Báo cáo khoa học: "A FOUNDATION FOR SEMANTIC INTERPRETATION " pot

Like Montague's, our semantics are compositional by design and strongly typed, with semantic rules in one-to-one correspondence with the meaning-affecting rules of a Marcus-style pars

A F O U N D A T I O N F O R S E M A N T I C I N T E R P R E T A T I O N

G r a e m e Hirst Department of C o m p u t e r Science

B r o w n University Providence, RI 02912

A b s t r a c t

Traditionally, translation from the parse tree repre-

senting a sentence to a semantic representation (such

as frames or procedural semantics) has a/ways been

the m o s t a d h o c p a r t of n a t u r a l language understand-

guistics, m o s t notably the system of formal semantics

k n o w n as M o n t a g u e semantics, suggest ways of putting

N L U semantics onto a cleaner and firmer foundation

W e are using a Montague-inspired approach to seman-

t h a t we are building Like Montague's, our semantics

are compositional by design and strongly typed, with

semantic rules in one-to-one correspondence with the

meaning-affecting rules of a Marcus-style parser We

have replaced Montague's semantic objects, functors

language Frail, and added a word sense and case slot

disambiguation system The result is a foundation for

semantic interpretation that we believe to be superior

~o previous approaches

I I n t r o d u c t i o n

m a p p i n g from a syntactically analyzed sentence of

natural language to a representation of its meaning

We exclude from semantic interpretation any con-

sideration of discourse pragmatics; rather, discourse

pragmatics operate upon the o u t p u t of the semantic

interpreter We also exclude syntactic analysis; the

integration of syntactic and semantic analysis becomes

very messy when c o m p l e x syntactic constructions are

considered, and, moreover, it is our observation t h a t

those who argue for the integration of the two are

usually arguing for subordinating the role of syntax, a

position we reject This is not to say t h a t parsing can

get by without semantic help; indirect object finding,

This work was supported by the Oflfice of Naval Research under

contract number N00014-79-C-0592

and prepositional phrase and relative clause attach- ment, for example, often require semantic knowledge Below we will show t h a t syntax and semantics m a y work well together while remaining distinct modules Research on semantic i n t e r p r e t a t i o n in artificial intelligence goes b a c k to Woods's dissertation (1967, 1968), which introduced procedural semantics in a natural-language front-end for an airline reservation system Woods's system had rules with patterns that,

w h e n they matched part of the parsed input sentence, contributed a string to the semantic representation

of the sentence This string was usually constructed from the terminals of the matched parse tree frag- ment T h e strings were combined to form a procedure call that, w h e n evaluated, entered or retrieved the ap- propriate database information This approach is still the predominant one today, and even though it has been refined over the years, semantic interpretation remains perhaps the least understood and most ad hoc area of natural language understanding (NLU).I However, recent advances in linguistics, most not- ably Montague semantics (Montague 1973; Dowry, Wall and Peters 1981), suggest ways of p u t t i n g NLU semantic interpretation on a cleaner and firmer foun- dation t h a n it now is In this paper, we describe such

a foundation 2

2 M o n t a g u e s e m a n t i c s

In his well-known " P T Q " paper (Montague 1973), Richard Montague presented the complete syntax and semantics for a small fragment of English Although

it was limited in vocabulary and syntactic com- plexity, Montague's fragment dealt with such impor- lit is also philosophically controversial For discussion, see

Fodor 1978, Johnson-Laird 1978, Fodor 1979, and Wilks 1982

2Ours is not the only current w o r k with this Ko~tl; in Section 7

we discuse other similarly motivated work,

t a n t semantic problems as opaque contexts, different

types of predication with the word be, and the "the

t e m p e r a t u r e is 90" problem; 3 for details of these, see

Dowty, Wall and Peters (1981)

