ANAPHORA AND LOGICAL FORM:
ON FORMAL MEANING REPRESENTATIONS
FOR NATURAL LANGUAGE
Bolt
B o n n i e Nash-Webber
B e r a n e k and Newman
C a m b r i d g e MA 02138
Inc.
Raymond R e i t e r
D e p a r t m e n t o f Computer S c i e n c e
U n i v e r s i t y o f B r i t i s h Columbia
V a n c o u v e r , B . C . , Canada
Abstract
We a r g u e ,
on
favor
of
repre sentations
We
t hen
adop
f unct ion"
for
repre sentation,
ident i f y i n g
th
of
anaphoric
c
sugge s t s
prope r t i e s of a
th
facil itate
ante
possi ble
re
appro p r i a t e
such
pro
with
t o de
possi ble
r i c h c l a s s of a
invoking
general
world
knowledge.
The
separate
issue
of
choosing
the
most
a p p r o p r i a t e antecedent from t h i s set w i l l ,
in
general,
require
plausible reasoning
b a s e d on s u c h g e n e r a l w o r l d k n o w l e d g e .
We
a r e a l s o aware
of
instances
where
such
knowledge
is
required
even
to
propose
possible
antecedents.
Nevertheless,"
In
this
p a p e r , our c o n c e r n i s t o e x p l o r e the
implications
of
a
purely
syntactic
appfoach
as
well
as
to
ascertain i t s
limitations.
It
turns
out
that
a
surprisingly
r i c h class of anaphora, both
p r o n o u n s and e l l i p s e s , i s a m e n a b l e t o s u c h
an
approach,
provided
that
an
appropriately
structured
logical
MRL i s
used.
W e s h a l l f i n d t h a t t h e use o f
such
an
MRL l e a d s t o p a r t i c u l a r l y s i m p l e r u l e s
for i d e n t i f y i n g possible antecedents,
and
that
the
structure
of
the
MRL
can be
exploited
computationally
to
preclude
c e r t a i n i n a p p r o p r i a t e ones.
We s h a l l also
find
that
this
task
of
identifying
p o s s i b l e r e f e r e n t s i s i n t i m a t e l y bound
up
with
an
ability
to
form
•PPLopriate
descriptions
of
them,
an3" tfiat
these
cfescrlptlons
are,
in
turn,
intimately
related to logical form.
I I .
1.
to provide a
computational
approach
to
certain
problems
i n anaphora i n
natural language;
2.
to argue in favor of
formal
meaning
representation
languages
(MRLs) f o r
natural language.
Why L o g i c a l
Meaning
Representations?
Although
there
i s u n i v e r s a l agreement
within
the
AI
community
that
natural
language
understanding
systems
must
provide
some
underlying
meaning
representation
onto which surface s t r i n g s
are
mapped,
the
nature
of
this
representation
remains
a
contentious
issue.
One a s p e c t o f t h i s d e b a t e
has
to
do
w i t h the form t h a t t h i s r e p r e s e n t a t i o n
should
take.'
There
appear
to
be
two
points
of
view:
logical
forms
[e.g.,
S a n d e w a l l , 1 9 7 1 ; Woods, e t a l ,
1972]
and
structured
networks
[e.g.,
W i l k s , 1975;
Schank, 1975; Simmons, 1 9 7 3 ] ,
These two o b j e c t i v e s a r e n o t
independent.
It
appears
t h a t the s o l u t i o n s to c e r t a i n
problems in anaphora are
best
formulated
with
respect
to
an
appropriately
structured
logical
MRL,
so
that
the
structural
entities
o u t o f w h i c h such a n
MRL
is
composed
suggest
possible
a n t e c e d e n t s f o r anaphor r e s o l u t i o n .
More
specifically,
we
have
set
ourselves
the
following
problem:
what
form
should
a
meaning
representation
assume
in
order
to
facilitate
the
identification
of possible antecedents of
anaphorlc
expressions,
and
~what
computational
mechanisms
does
t h i s task
require?
Moreover,
we
have
chosen
to
investigate
t h i s problem of i d e n t i f y i n g a
set
of
possible
antecedents
without
Natural
formal
Despite
this
lack
of
any
d i s t i n c t i o n between n e t w o r k s
and
logical
forms,
there
is a widespread bias w i t h i n
t h e A I c o m m u n i t y a g a i n s t l o g i c a l MRLs
for
natural
language.
[See
for
example,
C h a r n i a k and
Wilks,
1976].
We
suspect
Lan^uapo - 7 :
121
Nash-Wcbber
that
there
are
two i m p l i c i t assumptions
underlying t h i s a n t i - f o r m a l p o i n t of view:
1. t h a t
the c h o i c e o f
a
logical
form
necessarily
implies
a commitment to a
corresponding
proof
t h e o r y as o n e ' s
s o l e c o m p u t a t i o n mechanism;
2. t h a t
logical
forms must
have
their
"natural"
representation
at
the
implementation
level,
e.g.,
that
( x ) ( E y ) . Px,y & Qx,y
must
be
represented
by
the
S-expression
( ( X ) ( E Y)(AND (P X Y)(Q X Y ) ) ) .
N e i t h e r o f these assumptions i s j u s t i f i e d .
We have a l r e a d y observed t h a t networks can
be best viewed as
implementation l e v e l
r e p r e s e n t a t i o n s f o r l o g i c a l f o r m s , and
as
we
shall
show
in S e c t i o n I V . ,
the
c o m p u t a t i o n s t h a t we propose
for
anaphor
r e s o l u t i o n w i t h i n a l o g i c a l MRL are in no
way based on any k i n d of p r o o f t h e o r y .
I f i t i s the
case
then
that
logical
forms and networks are one and the same,
why p r e f e r one over the o t h e r ?
We f a v o r
l o g i c a l form o n the f o l l o w i n g g r o u n d s :
A.
Semantics
By
virtue
of
i t s being a
formal
l a n g u a g e , a l o g i c a l form i n h e r i t s a w e l l
cfefmea
semantics,
namely,
i t s Tarskian
semantics.
This
i s not
the case
for
network r e p r e s e n t a t i o n s p r e s e n t e d _in vacuo
(i.e.,
without
a
t r a n s l a t i o n mechanism
mapping the network to a l o g i c ) .
As Woods
[1975] p o i n t s
out,
networks
often
fall
short of t h i s requirement.
B.
Computation
The
high
level
conceptual
representatio n
of meaning p r o v i d e d
by
logical
form s
encourages th e f o r m u l a t i o n
o f a p p r o p r i a t e p r o c e s s i n g a l g o r i t h m s at an
e q u a l l y h i g h c o n c e p t u a l l e v e l , independent
of how these l o g i c a l forms ar e r e p r e s e n t e d
at
the
im p l e m e n t a t i o n
This
1e v e l .
provides
f o r p e r s p i c u o u s de s c r i p t i o n s o f
algorithms,
the
without
spec i f y i n g
irrelevant,
CONS
cell
le v e l ,
pointer
chasing
deta i l s
required
by
network
representatio ns.
The exampl es of S e c t i o n
I V i l l u s t r a t e the
such
ease
w i t h which
r u l e s can be f o r m u l a t e d , as w e l l a s t h e i r
conceptual cl a r i t y .
