\documentstyle[sober,fleqn,named]{article}
\author{Yasuhiro Katagiri \\ ATR Media Integration \& Communications
Research Laboratories \\ 
2--2 Hikari-dai Seika Soraku Kyoto 619-02 Japan\\
{\tt katagiri@mic.atr.co.jp}} 
\title{Implicit Argument Analysis of Japanese Zero's}
\date{}

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\begin{document}

\maketitle

\section{Introduction}

We propose a dynamic semantical framework of analyzing Japanese zero's
based on the assumption that Japanese zero's are implicit arguments of
underlying semantical predicates. We demonstrate that our analysis
captures the fundamental generality among various types of uses of
zero's.  We compare our analysis with the centering account, which is
based on the asssumption that Japanese zero's are null form anaphoric
pronouns, and argue that our analysis is superior in providing a
general framework in which to state priorities among discourse
salience, contextual salience and domain knowledge in obtaining
preferred interpretations.

Noun phrases are often omitted from Japanese sentences without
impairing their grammaticality.  Some of the omissions can be compared
to unexpressed subjects of English infinitival or gerundive phrases.
But there are far too many cases than those predicted from such simple
syntactic clues.  Identification of what zero's denote is important
for determining full semantical content of utterances as well as for
exploring ways to capture coherence in Japanese discourse.

There have been several attempts to elucidate denotation
identification and discourse functions of zero's.  \cite{Kame:japa88}
and \cite{cote:cent90} proposed to apply the centering mechanism
originally developed for English pronoun coreference
resolution\cite{gros:prov83} to Japanese zero antecedent
identification problem.  \cite{kuno:danw78} gave an analysis of zero's
in terms of ellipsis phenomena.  \cite{yosh:iden88} discussed the zero
antecedent identification by stipulating a hierarchical topic
structure.  All of these works start from the assumption that Japanese
zero's are null form discourse anaphoric pronouns, e.g., pronouns
without overt phonological realizations which have antecedents in the
preceding discourse.

We first examine zero usages and demonstrate that the discourse
anaphoric pronoun assumption for zero's is overly restrictive and
misses an important generality among various zero usages.  We then
propose an account based on the notion of implicit arguments.  We give
semantics for zero's within the framework of dynamic
semantics\cite{heim:sema82,kamp:theo81,groe:dyna91,barw:noun87,groe:upda94}
enriched by the notion of
circumstance\cite{stal:asse78,kapl:dtha78,barw:situ83}.
We will argue that zero usages can be accounted for by introducing two
circumstantial variables for the point of view and the focus of
attention.

\section{Usages of Zero's}

% discourse pronoun usage

The type of zero's that has been extensively studied is exemplified by
the example below.  Here, the zero shown as $\phi_{\cst{WO}}$ in the
second sentence works as if it is a discourse anaphoric pronoun and
takes as its antecedent Hanako, which was introduced into the discourse
by the first sentence.
\begin{enumerate}\itemsep0pt
\item[(1)]
\begin{tabular}[t]{l@{--}lr@{--}lllr@{--}lll}
Hanako & ga & Taro & wo & butta. & Sorede & sensei & ga &
$\phi_{\cst{WO}}$ & sikatta.\\
& \cst{SBJ} & & \cst{OBJ} & hit & and & teacher & \cst{SBJ} & &
scolded\\
\multicolumn{10}{l}{(Hanako hit Taro. And the teacher scolded her.)}
\end{tabular}
\end{enumerate}