Montague's semantic rules correspond to what we

have been calling semantic interpretation T h a t is, in

conjunction with a syntactic process, they produce a

semantic representation, or translation, of a sentence

There are four i m p o r t a n t properties of Montague

semantics t h a t we will examine here Below, we

will carry three of these properties over into our own

semantics

The first property, the one t h a t we will later drop,

is t h a t for Montague, semantic objects, the results

of the semantic translation, were such things as in-

from the cartesian product of points in time and pos-

sible worlds), properties of individual concepts, and

functions of functions of functions of functions At the

top level, the meaning, of a sentence was a t r u t h con-

dition relative to a possible world and point in time

These semantic objects were represented by expres-

sions of intensional logic; t h a t is, instead of translat-

ing English directly into these objects, a sentence was

first translated to an expression of intensional logic,

for which, in turn, there existed an interpretation in

terms of these semantic objects

Second, Montague had a strong theory of types for

his semantic objects: a set of types t h a t corresponded

to types of syntactic constituents Thus, given a par-

ticular syntactic category, such as proper noun or ad-

verb, Montague was able to say t h a t the meaning of

a constituent of t h a t category was a semantic object

of such and such a type 4 Montague's system of types

was recursively defined, with entities, t r u t h values and

intensions as primitives, and other types defined as

functions from one type to another in such a manner

t h a t if syntactic category X was formed by adding

category Y to category Z, then the type correspond-

ing to g would be functions from senses of the type of

3 T h a t is, t o e n s u r e t h a t " T h e t e m p e r a t u r e is ~0 a n d the t e m -

p e r a t u r e is rising* c a n n o t l e a d to t h e i n f e r e n c e t h a t "90 is ris-

ing"

4To be precise: t h e s e m a n t i c t y p e of a p r o p e r n o u n is set of

p r o p e r t i e s of i n d i v i d u a l c o n c e p t s ; t h a t of a n a d v e r b is f u n c t i o n

b e t w e e n set~ v[ i n d i v i d u a l c o n c e p t s ( D o w r y ¢~ a l Ig81: 183, 187)

Y to the type of X 5 Third, in Montague's system the syntactic rules and semantic rules are in one-to-one correspondence Each time a particular syntactic rule applies, so does the corresponding semantic rule; while the one operates on some syntactic elements to create a new element, the other operates on the corresponding semantic objects to create a new object t h a t will cor- respond to the new syntactic element Thus the two sets of rules operate in tandem

Fourth, Montague's semantics is compositional,

which is to say t h a t the meaning of the whole is a systematic function of the meaning of the parts At first glance this sounds trivial; if the noun phrase m y

namely the one sitting on my lap as I write this paper, then we do not expect it to refer to a different entity when it is embedded in the sentence [ love my pet

penguin, and a semantic system t h a t did not reflect this would be a loser indeed Yet there are alternatives

to compositional semantics

The first alternative is t h a t the meaning of the whole is a function of not just the parts but also the situation in which the sentence is uttered For ex- ample, the possessive in English is highly dependent upon pragmatics; the phrase Nadia's penguin could refer, in different circumstances, to the penguin t h a t Nadia owns, to the one t h a t she is carrying but doesn't actually own, or to the one t h a t she just bet on at the penguin races Our definition above of semantic inter- pretation excluded this sort of consideration, but this should not be regarded as uncontroversial

The second alternative to compositional semantics

is t h a t the meaning of the whole is not a systematic

function of the parts in any reasonable sense of the word This is exemplified by the interpretation of the word depart in Woods's original system, which varied greatly depending on the preposition it dominated (Woods 1967:A-43-A-46) For example, the interpreta- tion of the sentence:

AA-57 departs from Boston

is, not unreasonably:

5For e x a m p l e , t h e s e m a n t i c t y p e of p r e p o s i t i o n s is f u n c t i o n s

m a p p i n g s e n s e s of t h e t y p e of n o u n p h r a s e s to t h e s e m a n t i c t y p e

of p r e p o s i t i o n a l p h r a s e s

d e p a r ~ (as-57, b o s t o n )

That is, the semantic object into which depart is

translated is the procedure d e p a r t (AA-57 is an air-

line Right.) However, the addition of a prepositional

phrase changes this; Table 1 shows the interpreta-

tion of the same sentence after wrious prepositional

phrases have been appended For example, the addi-

tion of ~o Chicago changes the translation of depart;

to connect, though the intended sense of the word is

clearly unchanged, s

This is necessitated by the particular set of

database primitives that W o o d s used, selected for

their being %tom/c" (1967:7-4-7-11) rather than for

promoting compositions/Sty Rules in the system axe

able to generate non-compositional representations be-

cause they have the power to set an arbitrarily complex

parse tree as their trigger, and to return an axbitrary

representation that could modify or completely ignore

the components of the parse trees they are supposed to

be interpreting/ For example, a rule can say (1967:A-

44):