There
are
two
issues
here:
representational
perspicuity
and
r e p r e s e n t a t i o n a l adequacy.
The
first
is
l a r g e l y a s u b j e c t i v e m a t t e r . We b e l i e v e
l o g i c a l forms to be more r e a d a b l e and
comprehensible
than
their
corresponding
network f o r m s , e s p e c i a l l y when
the
usual
Natural
network
primitives
are
considerably
augmented i n o r d e r t o c o r r e c t l y
represent
logical
connectives
and
q u a n t i f i e r s and
t h e i r scopes
[Schubert,
1975;
Hendrix,
1975] .
i ssue
The se cond
r e p r e sent a t i o n a l
largely
adequacy - i s f a r more i m p o r t a n t ,
d e a l i n g wi t h the a b i l i t y of a g i v e n MKL t o
express t h e meani ng of s u r f a c e s t r i n g s . A
related
of
cToseTy
i s s ue
is
th at
r e p r e s e n t a t i o n a l c l o s u r e . Can one t e l l ,
from
the g i v e n
s p e c ! " F i c a t i o n of an MRL,
what can and wh at
ca nnot be ex pressed
Bee ause any l o g i c a l MRL has
within
it?
d e f i ned s yntax a nd a w e l l
b o t h a wel 1
d e f i n e d se m a n t i c s
i t ne c e s s a r i i y e x h i b i t s
a h i g h deg ree o f c l o s u r e .
T h i s i s not the
case f o r n etwork r e p r e s e n t a t i o n s pr esented
jj2 v a c u o , p r e c i s e l y b ecause t hey have no
Many o f
semantics,
the
net work based
meaning re p r e s e n t a t i o n s i n c u r r ent n a t u r a l
systems
language
19 75; W i l k s ,
[Sch ank,
1975; Norm an and Rumelh a r t ,
19 75]
suffer
from
t h i s def ec t ,
a f a c t t h at m akes i t
diffic ult
extremely
their
to
ass ess
content.
It
is inst ructive
i n con n e c t i o n w i t h
the above d i s c u s s i o n t o note
these
that
v e r y same i s s u e s were ho t l y de bated w i t h i n
the
data base managemen t comm u n i t y d u r i n g
the e a r l y 1970 s.
There t o o ,
the b a s i c
c h o i c e wa s bet ween a ne twor k view o f data
[CODASYL, 1971 ]
or
and
a
1o g i c a l
so-called
£el.a t i q n a ^ ,
view Codd , 1970] .
Moreover, the a rguments advanc ed i n f a v o r
of
the re l a t i o n a l view were i n many ways
i s o m o r p h i c t o t hose we h ave ma de
f a vor i n g
logical
f orm f or meanin g repr esenta t i o n s .
At l e a s t w i t h i n the d a t a base comm u n i t y ,
the
logical
view cu r r e n t l y pre v a i l s ,
primarily
i t s high
beca use
cone e p t u a l
level
s o - c a l l ed
p r o v i des
"data
independen c e , " i . e . ,
the
on e 's v iew o f
im piemen t a t i o n
data
i s inde pendent of
details.
III.
On A p p r o p r i a t e L o g i c a l MRLs
I n t h e p r e v i o us s e c t i o n , we ar gued on
fairly
g e n e r a l grounds i n f a v o r o f f o r m a l
for
meaning
represe n t a t i o n
languag es
n a t u r a l language
Of c o u r s e , not j u s t any
logical
MRL wi 11 d o . At the ver y l e a s t ,
any such f o r m a l language mu s t prov i d e
for
quantification
logical
and
the usual
connectives,
b ut
these
even
under
requirements
t here
broad
rema i n s
a
spectrum
of
logical
possib le
representations,
There a re a t 1e a s t two
dimensions t o t h i s spectrum c o r r e sponding
to
represent a t i o n a l
and
level
r e p r e s e n t a t i o n a l s t. r uc t ur e . wiTn' r e s p e c t
to
level,
r e p F e s e n t a t l o ns
in current
systems
range
surfacy"
from
ve r y
[Simmons,
197 0]
"deep",
to
very
ones
[ Wilks,
primitive-based
1975;
Lanpua** - 7 : Mash-WeKher
12 2
Schank,
1975].
Wilks,
i n Computational
Semantics [Charniak and W i l k s , l 9 7 6 ; 1761
provides
a
good d i s c u s s i o n of these
issues.
In t h i s p a p e r ,
we
use
as
illustration
an MRL t h a t keeps v e r y c l o s e
to t h e s u r f a c e syntax and l e x i c o n . We do
so because t h e c o m p u t a t i o n a l t a s k t h a t we
have taken as a " f o r c i n g f u n c t i o n " f o r an
adequate
representation,
namely,
identifying
possible
antecedents
for
anaphor r e s o l u t i o n , s e e m s not to r e q u i r e a
deep
l e v e l and is moreover f a c i l i t a t e d by
a " s u r f a c y " one, at l e a s t f o r a broad and
i n t e r e s t i n g c l a s s o f phenomena.
Our f o c u s in d i s c u s s i n g l o g i c a l MRLs is
on t h e i r r e p r e s e n t a t i o n a l s t r u c t u r e . We
have
found
t h a t the need
to
provide
appropriate
antecedents
for
anaphor
resolution
suggests
certain
structural
c o n s t r a i n t s on p o s s i b l e MRLs which g r e a t l y
facilitate
this
process.
In
this
c o n n e c t i o n , we emphasize t h a t we are not
here
proposing
a f u l l y developed l o g i c a l
MRL. To do so would r e q u i r e , at the v e r y
l e a s t , adequate r e p r e s e n t a t i o n s f o r t e n s e ,
modality,
mass t e r m s ,
events,
etc.
i s s u e s which we have so f a r c o m p l e t e l y
ignored.
The MRL used in t h i s paper is
merely a v e h i c l e
for
displaying
certain
f o r m a l s t r u c t u r a l p r o p e r t i e s which we have
found
necessary f o r the i d e n t i f i c a t i o n o f
antecedents.
Our b e l i e f i s t h a t any f u l l y
articulated logical
MRL w i l l
have
to
p r o v i d e these s t r u c t u r a l u n i t s i f i t i s t o
deal
effectively
with
anaphora.
Accordingly,
one way of
viewing
our
p r o p o s a l is as a s e t of design c o n s t r a i n t s
on
the s t r u c t u r e of p o s s i b l e l o g i c a l MRLs
for n a t u r a l language.
The
remainder
of
this
section
d e a l s w i t h these s t r u c t u r a l
properties.
A
*
Lambda-expressions
For a f o r m a l MRL to be adequate f o r the
resolution of
verb
phrase
ellipsis,
it
must
provide for constructions equivalent
to l a m b d a - e x p r e s s i o n s . ( T h i s has a l s o been
observed b y
several
linguists
recently,
i n c l u d i n g Sag [1976] and W i l l i a m s [ 1 9 7 7 ] . )
For example, the sentence p a i r
La. John l o v e s Mary.