% Indexical and demonstrative use

But, we can find other types of usages of zero's, which do not belong
to the category of discourse anaphoric pronouns.  In the example (2),
$\phi_{\cst{GA}}$ refers to the speaker, so it works as if it is an
indexical pronoun.  Sentences like (3) can be found on a label of a
bottle of medicine.  Since the situation of reading the label has to
be such that the reader is facing a bottle of medicine, the reading
situation dictates that both of the zero's in (3) are to be anchored
to that bottle.  Similarly, in (4), the speaker's act of presenting
something before the hearer dictates that the zero in (4) is to be
anchored to the thing presented.  In all of these examples, zero's do
not have antecedents previously introduced into the discourse.  Instead,
what zero's denote are provided by the circumstance in which the
utterance and its interpretation of the sentences takes place.
\begin{enumerate}\itemsep0pt
\item[(2)]
\begin{tabular}[t]{ll}
$\phi_{\cst{GA}}$ & tukareta.\\
& tired\\
\multicolumn{2}{l}{(I am tired.)}
\end{tabular}
\item[(3)]
\begin{tabular}[t]{lll}
$\phi_{\cst{GA}}$ & gekiyaku & desu.\\
& Strong-medicine & be\\
\multicolumn{3}{l}{(This is a strong medicine.)}
\end{tabular}\\
\begin{tabular}[t]{lr@{--}lr@{--}lr@{--}lr@{--}lr@{--}l}
$\phi_{\cst{WO}}$ & kodomo & no & te & no & todoka & nai & tokoro & ni
& oite & kudasai.\\ 
& children & \cst{GEN} & hand & \cst{GEN} & reach & \cst{NEG} & place
& \cst{IOBJ} & put & \cst{REQ}\\
\multicolumn{11}{l}{(Keep this out of children's reach.)}
\end{tabular}
\item[(4)]
\begin{tabular}[t]{llll}
Hora, & $\phi_{\cst{GA}}$ & kirei & desyo.\\
look & & pretty & be\\
\multicolumn{4}{l}{(Look, isn't this pretty?)}
\end{tabular}
\end{enumerate}

% Non subcategorized argument

Furthermore, zero's are related to other implicit arguments which are
not necessarily lexically subcategorized arguments. In (5) below, both
``ane'' (sister) and ``migi'' (to the right of) have implicit
arguments that are not lexically subcategorized arguments: the person
whom the referent of ``ane'' is the sister of, and the person or the
object from whose point of view to-the-right-of relation is
predicated.  The notable fact about (5) is that both of these implicit
arguments are filled with the same value as $\phi_{\cst{GA}}$, namely
Hanako, which also coincides with the point of view, indicated by the
favor giving verb ``kureru,'' from which the utterances of the
sentences are to be made.  This shows that uses of zero's and
occurrences of implicit arguments interact through, in this case, their
relationship with the discourse functional notion of point of view,
and a simple view of zero's as discourse anaphoric pronouns does not
capture this interaction.
\begin{enumerate}\itemsep0pt
\item[(5)]
\begin{tabular}[t]{l@{--}lr@{--}lr@{--}ll}
Ane & ga & Hanako & ni & terebi & wo & kureta.\\
sister & \cst{SBJ} & & \cst{IOBJ} & TV set & \cst{OBJ} & give\\
\multicolumn{7}{l}{(Her sister gave Hanako a TV set.)}
\end{tabular}\\
\begin{tabular}[t]{llr@{--}lr@{--}lr@{--}ll}
Sokode & $\phi_{\cst{GA}}$ & sore & wo & hondana & no & migi & ni & oita.\\
and & & it & \cst{OBJ} & bookshelf & \cst{GEN} & right-of & \cst{IOBJ}
& put\\
\multicolumn{9}{l}{(And she put it to the right of the bookshelf.)}
\end{tabular}
\end{enumerate}

\section{Implicit arguments in English}

There have been several attempts to give semantic accounts to implicit
arguments in English both in terms of situation semantics\cite{lars:impl88}
and dynamic semantics\cite{dekk:exis93}.  But they are restricted to
implicit arguments which either are existentially quantified away or
obtain values from optional adjunct phrases which can appear later in
the same sentences, and cases where implicit arguments obtain their
values from utterance contexts, which are the main focus of interest
of this paper, are not within the scope of their works.