If you have a sentence whose subject is a flight,

whose verb is leave or depart, and which has

two (or more) prepositional phrases modifying

t h e verb, one with /from and a place name, the

other with a~ and a time, then the interpretation

is equal (dtime (a, b), c), where a is the

flight, b is the place, and c is the time

Thus while Woods's semantics could probably be m a d e

• reasonably compositional simply by appropriate ad-

justment of the procedure calls into which sentences

are translated, it would still not be compositional by

8~Ve h a v e simplified a Little h e r e in order to m a k e our point In

fact, s e n t e n c e s like t h o s e in Table I w i t h p r e p o s i t i o n a l p h r a s e s

will ~ c t u a l l y c a u s e t h e e x e c u t i o n of t w o s e m a n t i c rules: o n e for

t h e c o m p l e t e s e n t e n c e , a n d o n e for t h e s e n t e n c e it h a p p e n s to

c o n t a i n , A.A-57 d e p c r t s f r o m 8 o s ~ o ~ T h e r e s u l t i n g i n t e r p r e t a -

t i o n will be t h e c o n j u n c t i o n of t h e o u t p u t f r o m e a c h rule ( W o o d s

1967~9-5):

A A - 5 7 depLrts from B o s t o n to C h i c a g o

depar~ (aa-ST, boston) and connec~ (aa-57 boston, c~icago)

W o o d s l e a v e s it open ( 1 9 6 7 : 9 - 7 ) a,s to h o w the s e m a n t i c r e d u n -

d a n c y in s u c h e x p r e s s i o n s s h o u l d be h a n d l e d , thou~,h one of hie

s u g g e s t i o n s is a filter t h a t w o u l d r e m o v e c o n j u n c t s i m p l i e d b y

others, g i v i n g , in this case, t h e i n t e r p r e t a t i o n s h o w n in Table 1

7 N o r is t h e r e &nything t h a t p r e v e n t s t h e c o n s t r u c t i o n of rules

t h a t w o u l d r e s u l t in c o n j u n c t i o n s w i t h conflicting, r a t h e r t h a n

merely redund~tnt, terms

T A B L E 1

NONCOMPOSITIONALITY IN WOODS'S SYSTEM AA-57 departs from Boston

d e p a r t (aa-57, bos~on) A.A-57 departs from Boston to Chicago

conltecT, (aa-5T, besT, on chicago) AA-57 departs from Boston on Monday

dday (aa-57, boston, monday)

A A - 5 7 departs from Boston at 8:00am

equal (dtlme (aa-5T boston), 8:00am) AA-57 departs from Boston after 8:00am

greater (dtime (aa-5T, boston), 8:00am) A.A-57 departs from Boston before 8:00am greater (8:00am, dtlme (aa-5T boston))

Although Montague semantics has m u c h to recom-

m e n d it, it is not possible, ho~vever, to implement it directly in a practical N L U system, for two reasons The first is that Montague semantics as currently for- mulated is computationally impractical It throws around huge sets, infinite objects, functions of func- tions, and piles of possible worlds with great abandon Friedman, M o r a n and Warren (1978a) point out that

in the smallest possible Montague system, one with two entities and two points of reference, there are, for example, 22"s= elements in the class of possible denota- tions of prepositions, each element being a set contain- ing 2512 ordered pairs, s

T h e second reason we can't use Montague seman- tics directly is that truth-conditional semantics are not useful in AI; A/uses know/edge semant.ics (Tarnawksy 1982) in which semantic objects tend to be symbols or expressions in a declarative or procedural knowledge representation system Moreover, truth-conditional semantics really only deals with declarative sentences (Dowry eC al 1981:13) (though there has been work attempting to extend Montague's work to questions; e.g Hamblin 1973); a practical N L U system needs to

be able to deal with c o m m a n d s and questions as well

as declarative sentences

8 D e s p i t e t h i s p r o b l e m , F r i e d m a n et ¢I ( 1 9 7 8 b , 1978c) h a v e i m p l e -

m e n t e d M o n t ~ g u e s e m a n t i c s c o m p u t a t i o n a l l y b y u s i n g t e c h - n/ques for m a i n t a i n i n g p a r t i a l l y specified m o d e l s H o w e v e r , their

s y s t e m is i n t e n d e d ~s ~ t o o l for u n d e r s t a n d i n g M o n t a g u e s e m a n - tics better, r~ther than &s ~ usable N L U system (1978b:26)