2. So does B i l l .
requires,
as
the
antecedent of the
e l l i p s e d verb phrase, the formal c o n s t r u c t
M x ) [Love x ,
Mary]
corresponding
to
"loving
Mary",
whence
the
resolved
sentence l b . becomes
B i l l , >s(x) [Love x , Mary]
which s i m p l i f i e s t o
Love B i l l , Mary
Natural
(Note
that
our
preferred
notation
for
applying
a
lambda-expression
to
an
argument is to f o l l o w the argument by
the
lambda e x p r e s s i o n , c o r r e s p o n d i n g to normal
subject-predicate
word order i n E n g l i s h . )
B.
Separation
of
Descr i g t i ^ o n a l
A s s e r t i o n ' a l I n f o r m a t i o n :'~Types
and
Since the antecedents of many anaphoric
expressions
are d e s c r i p t i o n s , an adequate
f o r m a l i s m must be so o r g a n i z e d t h a t
these
descriptions
stand
out c l e a r l y .
For
example, c o n s i d e r the p a i r of sentences
2a. Some c o t t o n T - s h i r t s are e x p e n s i v e ,
b. but not the one Mary gave J o h n .
In a
"flat"
predicate
calculus
MRL
(ignoring
the d i s t i n c t i o n between "some"
p l u r a l and "some" s i n g u l a r ) , sentence
2a.
might be r e p r e s e n t e d by
(Ex) . Cotton x & T - s h i r t x
& Expensive x
Now i n t u i t i v e l y ,
the antecedent of "one"
i n sentence 2b. i s something l i k e
"cotton
T-shirt",
but
from
the
flat
predicate
c a l c u l u s r e p r e s e n t a t i o n , t h e r e is no more
reason
to suppose t h a t Cotton and T - s h i r t
form a p o s s i b l e antecedent than Cotton and
Expensive, or T - s h i r t and
Expensive,
or
any one o r a l l t h r e e .
That i s , t h e r e i s
no
structural
indication
that
Cotton
T-shirt
is a r e f e r e n c e a b l e u n i t .
We
b e l i e v e such a n i n d i c a t i o n i s necessary i n
any
formalism
adequate
for
anaphor
resolution.
Using
the
s t r u c t u r e of a t.yped l o g i c ,
p r e d i c a t e s t h a t c o n s t r a i n the range
of
a
quantified
v a r i a b l e - i . e . , types - ( l i k e
T-shirt
here)
can
be
structurally
distinguished
from p r e d i c a t e s t h a t a s s e r t
things
(as
"Expensive"
does
here).
Moreover,
using
the lambda o p e r a t o r , the
n o t i o n of type can be extended from s i m p l e
o n e - p l a c e p r e d i c a t e s to more complex
ones
to
yield
all
and o n l y the a l l o w a b l e
referenceable e n t i t i e s .
For example, we
can r e p r e s e n t
"T-shirt"
as T - s h i r t
"cotton T - s h i r t "
as X ( u : T - s h i r t ) [ C o t t o n u]
" T - s h i r t t h a t Mary gave F r e d "
as > , ( u : T - s h i r t ) [Gave Mary, F r e d , u]
(The
first
is merely a
shorthand
for
> , ( u : T - s h i r t ) [True] .)
N o t i c e t h a t we are
postulating
a
representation for "cotton
T - s h i r t " that is
more
highly
structured
than a s i m p l e c o n j u n c t i o n of C o t t o n and
T-shirt,
i.e.
( T - s h i r t x & C o t t o n x) .
Specifically,
w e are s e p a r a t i n g t h a t p a r t
of the noun phrase d e n o t i n g
the p r i m a r y
Lan*ua£e-7: Mash-W*bber
123
manipulations
of
s t r u c t u r e d MRL;
class
to which an e n t i t y belongs ( u s u a l l y
t h e head n o u n ) f r o m t h o s e
parts
denoting
restrictions
on
that
c l a s s (conveyed by
a d j e c t i v e s and r e l a t i v e c l a u s e s ) .
This provides
yet
another
structural
p r o p e r t y t h a t a l o g i c a l MRL s h o u l d p o s s e s s
in
order t o f a c i l i t a t e the i d e n t i f i c a t i o n
of
antecedents
for
anaphor
resolution.
Consider
3.
Mary
and
bought a t i e - d y e d c o t t o n T - s h i r t
Fred
bought an e m b r o i d e r e d o n e .
Whether
intuitively
"one"
refers
to
"cotton
T-shirt"
or
"tie-dyed
cotton
T-shirt"
or
merely
"T-shirt",
it
must
refer
at
l e a s t t o " T - s h i r t " , the primary
c l a s s d e n o t e d b y t h e noun
phrase.
There
is
no
way
(pronominally)
in E n g l i s h to
refer
to
a
restriction
without
also
referring
to
the primary c l a s s .
This is
o u r m a i n r e a s o n f o r k e e p i n g them
distinct
i n o u r l o g i c a l MRL.
Another
consequence o f t h i s s e p a r a t i o n
of
descriptional
from
assertional
information
is
that
it
avoids problems
t h a t Woods [ 1 9 7 5 ] d i s c u s s e s
with
respect
to
adequate
representations for r e l a t i v e
clauses.
First,
sentences
like
"A
dog
t h a t had r a b i e s b i t a man" and " A dog t h a t
bit
a
man
had
rabies"
can be a s s i g n e d
d i s t i n c t r e p r e s e n t a t i o n s , f o r example
(Ex: X(u:Dog)
. Bit x,
(Ex: MurDog)
. Have x,
[Have u , R a b i e s l ) ( E y : M a n )
y
[ (Ey:Man) . B i t u, y] )
Rabies
More i m p o r t a n t l y , p r o c e s s i n g r u l e s s u c h a s
those proposed in S e c t i o n
IV,
can
treat
t h e s e two r e p r e s e n t a t i o n s d i f f e r e n t l y .
As
Woods
point
out,
conventional
semantic
networks f a i l t o represent the d i s t i n c t i o n
between these
two
sentences.
From
the
perspective
of
t h e a b o v e d i s c u s s i o n , one
reason for t h i s is c l e a r :
a
conventional
network
is
a
representation,
at
the
implementation
level,
of
a
"flat"
predicate logic.
IV
*
Examples
In
this
section,
we
present several
fragments of
discourse,
each
containing
anaphoric
expressions
pronouns and/or
ellipses.
Recall
that
we
are
not
concerned
here w i t h the kinds of e x t e r n a l
k n o w l e d g e n e e d e d t o c h o o s e among
possible
antecedents
for
an anaphoric e x p r e s s i o n .
What w e a r e
concerned
with
is
insuring
that
1.
i n cases where t h e a n t e c e d e n t
of
an
anaphoric
expression
is
not
e x p l i c i t l y p r e s e n t , i t can
often
be
derived
through
purely
syntactic
Natural
2.
an
appropriately
t h e p r o p e r t i e s we have p r o p o s e d f o r a
logical
MRL
make s u c h m a n i p u l a t i o n s
s i m p l e t o e x p r e s s and a p p l y .
our
approach
to
Since
d e v e l o p ing
form,
we
have
anaphora
and
1o g i c a l
one
discovered that i t is compatible with
major
l i n e of thought in t r a n s f o r m a t i o n a l
linguistics
today
(For
a
survey
of
current
ideas
on anaphora i n l i n g u i s t i c s
and p s y c h o l o g y , s e e [ N a s h - W e b b e r ,
1977].)