Partee\shortcite{part:bind89} showed that implicit arguments in
words like ``local'' can take their values either from a quantifier,
discourse context, or utterance context.  So in (6) below, the paper
can be about an athlete local either to each student, to the teacher,
or to the speaker.
\begin{enumerate}\itemsep0pt
\item[(6)] The teacher announced a new homework task.\\
Every student must write a paper about a local athlete.
\end{enumerate}
Gawron\shortcite{gawr:cont94} argues that some implicit arguments behave like
definite descriptions and can take values even from within the scope
of a negation.  So, (7) below means that no one could apply to the job
whose announcement was posted, and not that no one could apply to any
jobs.
\begin{enumerate}
\item[(7)] Until the job announcement was posted, no one could apply.
\end{enumerate}
These findings suggets that, similar to Japanese zero's we saw in the
previous section, range of functions implicit arguments in
English serve is broader than those of anaphoric pronouns.  

\section{Implicit argument analysis of zero's}

\subsection{Dynamic semantics framework}

Observations on the usages of zero's in the previous section suggest
close relationship between zero's and implicit arguments of underlying
semantic predicates.  We present in this section a dynamic semantical
analysis of zero's based on the assumption that zero's are in fact
implicit arguments.  The basic picture is as follows.  An utterance of
a sentence has the potential of changing discourse states modeled here
by the notion of information states\cite{heim:sema82,kamp:theo81,%
groe:dyna91,barw:noun87,groe:upda94}, either by
introducing new discourse referents into the ongoing discourse, or by
adding a piece of information to the discourse.  It has also been
argued that part of the information can also be provided through the
access to the circumstance of the
utterance\cite{stal:asse78,kapl:dtha78,barw:situ83,perr:cogn88}.
Circumstantial information is information about utterance events, and
includes such information as who the speaker in the utterance event
is, who the addressee is, when and where the utterance event takes
place, and which objects the speaker is making a demonstrative
reference to.  We further assume that the circumstance also provides
information on the point of view location and the focus of attention
of the speaker, and we will claim that zero's can refer either of
these two circumstantial objects.

Different from other dynamic semantical formulations, we take that
circumstantial information to be part of discourse information states.
Circumstantial information works not only as input, thereby
contributing relevant information from circumstances to utterance
contents, but also as output itself of the changes brought about by
the utterance.  This is to capture what Barwise and
Perry\shortcite{barw:situ83} called inverse information of the utterance in
dynamical terms.  Utterance of indexical expressions like ``I'' not
only contributes an individual to the content information by picking
it out from the utterance context but it also conveys the information
that that individual is the speaker of the utterance.  Japanese favor
giving expression ``kureru'' appeared in (5) is usually considered to
be used to convey speaker's assumed perspectives, which should better
be considered as inverse information rather than content information,
since it is information about the speaker, about the utterance
event.  \cite{perr:cogn88} also noted this aspect of inverse
information of an utterance by making a distinction between the
proposition created as contrasted with the proposition expressed by
the utterance.

Admittedly, change in circumstantial information is not caused by the
utterance itself only.  You can see who is speaking by being involved
in the utterance event, or you can know what time it is by looking at
the watch quite independent from what expression is being uttered.
Also, an act of pointing is a non-linguistic means of giving
circumstantial information by indicating and directing the focus of
attention.  Information thus aquired through non-linguistic means
nevertherless can contribute utterance content information when
combined with utterance inverse information.

\subsubsection*{Information state}
We first assume a domain of discourse $D$, a set of possible worlds
$W$, a set of discourse referents $V$, and a set of context roles $C$.
An information state $s$ is a set of triples \tuple{w,f,c} of possible
worlds $w\in{W}$, partial assignments $f\in{D^V}$, such that all
assignments $f$ in $s$ have the same domain of discourse referents,
and circumstance of utterances $c$, which is a sequence of utterance
events $u$, each of which is a partial functions from $C$ to $D$.
Possible world part of $s$ corresponds to the information content or
the truth condition of the discourse, and assignment part signifies
what discourse referents are available in the ongoing discourse.  The
circumstance part stands for the history of utterance events, each
component event specifies context role assignments.  We assume that
context roles $C$ includes $x_{\cst{SP}}$, $x_{\cst{HR}}$,
$x_{\cst{NOW}}$, $x_{\cst{HERE}}$, $x_{\cst{POV}}$ and
$x_{\cst{FOA}}$, which stand for the speaker, the addressee, the
utterance time, the utterance place, the point of view location and
the focus of attention, respectively.