There have, however, been attempts to take the

intensional logic t h a t Montague uses as an inter-

mediate step in his translations, and give it a new in-

terpretation in terms of A I - t y p e semantic objects, thus

preserving all other aspects of Montague's approach;

see, for example, Hobbs and Rosenschein 1977, and

Smith's (1979) objections to their approach There has

also been interest in using the intensional logic itself

(or something similar) as an AI representation ~ (e.g

Moore 1981) But while it may be possible to make

limited use of intensional logic expressions, I° there are

many problems that need to be solved before inten-

sional logic or other flavors of logical forms could sup-

port the type of inference and problem solving that

AI requires of its semantic representations; see Moore

1981 for a useful discussion Moreover, Gallin (1975)

has shown Montague's intensional logic to be incom-

plete (See also the discussion in Section 7 of work

using logical forms.)

Nevertheless, it is possible to use many aspects of

Montague's approach in semantics in AI The seman-

tic interpreter that we describe below maintains three

of the four properties of Montague semantics that

we described above, and we therefore refer to it as

"Montague-inspired"

TABLE 2

TYPES IN THE AHSITY SEMANTIC INTERPRETER

BASIC TYPES Frame a

( p e n g u i n ?x), Clove ?x) Slot

color, agent Frame determiner b (t~e ?x), Ca ?x)

O T H E R TYPES Slot-filler pair = slot ~ frame statement (color=red), (agent=(the ?x (f±sh ?x))) Frame descriptor = frame ~ slot-filler pair*

(pen~uln ?x (owner=Nadla)), (love ?x (agent=Ross) (patient=Nadla)), (dog ?x)

Frame statement [or instance c]

= frame determiner -~ frame descriptor (the ?x (penguin ?x (owner=Nadla))), (a ?x (love ?x (agent=Ross)

(pail ent=Nadl a) ) ), ( t h e ?x (dog ? x ) )

p e n ~ l n 8 7 [an instancel

3 Our semantic interpreter

Our semantic interpreter is a component of a system

that uses a frame-like representation for both story

comprehension and problem-solving The system in-

cludes a frame language, n a m e d Frail, a problem sol-

ver, and a discourse pragmatics component; further

details m a y be found in Charniak 1981, W o n g 1981a,

and W o n g 1981b The natural language front-end in-

cludes Paragram, a deterministic parser based on that

of Marcus (1980) Unlike Marcus's parser, Paragram

has two types of rule: base phrase structure rules and

transformational rules It is also able to parse un-

grammatical sentences; it always uses the rule that

matches best, even if none match exactly Paragram

is described in Charniak 1983

9 1 t o n i c a l l y , M o n t a g u e r e g a r d e d i n t e n s i o n a l l o g i c m e r e l y as a c o n -

v e n i e n c e in s p e c i f y i n K his t r a n s l a t i o n , a n d o n e t h a t w a s c o m -

p l e t e l y i r r e l e v a n t t o t h e s u b s t a n c e of his s e m a n t i c t h e o r i e s

l O G o d d e n ( 1 9 8 1 ) in f ~ c t u s e s t h e m for s i m p l e t r a n s l a t i o n b e t -

w e e n T h a i a n d E n g l i s h

a T h e q u e J t i o n - m ~ r k p r e f i x i n d i c a t e s & v a r i a b l e W h e n e v e r a f r e e

v ~ i a b l e i n a f r a m e is b o u n d t o a v ~ i a b l e in a f r a m e d e t e r m i n e r , a

u n i q u e n e w n a m e is g e n e r a t e d for t h a t v a r i a b l e a n d its b i n d i n g s

In t h i s p a p e r , we s h a l l a s s u m e for s i m p l i c i t y t h a t v a x i a b l e n a m e s

~ r e m a K i c a l l y ~ c o r r e c t " f r o m t h e s t a r t bDo n o t be m i s l e d b y t h e f a c t t h a t f r a m e s a n d f r a m e d e t e r m i n e r s

l o o k s i m i l a r T h e y H e a c t u a l l y v e r y d i f f e r e n t : t h e first is a g t a t i c

d a t a s t r u c t u r e ; t h e s e c o n d is a f r a m e retrieva~l p r o c e d u r e CAn i n s t a n c e is t h e r e s u l t of e v a l u a t i n g a f r a m e s t a t e m e n t in F r a i l