That
line
of thought i s i l l u s t r a t e d both
by
Chomsky's
recent
remark
that
the
"general
p r i n c i p les
of anaphora apply to
logical
forms
rather
than
to
surface
structures
directly"
[ C h o m s k y , 1 9 7 5 : 241
f t . 3 1 ] , a s w e l l a s by r e c e n t r e s e a r c h
on
logical
t h e o r i e s of verb phrase d e l e t i o n
and g a p p i n g [ S a g , 1 9 7 6 ; W i l l i a m s , 1 9 7 7 ] .
set
W i t h o u r ex a m p l e s , w e g i v e a s m a l l
yield
the
of
man i p u l a t i on
r ules
whic h
needed a n t e c e d e n t s .
We make n o c l a i m s f o r
rules;
there
t h e com p l e t e n e s s
of
these
obvious ly
rem a i n s a g r e a t de a l o f w o r k t o
(see . S e c t i o n
be done a l o n g
thes e
lines
VI) .
b e l i eve,
We do
howe v e r , t h a t t h e
example s i n d i e a t e t h e u t i l i t y of our b a s i c
a p p r o a c h , and t h a t t h i s a p p r o a c h
provides
a
pr o m i s i n g
for
further
di rection
researc h.
A.
Implicit
Sets
Our f i r s t e x a m p l e i l l u s t r a t e s
one
way
of
deriving
a
set
as
a
candidate
antecedent
for
"they".
Consider
the
sentences
4a.
b.
The
Mary g a v e e a c h boy a T - s h i r t ,
She b o u g h t tjiem a t F i l e n e ' s .
f i r s t may b e
4c.
represented
as
(Vx:Boy)(Ey:T-shirt)
. Gave M a r y , x , y
(For
simplicity,
we w i l l ignore the f a c t
t h a t "each
boy"
is
probably
anaphoric,
referring
to
e a c h boy i n some p r e v i o u s l y
m e n t i o n e d s e t o r one i m p l i c i t l y d e f i n e d b y
context,
and
treat
it
rather
as
a
u n i v e r s a l l y q u a n t i f i e d noun p h r a s e . )
are
c o n s i d e r i n g each
Notice
that
we
since
we
want
to
sentence i n d i v i d u a l l y
that
is
assign
it
a
repr e s e n t a t i o n
c o r r e c t , b u t w h i c h d o es n o t depend on what
may f o l l o w .
The r e s u I t w i T l
often
be
a
reading
that
is
in
some
sense
n o n c o m m i t t a l : i t w i l l be vague
but
true.
If
subsequently
we
l e a r n more a b o u t t h e
situation,
we
wi l l
refine
this
reflect
our
new
representation
to
knowledge s t a t e .
Lan*iia * e - 7 :
12U
Nash-Wfthber
The second
sentence
we
initially
with
its
anaphoric
o v e r t l y marked, t h a t i s ,
represent
elements
4 d . Bought SHE^ THEY , F i l e n e ' s
(We s u b s c r i p t the pronoun
symbols merely
to
keep s e v e r a l i n s t a n c e s of the same one
d i s t i n c t , as would be the case in "They
thanked her f o r t h e m " . )
Next, we i d e n t i f y
possible
referents
for
the
anaphoric
terms.
Since
Mary
is
the o n l y female
a r o u n d , we t r i v i a l l y a s s i g n her as the
referent of SHE.
Regarding
candidate
antecedents
for
THEY, ,
we p o s t u l a t e two ways of d e r i v i n g
p o s s i b l e s e t s from sentences l i k e 4a.
1. Form the set d e s c r i p t i o n of any
type
restricting
a universally quantified
variable.
(toe r e p r e s e n t
the
set
d e s c r i p t i o n of type C by { x | C x } . )
2. Let to be a
(prior)
formula
not
containing
the
anaphoric element
THEY, nor any n e g a t i o n
in
the main
clause.
( I n the c u r r e n t example, 4c.
plays
the r o l e of W.)
Suppose to has
an e x i s t e n t i a l l y quantified
variable
y
that
lies within
the scope of a
universally
quantified
variable.
Form
the
set d e s c r i p t i o n of the set
of
y ' s s a t i s f y i n g W.
T h i s is a
straightforward
procedure, i n v o l v i n g
the t y p e of y r e s t r i c t e d by
an
e x p r e s s i o n d e r i v i n g from W.
Details
are
given
in
[Nash-Webber,
forthcoming],
but the example should
suggest i t s b a s i c o u t l i n e .
Thus, sentence 4a. y i e l d s {x|Boy x ) ,
the
set
o f b o y s , v i a the f i r s t p r o c e d u r e , and
{ v | T - s h i r t v & (Ew:Boy) . Gave Mary,w,v}
via
the second one,
i.e.
the
set o f
T - s h i r t s , f o r each of which t h e r e is
some
boy
to whom Mary gave i t .
Substituting
each of these
sets
in
turn
for
THEY^,
yields
4e. Bought Mary, {x|Boy x } , F i l e n e ' s
f. Bought Mary, ( v | T - s h i r t v & (Ew:Boy)
. Gave M a r y , w , v } , F i l e n e ' s
That
is,
e i t h e r Mary bought a l l the boys
a t F i l e n e ' s o r she bought
there
all
the
T-shirts
she
gave
out.
Real w o r l d
knowledge would now be needed to choose
the more p l a u s i b l e r e a d i n g .
Notice
that
in
English
the pronoun
"they",
as w e l l as many p l u r a l
noun
phrases,
are
ambiguous
between
a
c o l l e c t i v e r e a d i n g ( " a l l t o g e t h e r " ) and
a
distributive
one ( " t a k e n one at a t i m e " ) .
Sometimes, a l e x i c a l
item w i l l
indicate
that
a
plural
should be understood
d i s t r i b u t i v e l y , a s " e a c h " does i n
"Mary's
suitcases
were
each
weighed at
the
Natural
airport".
Sometimes, semantic s e l e c t i o n a l
r e s t r i c t i o n s can be used to choose between
the two.
For example, " p i l e "
requires a
collective
interpretation
of i t s object:
"She p i l e d them i n t o a heap" doesn't mean
"for
each one, she p i l e d i t i n t o a h e a p " .
But o f t e n , o n l y unknown
aspects of the
situation
can
furnish
the
appropriate
information.
For example,
if we l e a r n
t h a t " M a r y ' s s u i t c a s e s were weighed at the
airport",
we
cannot say f o r c e r t a i n
whether each one
was
weighed
there
s e p a r a t e l y o r j u s t the whole l o t t o g e t h e r .
In
l i n e then w i t h the p o l i c y d e s c r i b e d
above of always o p t i n g
for
a vague but
true
interpretation,
rather
than making
unsubstantiated choices, we w i l l i n t e r p r e t
plurals non-distributively,
unless
or
u n t i l a d d i t i o n a l i n f o r m a t i o n would l e a d u s
to an a l t e r n a t e d e c i s i o n .
B.