\subsubsection*{Meaning of a sentence $\varphi$}
Meaning of a sentence $\varphi$ is a partial function \trns{\varphi}
from information states to information states.  We follow the
dynamic semantics convention, and write the result of 
applying \trns{\varphi} to $s$ as $s\trns{\varphi}$, 
Meaning of a multi-sentence discourse $\varphi_1;\ldots;\varphi_n$ is
also a partial function defined by the composition of the meaning
function of each component $\varphi_i$.
\[s\trns{\varphi_1,\ldots,\varphi_n} =
s\trns{\varphi_1}\ldots\trns{\varphi_{n}} \]

\subsection{Contribution of various NPs}

Equipped with the mechanism of the previous section, we can now write
the contributions of various types of noun phrases including zero's to
the discourse.  We first categorize all NPs into the class of overt
NPs and zero's.  Since Japanese does not syntactically distinguish
indefinite and definite NPs among overt NPs, we make the distinction
in the usage types of overt NPs.  Their usages are divided into three
classes, new object introduction, information addition on already
introduced objects and reference to circumstantial objects.  Discourse
anaphoric pronouns always serve to add information on already
introduced objects. Indexical pronouns and demonstratives always
serve to refer to circumstantial objects.

\subsubsection*{Overt NP}
\begin{itemize}\itemsep0pt
\item New object introduction
\begin{quote}
When a new object is introduced into the discourse, the domain of the
assignment is extended to include the object, and the set of possible
worlds is narrowed down to reflect the addition of information content
brought about by the noun phrase description and
predication. Circumstance is extended by attaching the current
utterance event to the utterance history.
\end{quote}
\[s\trns{\cst{NP}_x\ \cst{Pred}} = \{\tuple{w,f,c.u}\ |
\begin{array}[t]{l}
\exists g [\tuple{w,g,c}\in{s}\wedge g <_x f ]\\
\wedge f(x)\in{w(\cst{NP})}\wedge f(x)\in{w(\cst{Pred})} \}
\end{array}\]
\item Information addition
\begin{quote}
In the case of information addition, only the set of worlds is
narrowed down without any extension of the domain of the assignment.
Circumstance is extended with the current utterance event.
\end{quote}
\[s\trns{\cst{NP}_x\ \cst{Pred}} = 
\{\tuple{w,f,c.u}\ |\ \tuple{w,f,c}\in{s}\wedge f(x)\in{w(\cst{NP})}\wedge f(x)\in{w(\cst{Pred})} \}\]
\item Circumstantial object reference
\begin{quote}
Here, the referent is specified through a context role $x_{r}\in{C}$
in the utterance event $u$, a new discourse referent $z$ is introduced
into the discourse if it is not yet already introduced, and the set of
possible worlds is narrowed down in accordance with the information
brought about by the NP description, if any, and predication.
\end{quote}
\[s\trns{\cst{NP}_{x_r}\ \cst{Pred}} = \{\tuple{w,f,c.u}\ |
\begin{array}[t]{l}
\exists g [\tuple{w,g,c}\in{s}\wedge g\leq_z f] \wedge
f(z) = u(x_r)\\
\wedge f(z)\in{w(\cst{NP})}\wedge f(z)\in{w(\cst{Pred})} \}
\end{array}\]
\end{itemize}

\subsubsection*{Zero}
A zero is either a bound variable or a contextual.  A bound variable
zero is either coindexed with another NP within a sentence, or it is
existentially quantified away.  A contextual zero is either
$x_{\cst{FOA}}$ or $x_{\cst{POV}}$.  The latter is the point of our
proposal on the use of zero's.  We regard all contextual use of zero's
as implicit arguments, and assume that they refer to objects in either
the focus of attention or the point of view of the speaker, both of
which are modeled here by the context roles.

\subsection{Information integration}

\subsubsection*{Non-linguistic update}

Information is provided not only by linguistic means.  When we get
involved in an utterance event, we most likely obtain various
information about that event through visual and other perceptual
means.  When we identify a known individual mentioned in the
utterance, we can probably use our own knowledge about it to further
refine the information.

