a n g u a g e s . I f t hese arguments a re v a l i d , t hen t h e p r o s p e c t s f o r a f r u i t f u l i n t e r a c t i o n between t h e o r e t i c a l l i n g u i s t i c s and A I a r e much b r i g h t e r t han t h e y would o t h e r w i s e be .