T
ype Antecedents
Our
next
example
i l l u s t r a t e s the
identification
of
descriptions
as
c a n d i d a t e antecedents f o r anaphoric " o n e " .
5a. Mary gave each boy a green T - s h i r t ,
b. She gave Sue a red one.
t o e i n t e r p r e t sentence 5a. l i k e 4a. above,
except f o r the a d d i t i o n a l m o d i f i e r " g r e e n "
on T - s h i r t .
(tfx:Boy) (Ey: X ( u : T - s h i r t ) [Green u])
. Gave Mary, x, y
Sentence 5b. can be r e p r e s e n t e d
as
(Ez: A(u:P?) [Red u ] )
initially
. Gave SHE 1 Sue,z
That
is,
there
is
something o f unknown
type P? t h a t should be d e r i v a b l e from
context,
which we know e x p l i c i t l y is r e d ,
which some known female SHE, gave Sue.
Our
task
is
now
to
i d e n t i f y possible
antecedents f o r SHE, and P?.
There are s i m p l e s y n t a c t i c c r i t e r i a f o r
r e j e c t i n g Sue as an antecedent for SHE,.
So again by d e f a u l t , t h e r e being no o t h e r
females a r o u n d , we a s s i g n Mary as the
r e f e r e n t f o r SHE,.
As
for
P?,
i t s p o s s i b l e antecedents
include a l l
"recently"
mentioned
types,
independent o f the p a r t i c u l a r q u a n t i f i e r s .
("Recent"
seems
to mean here the c u r r e n t
s e n t e n c e , the p r e v i o u s one,
and perhaps
the one b e f o r e t h a t .
It does not seem to
be a f f e c t e d by task s t r u c t u r e
[Deutsch,
1975]
or
s t o r y s t r u c t u r e , or any of the
o t h e r f a c t o r s t h a t seem to change the
set
of
available
antecedents
for
definite
pronouns, "he", " i t " , e t c . )
Languare-7: Nash-Webber
125
The t y p e s e x p l i c i t l y g i v e n in example 5
are:
M u : T - s h i r t ) [Green u] ,
T-shirt
and
Boy.
Notice
t h a t when one
type
is
constructed out of
other
types
via
the
lambda o p e r a t o r ,
we i n c l u d e them a l l as
possible
candidate
antecedents.
Prescribing
e x a c t l y what
criteria
one
would use t o i d e n t i f y t h e
most
plausible
antecedent
for
P?,
or
in what way one
would a p p l y them, i s n o t w i t h i n the
scope
of t h i s paper.
But they would i n c l u d e the
semantic c r i t e r i o n
t h a t one be a b l e to
p r e d i c a t e Red of an e n t i t y of type P?
T h i s would e l i m i n a t e ) v ( u : T - s h i r t ) (Green u]
through
a p p l i c a t i o n of a "clashing c o l o r "
a x i o m : i f something i s g r e e n ,
it
is
not
red.
(Notice
that
if
sentence 5b. had
been " F r e d , she gave a l a r g e o n e . "
there
would be no reason to e l i m i n a t e t h i s
d e s c r i p t i o n as a p l a u s i b l e a n t e c e d e n t . )
Under r h e t o r i c a l c r i t e r i a , we would expect
parallelism
to
argue
for
plausibility.
That i s , i f two s u c c e s s i v e
sentences
are
structurally
similar
( " p a r a l l e l " ) and i n
t h e l a t t e r , a n a p h o r i c "one" h e l p s t o
fill
role R
( h e r e , the o b j e c t ) , then i t has a
very p l a u s i b l e
antecedent
in
the
noun
phrase
filling
role R in
the p r e v i o u s
sentence ( h e r e ,
the p r e v i o u s o b j e c t
"a
green
T-shirt").
But
our p o i n t here i s
not t o
s p e c i f y procedures
for
choosing
among c a n d i d a t e a n t e c e d e n t s ;
it is rather
to
show how a s u i t a b l e l o g i c a l framework
p r o v i d e s in a s t r a i g h t f o r w a r d way a l l
and
o n l y the a p p r o p r i a t e p o s s i b i l i t i e s .
c
*
P r e d i c a t e Antecedents
1•
Simple Ve£b Phrase D e l e t i o n
The
next
few examples i l l u s t r a t e some
problems i n v o l v i n g verb
phrase
ellipsis,
which are handled r a t h e r n e a t l y w i t h i n our
framework.
6 a . Mary gave Sue a T - s h i r t ,
b . Jane d i d t o o .
The
representation
sentence 6 a . i s
(Ex:T-shirt)
that
we
assign
to
John h i t a l i n g u i s t
Fred d i d t o o .
as opposed to
John was h i t by a l i n g u i s t .
Fred was t o o .
*Fred"~~dTd t o o .
T h i s example i s s i m p l e i n t h a t t h e r e i s
only
one
such
one-place
predicate
abstractable off a subject:
X(r)[(Ex:T-shirt)
. Gave r, Sue, x]
that
is,
giving
Sue
S u b s t i t u t i n g f o r P? y i e l d s
Jane, X ( r ) I ( E x : T - s h i r t )
which
is equivalent
a
T-shirt.
. Gave r , S u e , x J
to
( E x : T - s h i r t ) . Gave J a n e , Sue
Note t h a t
this
representation
commit
us
to b o t h g i r l s h a v i n g
the same T - s h i r t ,
nor need
different.
The
description of
one i s
x
does
not
g i v e n Sue
they
be
the f i r s t
flz: T - s h i r t z & Gave Mary, Sue,
z
"a T - s h i r t t h a t Mary gave S u e " ,
where ^
indicates
the
i n d e f i n i t e operator.
(z
might be c a l l e d in
English
"the
T-shirt
which
Mary
gave Sue" if no o t h e r T - s h i r t
i n t h e d i s c o u r s e meets t h i s
description.)
The second T - s h i r t is d e s c r i b a b l e as
V\w: T - s h i r t w & Gave J a n e , Sue w
. Gave Mary, Sue, x
Sentence 6 b . , we i n t e r p r e t as p r e d i c a t i n g
something (P?) of Jane t h a t had p r e v i o u s l y
been p r e d i c a t e d of someone e l s e :
P? Jane
To
i d e n t i f y possible
antecedents for
P?, we f i n d t h e o n e - p l a c e p r e d i c a t e s
that
either
are g i v e n e x p l i c i t l y o r
can b e
d e r i v e d v i a lambda a b s t r a c t i o n o n
the
subject p o s i t i o n .
(Again,
one p r o b a b l y
need o n l y search f o r
such
predicates
in
the
current
sentence
if
i t has s e v e r a l
c l a u s e s or in t h e one or
two
sentences
Natural
i m m e d i a t e l y p r e c e d i n g i t , a s the h a l f - l i f e
of
predicate
antecedents,
like
that of
t y p e a n t e c e d e n t s , seems to be v e r y
short.
Note
that
we
are v i e w i n g t h e
first
argument
place
of
a
predicate
as
corresponding to surface subject p o s i t i o n .
Though
this
requires
a
different
representation
for
active
and
passive
sentences,
we see the need f o r t h i s on
other
grounds,
for
example,
their
difference
with
respect
to
simple verb
phrase d e l e t i o n :
" a T - s h i r t t h a t Jane gave S u e " .