It is a s y m b o l t h a t d e n o t e s t h e o b j e c t r e f e r e n c e d b y t h e f r a m e

s t a t e m e n t To A b s i t y , t h e r e is n o d i s t i n c t i o n b e t w e e n t h e t w o ; ~n

i n s t a n c e c a n b e u s e d w h e r e v e r ~ f r a m e I t a t e m e n t c~n

The semantic interpreter is named Absity (for reasons too obscure to burden the reader with) As

we mentioned above, it retains three of the four properties of Montague semantics t h a t we discussed The property that we have dropped is, of course, t r u t h conditionality and Montague's associated treasury of semantic objects We have replaced them with AI- style semantics, and our own repertory of objects,

T A B L E 3

T Y P E C O R R E S P O N D E N C E S IN ABSITY

Major sentence

Sentence

Noun

Adjective

Determiner

Noun phrase

Preposition

Prepositional Phrase

Verb

Adverb

Auxiliary

Verb phrase

Clause end

F r a m e statement, instance Frame descriptor

Frame Slot-filler pair

F r a m e determiner

F r a m e statement, instance Slot name

Slot-filler pair (Action) frame Slot-filler pair Slot-filler pair

F r a m e descriptor Frame d e t e r m i n e r

which are components of the frame language Frail 11

We do, however, retain a strong typing upon our

semantic objects, t h a t is, each syntactic category has

an associated semantic type Table 2 shows the types

of components of Frail, how t h e y m a y be combined,

and examples of each; the nature of the components

listed will become clearer with the examples in the

next section Table 3 gives the component of Frail t h a t

corresponds to each syntactic type As a consequence

of the kind of semantic objects we are dealing with,

the system of types is not recursively defined in the

Montague style, but we retain the idea t h a t the type

of a semantic object should be a function of the types

of the components of t h a t object

We have also carried over from Montague seman-

tics the operation of syntactic and semantic rules in

t a n d e m upon corresponding objects However, it is not

possible to maintain the one-to-one correspondence of

rules when we replace Montague's simple syntax with

the much larger English g r a m m a r of the P a r a g r a m

parser This is because in Montague's system each syn-

tactic rule either creates a new node from old o n e s - -

for example, forming an intransitive verb phrase from

a transitive verb and a noun p h r a s e - - o r places a new

l l A l t h o u ~ h t h e o b j e c t t h a t r e p r e s e n t s a S e n t e n c e is • p r o c e d u r e

call in Frail u p o n a knowledge basej this is n o t p r o c e d u r ~ l s e m ~ n -

tics in t h e s t r i c t W o o d s s e n s e , as t h e mes~aing i n h e r e s n o t in t h e

p r o c e d u r e s b u t in the o b j e c t s t h e y m a n i p u l a t e

node under an existing o n e - - s u c h as adding an adverb

to an existing intransitive verb phrase These are" ac- tions t h a t clearly have semantic counterparts W h e n

we s t a r t to add m o v e m e n t rules such as passivizatioa and dative m o v e m e n t to the g r a m m a r , we find our- selves with rules t h a t have no clear semantic counter- part; indeed with rules t h a t , it is often claimed (e.g

C h o m s k y 1965:132), leave the meaning of a sentence quite unchanged

We therefore distinguish between parser rules t h a t should have corresponding semantic rules and those

t h a t should not As the above discussion suggests, rules t h a t a t t a c h nodes are the ones t h a t have seman- tic counterparts In P a r a g r a m , these are the base structure rules For this subset of the syntactic rules, semantic rules run in t a n d e m , j u s t as in Montague's semantics, m

It is a consequence of the above properties of our semantic interpreter t h a t we have also retained the p r o p e r t y of compositionaiity by design This fol- lows from the uniform typing; the correspondence bet- ween syntactic and semantic rules t h a t maintains this uniformity; and there being a unique semantic object corresponding to each word of English i~ (see Dowty e~

al 1981:180-181) Unlike those of Woods's (1967) air- line reservation system front-end discussed in Section

2, our semantic rules are very weak: t h e y cannot change or ignore the components upon which t h e y operate, nor can more t h a n one rule volunteer an inter- pretation for any node of the parse tree The power of the system comes from the nature of the semantic ob- jects and the syntax-directed application of semantic rules, rather t h a n from the semantic rules themselves

4 Examples

Some examples will make our semantic interpreter clearer First, let's consider a simple noun phrase, the book From Table 3, the semantic type for the determiner She is a frame determiner function, in this case ( t h e ? x ) , and the type for the noun book is a kind of frame, here (book ?x) These are combined