One useful and common way to obtain information non-linguistically is
through preceptual identification of context roles.  We can know who
is talking to whom by simply observing the utterance.  Perceptual
identification of the context role $x_r$ through involvement to
utterance event $u$ of $\varphi$ can be captured as an additional
condition on the circumstance component of information states.
\begin{quote}
For all $\tuple{w,f,c.u},\tuple{w',f',c'.u'}\in{s}\trns{\varphi}$\ 
$[u(x_r) = u'(x_r)]$
\end{quote}

\subsubsection*{Role merging}

Suppose we overhear an exchange between two people: ``You eat large
pizza,'' and then ``I am hungry.''  We can integrate these two pieces
of information and obtain the information that the second speaker both
eat large pizza and is hungry, without knowing the identity of either
persons.  This is because we form a merge between the addressee role
of the first utterance and the speaker role of the second utterance.

We say the role $x_r$ in the $i$-th utterance and the role $x_{r'}$ in
the $j$-th utterance are merged in an information state $s$ when for
all $i$-th utterance event $u$ and $j$-th utterance event $u'$ in $s$,
$u(x_r)$ is equal to $u'(x_{r'})$.

This way of information integration at the level of context roles is
also often exploited as default role persistence between successive
utterances.

\subsection{Point of view and focus of attention}

Different from circumstantial variables corresponding to indexicals
like ``I,'' ``now'' and ``here,'' whose values are solely determined by the
event of the making of the utterance, both the point of view and the
focus of attention can freely take their value from both the
circumstantial objects and discourse objects.  They are more like
demonstratives for that respect.  We can distinguish three cases of
the value assignment for both $x_{\cst{POV}}$ and $x_{\cst{FOA}}$.

\subsubsection*{Three types of value assignment to the point of view}
\begin{itemize}\itemsep0pt
\item[(a)] $x_{\cst{POV}}$ is anchored to the speaker.  This is the
most dominant and therefore the default assignment for $x_{\cst{POV}}$.
\item[(b)] $x_{\cst{POV}}$ is anchored to an object in the utterance
circumstance other than the speaker. 
\item[(c)] $x_{\cst{POV}}$ is anchored to an object in the discourse.
This is the case where the speaker expresses certain information
content by taking on the point of view of a person in the discourse
other than herself.  This type of point of view shift is often
indicated by the use of specific lexical items, including favor giving
verbs, directional verbs and attitude verbs.
\end{itemize}

\subsubsection*{Three types of value assignment to the focus of attention}
\begin{itemize}\itemsep0pt
\item[(a)] $x_{\cst{FOA}}$ is anchored to an object in the utterance
circumstance. Focus of attention in the utterance circumstance can be
indicated by pointing and presenting.
\item[(b)] $x_{\cst{FOA}}$ is anchored to an object in the discourse
domain which is in the focus of attention of the speaker. This
corresponds to the discourse salience.  Discourse topic is a typical
case of this type of focus of attention.
\item[(c)] $x_{\cst{FOA}}$ is anchored to an object in the discourse
domain which is in the focus of attention of $x_{\cst{POV}}$.  This
presupposes the case (c) with the point of view.
\end{itemize}

\subsection{Account for examples}

% Explanation of examples (1)-(5)

Let us examine how our analysis account for some of our previous examples.
First, look at our first example (1).  For its most natural
interpretation, zero $\phi_{\cst{WO}}$ in the second sentence has to
work as the focus of attention $x_{\cst{FOA}}$.  Then, from the
definitions given in 4.2, 
\begin{eqnarray*}
\lefteqn{s\trns{\mbox{$\mbox{Hanako}_{x}$-ga $\mbox{Taro}_{y}$-wo butta};
\mbox{sorede $\mbox{sensei}_{z}$-ga $\phi_{\cst{FOA}}$ sikatta}}}\\
& = & \{\tuple{w,f,c.u.u'}\ |
\begin{array}[t]{l}
\exists g [\tuple{w,g,c}\in{s}\wedge Dom(f)=Dom(g)\cup\{x,y,z\}]\\
\wedge f(x)=\cst{Hanako}\wedge f(y)=\cst{Taro}\wedge
f(z)\in{w(\cst{teacher})}\\
\wedge\tuple{f(x),f(y)}\in{w(\cst{hit})}
\wedge\tuple{f(z),u'(x_{\cst{FOA}})}\in{w(\cst{scold})} \},
\end{array} 
\end{eqnarray*}
This gives an anaphoric interpretation only when
$u'(x_{\cst{FOA}})=f(x)$.  That is, the utterance event u' in the
circumstance component of $s$ has to give enough contextual salience
and prepare an adequate value assignment of $x_{\cst{FOA}}$ to Hanako
to establish an anaphoric link.