It
is
important
to be able to d e r i v e
such d e s c r i p t i o n s , s i n c e t h e e n t i t i e s they
d e s c r i b e may serve as a n t e c e d e n t s f o r
later
a n a p h o r i c e x p r e s s i o n s , f o r example,
6 c . N e i t h e r o f them f i t h e r .
where "them" r e f e r s t o t h e i m p l i c i t s e t o f
T - s h i r t s g i v e n t o Sue,
who
is
also
the
most p l a u s i b l e a n t e c e d e n t o f " h e r " .
L a n * u a * e - 7 : Nash-Webber
126
2
*
sloppy
identity
Our
ne x t
ex ample
illustrates
a
phenomenon t h a t
h as been
called
the
"sloppy
iden t i t y
problem"
[Ross, 1967] .
I t i n v o l v e s account i n g f o r the
appearance
of
an
addi t i o n a l
reading
f o r sentences
c o n t a i n i n g d e l e t e d verb phr ases.
That i s ,
while
sente nee 7a
seems
unambiguous,
sentence 7b
migh t mean e i t h e r t h a t Fred
beats G a r t h ' s w i f e or t h a t he beats h i s
own. How do we ace ount f o r t h i s ?
7a. Garth beats h i s w i f e ,
b. Fred does t o o .
We a s s i g n 7a. an i n i t i a l r e p r e s e n t a t i o n
in
which
its
anaphoric
term i s o v e r t l y
marked.
7 c . Beat G a r t h ,
'S(Wife)HE1
(where 'S is d e f i n e d to be a f u n c t i o n t h a t
takes a unary p r e d i c a t e l i k e W i f e , S c h o o l ,
etc.,
and
returns
a
function
like
"wife-of",
"school-of",
etc.
'S(Wife),
f o r example, is a f u n c t i o n
that
takes a
man
as i t s argument and r e t u r n s h i s w i f e :
'S(Wife)John is John's w i f e .
Having a
function
like
' S e l i m i n a t e s the need t o
p o s t u l a t e a separate "Y-of"
function
for
every unary p r e d i c a t e Y.)
With no o t h e r male a r o u n d , we can
a s s i g n HE, t o Garth b y d e f a u l t , t h a t i s ,
7 d . Beat G a r t h ,
'S(Wife)Garth
We a s s i g n sentence 7b. the
representation
7 e . P? Fred
Now, w h i l e t h e r e are no e x p l i c i t o n e - p l a c e
predicates
around
to
serve
as
an
antecedent f o r P?, t h e r e are two ways
in
which to a b s t r a c t one from 7 d .
(i) >(r)[Beat r,
( i i ) y ( r ) [Beat r ,
>(r)[Beat r,
y(r)[Beat r,
(While we have not done so h e r e , it
is
actually
i m p o r t a n t t o d i s t i n g u i s h whether
an argument p l a c e was o r i g i n a l l y f i l l e d by
an a n a p h o r i c e x p r e s s i o n or by a f u l l noun
phrase.
Observe
t h a t i f sentence 7a. had
read " G a r t h b e a t s G a r t h ' s w i f e " ,
which
would be r e p r e s e n t e d s i m p l y as 7 d . , the
from
3a. John h i t a c o p .
b. L i k e w i s e , a CIA a g e n t .
The f i r s t sentence may be r e p r e s e n t e d as
8c.
(Ex.-Cop)
. H i t John, x
and the second one as
(Ey:CIA-agent)
8d.
. Q? y
where G? stands f o r an anaphoric p r e d i c a t e
like
P?,
but one whose argument f i l l s a
non-subject r o l e .
To r e s o l v e 6>?, we must i d e n t i f y the
one-place
predicates
that
can
be
abstracted
from n o n - s u b j e c t
positions.
From 8 c . , we g e t
> ( r ) [ H i t John,
r]
substituted for $? in 8 d . , y i e l d s
. y,
>,(r)[Hit John,
r]
or more s i m p l y ,
(Ey:CIA-agent)
that i s ,
That i s , e i t h e r Fred beats G a r t h ' s w i f e o r
he beats h i s own.
Predicates
The p o i n t of the next ex ample
is
to
illustrate
abstracting
one-place
p r e d i c a t e s from p o s i t i o n s o t h er
than
the
surface
subject.
In English , "1ikewise",
"too",
"similarly",
etc. ,
followed
o p t i o n a l l y by a p r e p o s i t i o n and then by a
noun phrase i n d i c a t e t h a t the r o l e
filled
by the new noun phrase in a p r e v i o u s l y
mentioned p r e d i c a t e is a n o n - s u b j e c t
one.
(As i l l u s t r a t e d
earlier,
a noun phrase
f o l l o w e d by an a u x i l i a r y requ i r e s t h a t the
noun phrase f i l l s the s u b j e c t r o l e of
a
previous predicate.)
(Ey:CIA-agent)
'S(Wife)Garth]
'S(Wife)r]
Natural
3'
Abstracting
Non-Subject P o s i t i o n
which,
'S(Wife)Garth]
'S(Wife)r]
The
f i r s t represents beating Garth's wife
and t h e second,
beating
one's
own.
Substituting
f o r P?, we get the two
p l a u s i b l e readings
Fred,
Fred,
f o l l o w i n g s e n t e n c e , "Fred does t o o " , c o u l d
o n l y mean t h a t Fred
beats G a r t h ' s w i f e .
Thus in an a c t u a l i m p l e m e n t a t i o n , we would
have to i n d i c a t e "he = G a r t h " , r a t h e r than
simply replacing
"he"
with
"Garth", in
order t o d e r i v e a l l and o n l y
the
correct
lambda a b s t r a c t i o n s . )
. H i t John, y
" L i k e w i s e , John h i t a CIA a g e n t . "
It
might appear t h a t one c o u l d r e s o l v e
" l i k e w i s e " - e l l i p s e s a t the
level
of
the
surface
s t r i n g a l o n e , but t h i s i s not the
case.
Consider the f o l l o w i n g example:
9a. John gave S a l l y her p r e s e n t ,
b. Likewise, Fred.
Obviously, while s u b s t i t u t i n g
"Fred"
for
"Sally"
i n the s u r f a c e s t r i n g would y i e l d
an i n t e r p r e t a b l e s e n t e n c e , "John gave Fred
Langua g e - 7 : Nash-Webber
127
her p r e s e n t " , t h i s i s n o t
the
only,
nor
the most p l a u s i b l e r e a d i n g o f the e l l i p s i s
in 9b.
That i s , f o r e v e r y man f o r whom
there
is
some donkey t h a t he owns, he b e a t s i t .
Now w h i l e t h e r e
is
nothing
explicit
to
serve a s the antecedent f o r " i t " , i t t u r n s
out t h a t " i t " can a l s o r e f e r e n c e a c e r t a i n
kind
of
functional
entity
which a r i s e s
from e x i s t e n t i a l s .
We r e p r e s e n t 9 a . as
9 c . Gave J o h n , S a l l y ,
'S (Present) SHE.
which we i n t e r p r e t as
9 d . Gave J o h n , S a l l y ,
'S(Present)Sally
she
being
the
only
female
Sentence 9 b . , we r e p r e s e n t as
9e.