12In her s y n t h e s i s of t r a n s f o r m a t i o n a l s y n t a x w i t h Monta6,ue

a c r o s t i c s , P a r t e e (1973, 1975) observes t h a t the s e m a n t i c rule

c o r r e s p o n d i n g to m a n y t r a n s f o r m a t i o n s will simply be the iden- tity m a p p i n g

13We show in Section 6 how this may be reconciled w i t h lexical ambiguity

in the canonical w a y - - t h e frame name is added as an

argument to the frame determiner f u n c t i o n - - a n d the

result, ( t h e ?x (book ? x ) ) , is a Frail frame state-

ment (which evaluates to an instance) that represents

the unique book referred to 14

A descriptive adjective corresponds to a slot-filler

pair; for example, red is represented by ( c o l o r = r e d ) ,

where c o l o r is the name of a slot and r e d is a frame

instance, the name of a frame A slot-filler pair

can be added as an argument to a frame, so the red

book would have the semantic interpretation ( t h e ?x

(book ?x (color=red)))

Now let's consider a complete sentence:

Nadia bought the book from a store in the mall

Table 4 shows the representation for each component

of the sentence; note t h a t the basic noun phrases

have already been formed in the manner described

above Note also that we have inserted the pseudo-

prepositional subject and object markers s u s J and

osJ, which are then treated as if they were real

prepositions; see Hirer and Charniak 1982 or Hirst

1983 for details of this For simplicity, we assume that

each word is unambiguous (we discuss our disambigua-

tion procedures in Section 6); we also ignore the tense

cn the verb Table 5 shows the next four stages in the

interpretation First, noun phrases and their preposi-

tions are combined, forming slot-filler pairs Then the

prepositional phrase in the mall can be attached to a

store (since a noun phrase, being a frame, can have

a slot-filler pair added to it), and the prepositional

phrase from a store in the marl is formed The third

stage shown in the Table is the attachment of the slot-

filler pairs for the three top-level prepositional phrases

to the frame representing the verb Finally, the period,

which is translated as a frame determiner function,

causes instantiation of the buy frame, and the trans-

lation is complete

5 Semantic help for the parser

As we mentioned earlier, any parser will occasionally

need semantic help In Marcus-type parsers, this need

occurs in rules that have the form "If semantics prefers

1 4 N o t e ~ h a t i t is t h e r e s p o n s i b i l i t y " of t h e f r a m e s y s t e m t o d e t e r -

m i n e w i t h t h e h e l p of t h e p r a g m a t i c s m o d u l e w h i c h o n e o f t h e

b o o k s t h a t i t m~ty k n o w a b o u t is t h e c o r r e c t o n e in c o n t e x t

TABLE 4

ABSITY EXAMPL E

W O R D O R P H R A S E S E M A N T I C O B J E C T

Nadia ( t h e ?x ( t h i n g ?x

(propername="Nadla")))

the mall (the ?w (mall ?w))

• [period I (a ?u)

X over Y then do X ' ; otherwise do Y " To answer

such questions, we have a Semantic Enquiry Desk r, hat

operates upon the same semantic objects as the seman- tic interpreter Because these objects are components

of the Frail frame language, the Enquiry Desk can use the full retrieval and inference power of Frail in answering the enquiry

6 Word sense disambiguation

O n e problem that Montague semantics does not ad- dress is that of word disambiguation Rather, there is assumed to exist a function that m a p s each word to a unique sense, and the semantic formalism operates on the values of this function.Is Clearly, however, a prac- tical NLU system must take account of word sense am- biguity, and so we must add a disambiguation facility

to our interpreter Fortunately, the word translation function allows us to ~dd this facility transparently Instead of simply mapping a word to an invariant unique sense, the function can map it to whatever sense is correct for a particular instance

Our disambiguation facility is called Polaroid

Words Is Each word in the system is represented by

15This is not quite true Specified unique translations axe given

for p r o p e r n a m e s a n d f o r a f e w i m p o r t a n t f u n c t i o n w o r d s , s u c h as

t h e a n d be; s e e M o n t a ~ e 1 9 7 3 1 2 ] : 2 6 1 , o r D o w r y ~ ~l 1 9 8 1 : 1 9 2 f f