We p o s t u l a t e
the
following rule
identifying
a p o s s i b l e antecedent f o r
around.
1 . Find a t y p e r e s t r i c t i o n which c o n t a i n s
an
existentially quantified variable y
not w i t h i n
the
scope of e i t h e r
a
universal q u a n t i f i e r or negation.
Fred
To r e s o l v e ( , we i d e n t i f y the o n e - p l a c e
p r e d i c a t e s t h a t may be a b s t r a c t e d from
non-subject p o s i t i o n s .
From 9 d . , we g e t
s u b s t i t u t i n g for
c l a r i t y , we g e t
and
flattening
2 . Determine t h e d e s c r i p t i o n o f
y with
respect
to t h i s type r e s t r i c t i o n :
any
e n t i t y which s a t i s f i e s t h i s d e s c r i p t i o n
is
a possible
antecedent
for
IT.
(Again,
we o m i t the s p e c i f i c a t i o n of
the
rule
for
determining
y's
description,
a l t h o u g h one should be
c l e a r from t h e example.)
for
For i . , t h e r e i s one such type r e s t r i c t i o n
- [(Ey:Donkey) . Own u , y ] . The d e s c r i p t i o n
o f the e x i s t e n t i a l l y q u a n t i f i e d y i s
-Gave J o h n , F r e d , ' S ( P r e s e n t ) S a l l y
" L i k e w i s e , John gave Fred S a l l y ' s
present."
ii .
-Gave J o h n , F r e d , ' S ( P r e s e n t ) F r e d
" L i k e w i s e , John gave Fred F r e d ' s
present."
That i s , i t i s a f u n c t i o n w h i c h ,
given
a
u,
r e t u r n s a donkey t h a t u owns if u owns
a donkey.
For a g i v e n x t h e n ,
-Gave J o h n , S a l l y , Fred
" L i k e w i s e , John gave S a l l y F r e d . "
Donkey
Again,
the p r e f e r r e d i n t e r p r e t a t i o n would
be chosen by u s i n g w o r l d knowledge.
C.
Own
is a donkey
that
into i. yields
"Donkeys"
x
owns,
Substituting
iii .
As our
f i n a l example
of
how
an
appropriate
logical
representation
of a
sentence can y i e l d
antecedents
necessary
f o r anaphor r e s o l u t i o n , we w i l l c o n s i d e r a
particularly
bothersome
class
of
s e n t e n c e s , i l l u s t r a t e d by example 1 0 .
N o t i c e t h a t t h i s r u l e i s independent o f
how
the
type
c o n t a i n i n g the e x i s t e n t i a l
has been q u a n t i f i e d .
Thus, in
1 1 . Some man who owns a donkey b e a t s i t .
1 2 . Which man who owns a donkey b e a t s T t ?
10. Every man who owns a donkey
beats i t .
the
antecedent
of
"it"
is
the donkey
obtained by applying function ( i i ) to
the
q u a n t i f i e d v a r i a b l e associated w i t h "man".
in
identifying
the
The
problem
lies
I t i s not " a donkey",
antecedent o f " i t " .
The sentence do es n o t mean t h a t every man
but
who owns a donk ey b e a t s a donkey,
rather
that
he b e a t s any donkey t h a t he
there
is •no way
of
owns.
Moreover ,
existential
quantifier
construing
the
"it"
a s s o c i a t e d w i t h " a donkey" such t h a t
How does the
falls
within
i ts scope.
"it"
emerge
from
c o r r e c t antecede n t f o r
our framework?
We
first
interpretation
for
IT.
assign
sentence
Natural
10
v
•
Discussion
The
examples
of
Section
IV were
designed t o i l l u s t r a t e t h e f e a s i b i l i t y
of
deriving
possible
antecedents
for
anaphoric expressions d i r e c t l y
from a n
appropriately
structured
logical
representation.
Notice t h a t basic to t h i s
r e p r e s e n t a t i o n is an
adequate
indication
of
the
scope of
logical
operators
-
the
Langua
128
Nash-Webber
q u a n t i f i e r s , conjunction
and
negation f o r o t h e r w i s e , we c o u l d not d e a l c o r r e c t l y
w i t h a n t e c e d e n t s a r i s i n g from e x i s t e n t i a l s
(e.g.,
t h e examples i n S e c t i o n s IV.A and
IV.D).
A l s o b a s i c i s the r e c o g n i t i o n
and
c o r r e c t attachment of m o d i f i e r s - r e l a t i v e
c l a u s e s , p r e p o s i t i o n a l p h r a s e s , prenominal
modifiers,
e t c . - necessary f o r c o r r e c t l y
handling
"one"
anaphora
(e.g.,
Section
IV.B).
Taken
t o g e t h e r , these impose the
r e q u i r e m e n t of a p r e - p r o c e s s o r f o r mapping
surface
strings
onto
logical
forms a t
least
as p o w e r f u l
as t h a t of the LUNAR
system [Woods e t a l ,
1972].
It
follows
t h a t much of
the burden of antecedent
i d e n t i f i c a t i o n is
actually
being
placed
upon
t h i s pre-processor,
g i v e n the need
f o r an a p p r o p r i a t e l o g i c a l form b e f o r e our
approach can be a p p l i e d .
It should a l s o be c l e a r
t h a t what we
are
describing
in
this
paper
is
e s s e n t i a l l y a competence model f o r anaphor
resolution.
In
~ Its
crudest
i m p l e m e n t a t i o n , one would f i r s t generate a
s e t of p o s s i b l e a n t e c e d e n t s , and then t e s t
each of these by p l a u s i b l e reasoning using
g e n e r a l w o r l d knowledge.
Of c o u r s e , we
are n o t s e r i o u s l y
proposing
such
a
g e n e r a t e and
t e s t implementation.
There
are a v a r i e t y of h e u r i s t i c s
t h a t can be
invoked to a i d
the c h o i c e of
a most
p r o b a b l e antecedent
and
any performance
model must make use of such knowledge.
( H e u r i s t i c s f o r a s s i g n i n g antecedents have
been proposed t h r o u g h o u t the
linguistics,
psychology
and
AI
literatures.
See
[Nash-Webber, 1977] f o r
a d i s c u s s i o n of
many
of
them.)
Nevertheless,
even a
performance model must have the a b i l i t y to
determine
the
space
of
possible
alternatives
from which
such h e u r i s t i c s
are to make t h e i r c h o i c e .
Since some of
these
a l t e r n a t i v e s may not be p r e s e n t
e x p l i c i t l y , i t i s here t h a t
the
approach
o f t h i s paper becomes r e l e v a n t .
VI,
Further
Problems
As t h i s paper i s n e c e s s a r i l y b r i e f , w e
do n o t have the space to d i s c u s s at l e n g t h
such i n t e r e s t i n g i s s u e s a s the e f f e c t s o f
n e g a t i on or v a r i o u s opaque c o n t e x t s on the
a n t e c e d e n t s evoked.
These are
k i rid s of
[Nash-Webber,
forthcoming].
d i s c u s sed in
We wi 1 1 , however, mention one such i s s u e
— t h e problem o f e x i s t e n t i a l noun phrases
i n neg a t i v e c o n t e x t s .
The scope of
negation
is
inherently
ambiguous,
and
as
with quantifiers,
different
scope
interpretations
yield
different
antecedents.
Moreover,
some
i n t e r p r e t a t i o n s may y i e l d no antecedent at
all.
For example, we know t h a t in a
positive
context,
a n e x i s t e n t i a l noun
phrase w i l l always
result
in
an e n t i t y
which can be d e s c r i b e d in terms of t h a t
Natural
context.
So, if John m a r r i e d
a Swedish
girl,
"she" can r e f e r t o the Swedish g i r l
t h a t John m a r r i e d .
However, in a n e g a t i v e
c o n t e x t , "she" may r e f e r t o o t h e r e n t i t i e s
depending on how the scope of n e g a t i o n
is
interpreted.
Negated Vejrb
13a. John d i d n ' t marry a Swedish g i r l ,
b. He l i v e d w i t h he_£ f o r t h r e e y e a r s .
"she" = the Swedish g i r l John was
involved with
Negated Modi f i e £
14a. John d i d n ' t marry a Swedish
b. She was from Denmark.
girl,
"she" = the g i r l John m a r r i e d
Negated Main Desc£ijDtor
15a. John d i d n ' t marry a Swedish g i r l ,
b. She was at l e a s t 15 years
HTs s e n i o r .
"she" = the Swedish female John
married
Negated
Proposition
16a. John d i d n ' t c a t c h a t r o u t ,
b . *He ate i t f o r d i n n e r .
A g a i n , we would want
to
post u l a t e a
for
neutral
initial
r e p r e s e n t a t i on
but
n e g a t i o n , one t h a t might be vag ue,
would
n e v e r t h e l e s s be t r u e . Only when we
to
were r e q u i r e d
to
e.g.,
i n o rder
r e s o l v e an anaphoric e x p r e s s i o n - would we
then a t t e m p t to ma ke a commitmen t t o the
scope of n e g a t i o n ,
(Note t h a t a b e l i e f
context
poses much the same pr oblem as
the
negation, i . e .
t h a t of
determin ing
scope of b e l i e f .
Fo r example, in
17a. John t h o u g h t he m a r r i e d a Swedish
girl,
b. but she was r e a l l y
from Denmark.
as in Example 14 above, " s h e " is the g i r l
t h a t John m a r r i e d .
Here a g a i n , i t i s o n l y
the m o d i f i e r
"Swedish"
that
should be
taken a s
f a l l i n g w i t h i n the scope o f
belief.)
As we mentioned e a r l i e r , b e f o r e one can
f i x on a p a r t i c u l a r MRL, one must p r o v i d e
for
mass
concepts;
for
tense;
for
q u a n t i f i e r s other
than
universals
and
e x i s t e n t i a l s ; for f a c t s , events, states or
acts;
and
for generics,
among o t h e r
things,
as
the
following
examples
illustrate.
L a n * u a * e - 7 : Nash-Webber
129
18a. When John s p i l l s beer,
b. his dog l i c k s i j : up.
" i t " = the b i t of beer
19.
that John s p i l l s
John drinks beer because i.t tastes
good .
" i t " = beer
20a. Many l i n g u i s t s smoke, although
b
* t n e v know r t c a u s e s c a n c e r .
"they"
"it"
21a.
b.
b.
smoke
=
linguists
A b e a g l e s m i l e d at me y e s t e r d a y ,
They a r e v e r y f r i e n d l y d o g s .
"they"
23a.
t h e l i n g u i s t s who
smoking
Few l i n g u i s t s s m o k e , s i n c e
t h e y know i t c a u s e s c a n c e r .
"they"
22a.
b.
=
=
=
the
generic
class
John dunked M a r y ' s
inkwell.
r t made h e r c r y .
of
braids
beagles
in
"it"
=
24a.
John dunked M a r y ' s b r a i d s i n
inkwell.
He d i d n ' t regret doing i t .
b.
"it"
=
b.
Acknowledcjem e n t s
the
the event of John's dunking
Mary's b r a i d s in the i n k w e l l
the
This
research
was
supported
by
the
National
Institute
of
Education
under
Grant
MS-NIE-C-400-76-0116
and
by
the
N a t i o n a l R e s e a r c h C o u n c i l o f Canada
under
Grant
A-7642.
The a u t h o r s w i s h t o t h a n k
D r . A l a n M a c k w o r t h and D r .
William
Woods
f o r r e a d i n g and s u g g e s t i n g i m p r o v e m e n t s t o
e a r l i e r d r a f t s of t h i s paper.
References
the act of dunking Mary's
braids in the i n k w e l l
F i n a l l y , a l t h o u g h w e have i n d i c a t e d t h e
need
for p l a u s i b l e inference for choosing
an a p p r o p r i a t e c a n d i d a t e
from
a
set
of
possible
antecedents
(e.g.,
Section
IV.B.), it is
also
the
case
that
such
inferencing
may
be
needed
to
derive
Possible
antecdents.
That
is,
not
all
possible
antecedents
are
structurally
derivable.
25a.
N o t i c e t h a t our t r e a t m e n t o f a l l o f t h e
examples
of
Section
IV
has a d e c i d e d l y
s y n t a c t i c c h a r a c t e r : d e s c r i p t i o n s of those
e n t i t i e s proposed as p o s s i b l e
antecedents
are
either
explicitly
present
in
some
f o r m u l a o f t h e MRL, o r c a n b e d e r i v e d f r o m
such
a
formula
by
appropriate
local
operations
on
i t s s t r u c t u r e , independent
of
the
availability
of
general
world
knowledge.
While
there
a r e cases where
w o r l d knowledge i s r e q u i r e d even t o d e r i v e
possible
antecedents
for
pronouns,
we
believe
we
have
shown
that
the
vast
majority
can
be
derived
by
purely
syntactic
considerations
and
that world
k n o w l e d g e i s used ( p e r h a p s
heuristically)
to
select
from
among
these.
The
determination
of
possible
antecedents
based
on
such
purely
syntactic
considerations
and
the
formulation
of
design
constraints
o n MRLs t o f a c i l i t a t e
t h i s process best describe the
objectives
of t h i s paper.
Y e s t e r d a y I saw a c o u p l e i n t h e
park.
H e was w e a r i n g s h o r t s and she had
on a d a s h i k i .
C l e a r l y , w h a t i s r e q u i r e d i s some s o r t
of
general
knowledge of the f o r m :
"A couple
u s u a l l y c o n s i s t s o f two
individuals,
one
m a l e and one f e m a l e . "
A l t h o u g h we c a n see no a
priori
reasons
why
a
f o r ma l
approach
could
not
a c c o m m o d a t e t h e use o f p l a u s i b l e r e a s o n i n g
i n the d e r i v a t ion of p o s s i b l e a n t e c e d e n t s ,
we h a v e c h o s e n n o t t o e x p l o r e t h e s e i s s u e s
in t h i s paper,
R a t h e r , our
intention
in
this
work
i s t o f i r s t d e t e r m i n e j u s t how
far an e s s e n t i al l y s y n t a c t i c approach
can
be p u s h e d .
Natural
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]
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