16polaroid is a t r a d e m a r k of the Polaroid C o r p o r a t i o n

T A B L E 5

A B S I T Y E X A M P L E (CONTINUED)

SUBJ N a d i a

( a g e n t , = ( t h e ?x

(thlng ?x (propername="Nadla"))))

OSJ the book

( p a t l e n l ; = ( t h e ? y ( b o o k ? y ) ) )

in t h e m a l l

(loca~lon:C1;he ?~ (mall ?w)))

a store in the mall

(a ?z (s~core ?z

(loca~ion=C~he ?w (mall ?w)))))

from a store in the mall

(source=Ca ?z (s~ore ?z

(locatlon=(the ?w (mall ?W))))))

N a S a b o u g h t t h e b o o k f r o m a s t o r e i n t h e m a l l

( b u y ? u

( a g e n t = ( t h e ?x ( t h l n g ?x

(propername="Sadia"))))

(patient=(the ?y (book ?y)))

(source=(a ?z (store ?z

(location=(the ?w (m~ll ?w)))))))

Nadia bought the book from a store in the mail

(a ?u

(buy ?u

( a g e n r , = ( t h e ? x (thing ? x

(propername=" N adla" ) ) ) )

( p a t i e n t = ( t h e ? y ( b o o k ? y ) ) )

(source=(a ?z (store ?z

(locatlon=(1;he ?w (marl ?w)))))))

a separate process that, by talking to other processes

and by looking at paths m a d e by spreading activation

in the knowledge base, figures out the word's mean-

ing Each word is like a self-developing photograph

that can be manipulated by the semantic interpreter

even while the picture is forming; and if some other

process needs to look at the picture (e.g if the

Semantic Enquiry Desk has an "if semantics prefers ~

question from the parser), then a half-developed pic-

ture m a y provide enough information Exactly the

same process, without the spreading-activation phase,

is used to disambiguate case roles as well Polaroid

W o r d s are described more fully in Hirst and Charniak

1982 and Hirst 1983

7 Comparison with other work

O u r approach to semantic interpretation m a y usefully

be compared with other recent work with similar goals

to ours

O n e such project is that of Jones and W a r r e n (1982), w h o attempt a conciliation between Montague semantics and a conceptual dependency representation (Schank 1975) Their approach is to modify Montague's translation from English to intensional logic so that the resulting expressions have a canonical interpreta- tion in conceptual dependency T h e y do not ad- dress such issues as extending Montague's syntax, nor whether their approach can be extended to deal with more m o d e r n Schankian representations (e.g Schank 1982) Nevertheless, their work, which they describe

as a hesitant first step, is similar in spirit to ours, and

it will be interesting to see h o w it develops

Important recent work that extends the syntac- tic complexity of Montague's work is that on general- ized phrase structure g r a m m a r ( G P S G ) (Gazdar 1982) Such g r a m m a r s combine a complex transformation- free syntax with Montague's semantics, the rules again operating in tandem G a w r o n et al (1982) have imple- mented a database interface based on G F S G In their system, the intensional logic of the semantic com- ponent is replaced by a simplified extensional logic, which, in turn, is translated into a query for database access Schubert and Peiletier (1982) have also sought

to simplify the semantic output of a G P S G to a more

~conventional" logical form; and Rosenschein and Shieber (1982) describe a similar translation process into extensional logical forms, using a context-free

g r a m m a r intended to be similar to a G P S G Iv

T h e G P S G approaches differ from ours in that their output is a logical form rather than an im- mediate representation of a semantic object; that

is, the output is not tied to any representation of knowledge In G a w r o n et al's system, the database

17 Rosenschein and Shieber's semaxltic translation fonow~ pars-

ing rather than running in parallel with it, but it iv strongly

syntax-dLrected, and is, it seems, isomorphic to ~n in-t~ndem translation that provides no feedback to the p~rser

provides an i n t e r p r e t a t i o n of the logical form, but

only in a weak sense, as the form must first pass

t h r o u g h another ( a p p a r e n t l y somewhat ad hoc) trans-

lation and disambiguati0n process Nor do these ap-

proaches provide any semantic feedback to the par-

set is These differences, however, are independent of

the choice of GPSG; it should be easy, at least in prin-

ciple, to modify these approaches to give Frail output,

or, conversely, to replace P a r a g r a m in our system with

a G P S G parser 19

The PSX-KLON~- system of Bobrow and Webber

(1980a, 1980b) also has a close coupling between syn-

t a x and semantics R a t h e r t h a n operating in tandem,

though, the two are described as " c a s c a d e d ' , with an

ATN parser handing constituents to a semantic in-

terpreter, which is allowed to return t h e m (causing

the ATN to back up) if the purser's choice is found

to be semantically untenable Otherwise, a process

of incremental description r e f i n e m e n t is used to in-

t e r p r e t the constituent; this relies on the fact t h a t

the syntactic constituents are represented in the same

formalism, KL-OSZ (Brachman 1978), as the system's

knowledge base The semantic interpreter uses projec-

tion rules to form an interpretation in a language

called JAaGON, which is then translated into KL-ONZ

Bobrow and Webber are particularly concerned with

using this framework to determine the combinatoric

relationship between quantifiers in a sentence

Bobrow and Webber's approach addresses several

of the issues t h a t we do, in particular the relationship

between syntax and semantics The information feed-

back to the parser is similar to our Semantic Enquiry

Desk, though in our system, because the parser is

deterministic, semantic feedback cannot be con fluted

with syntactic success or failure Both approaches rely

on the fact t h a t the objects manipulated are objects of

a knowledge representation t h a t permits appropriate

judgments to be made, though in rather a different

manner

Hendler and Phillips (1981; Phillips and Hendler

1982) have implemented a control structure for NLU

18Gawron et al p r o d u c e all p o s l i b l e t r e e s a n d t h e i r t r a n i l a t i o n s

for t h e i n p u t s e n t e n c e , s.nd t h e n t h r o w a w a y a n y t h a t d o n ' t m a k e

s e n s e t o t h e d a t a b a s e

I f ' O u r c h o i c e o f P a r a g r a m w a s l a r g e l y p r a g m a t i c ~ i t w&s avL/l-

• b l e - - a n d d o e s n o t r e p r e s e n t &ny p a r t i c u l a r c o m m i t m e n t t o

t r a n s f o r m a t i o n a l g ~ a m m a r s

based on message passing, with the goal of running syntax and semantics in parallel a n d providing seman- tic feedback to the parser A ~moderator" trans- lates between syntactic constructs and semantic repre- sentations However, their a p p r o a c h to interpretation

is essentially ad hoc (James Hendler, persoaoi cum- munication), and they do not a t t e m p t to put syntactic and semantic rules in strict correspondence, nor type their semantic objects

None of the work mentioned above addresses issues of lexical ambiguity as ours does, t h o u g h Bobrow and Webber's incremental description refine- ment could possibly be extended to cover it Also, Gawron et al have a process to disambiguate case roles

in the logical form after it is complete, which operates

in a manner not dissimilar to the case-slot part of Polaroid Words

8 C o n c l u s i o n

We have described a new a p p r o a c h to semantic inter- pretation, one suggested by the semantic formalism

of Richard Montague We believe this work to be a clean and elegant foundation for semantic interpreta- tion, in contrast to previous ad hoc approaches At the moment, though, the work is only a foundation; the test of a foundation is w h a t can be constructed

on top of it We do not expect the construction to be unproblematic; here are some of the problems we will have to solve

First, the approach is not just compositional but almost too compositional At present, noun phrases are taken to be invariably and unalterably specific and extensional, t h a t is to imply the existence of the unique entity or set of entities t h a t they specify In English, this is not always correct A sentence such as:

Nadia owns a unicorn

implies t h a t a unicorn exists, but this is not true of:

Nadia talked abou~ a unicorn

which also has a non-specific reading Montague's solution to this problem does not seem easily adaptable

to Absity 2° Similarly, a sentence such as:

The lion is not a beast to be trifled w/th

can be a generic statement intended to be true of all

lions; Montague did not treat generics

Second, the approach is heavily dependent upon

the expressive power of the underlying frame language

For example, our language, Frail, is yet deficient in

its handling of time, and this is clearly reflected in

Absity Further, the approach makes certain claims

about the nature of frame representations~that a

descriptive adjective in some sense is a slot-filler pair,

for example that might be shown to be untenable

W e will also have to deal with problems in

quantification, anaphoric reference, and m a n y other

areas Nevertheless, we believe that this approach to

semantic interpretation shows considerable promise

Acknowledgemems

I am grateful to Eugene Charniak, C~role Chaski, Jim

Hendler, Polly Jacobson, and Nadia Talent for their

comments upon earlier versions of this paper

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