Next, take a look at our example (3).  Here again, we assume
$\phi_{\cst{GA}}$ works as the focus of attention $x_{\cst{FOA}}$.
\[s\trns{\mbox{Hora, $\phi_{\cst{FOA}}$ kirei desyo}} =
\{\tuple{w,f,c.u}\ | \tuple{w,f,c}\in{s}\wedge
u(x_{\cst{FOA}})\in{w(\cst{pretty})} \}\] 
In this example, without further information the hearer can only
conclude that whatever the speaker is focusing her attention to is
pretty.  When the utterance event $u$ involves the speaker's act of
demonstration of a certain object $a$, then the hearer can obtain
further information through the operation of a non-linguistic update
that $a$ is pretty. 

Lastly, we examine the example discourse (5).  Here, the use of the
favor-giving verb ``kureru'' in the first sentence indicates
that the point of view of the speaker $x_{\cst{POV}}$ is placed on the
referent of the ``{\it ni}''-marked NP, that is, in this case, Hanako.
\begin{eqnarray*}
\lefteqn{s\trns{\mbox{$\mbox{Ane}_{x}$-ga $\mbox{Hanako}_{y}$-ni
$\mbox{terebi}_{z}$-wo kureta}}\ ( = s')}\\
& = & \{\tuple{w,f,c.u}\ |
\begin{array}[t]{l}
\exists g [\tuple{w,g.c}\in{s}\wedge Dom(f)=Dom(g)\cup\{x,y,z\}]\\
\wedge\tuple{u(x_{\cst{POV}}),f(y)}\in{w(\cst{sister})}\wedge
f(y)=\cst{Hanako}\wedge\\
f(z)\in{w(\cst{TV-set})}\wedge u(x_{\cst{POV}})=f(y) \}
\end{array}
\end{eqnarray*}
The implicit argument in \cst{sister} obtains the value \cst{Hanako}
from the context role $x_{\cst{POV}}$.

Next, we take the zero
$\phi_{\cst{GA}}$ in the second sentence to be the point of view of the
speaker $x_{\cst{POV}}$.  
\begin{eqnarray*}
\lefteqn{s'\trns{\mbox{Sokode $\phi_{\cst{POV}}$ $\mbox{sore}_{z}$-wo
$\mbox{hondana}_{t}$-no $\mbox{migi}_{l}$-ni oita}}}\\
& = & \{\tuple{w,f,c.u'}\ |
\begin{array}[t]{l}
\exists g [\tuple{w,g,c}\in{s'}\wedge Dom(f)=Dom(g)\cup\{t,l\}]\\\
\wedge\tuple{u'(x_{\cst{POV}}),f(z),f(v)}\in{w(\cst{put})}
\wedge f(t)\in{w(\cst{bookshelf})}\\
\wedge\tuple{f(v),f(t),u'(x_{\cst{POV}})}\in{w(\cst{right-of})} \}
\end{array}
\end{eqnarray*}
Both the zero and the implicit argument of \cst{right-of} obtain the
value \cst{Hanako}, provided that $u(x_{\cst{POV}}) =
u'(x_{\cst{POV}})$, that is, when successive utterances $u$ and $u'$
share their point of view roles, which usually holds by default by
role merging between successive utterances.  So, this default condition 
leads to the most preferred interpretation.

\subsection{Topic marker ``wa''}

It has been pointed out\cite{yosh:iden88} that discourse anaphoric
zero's often take as their antecedents topic NPs introduced by the
topic marker ``wa.''  We can accommodate this observation by assuming that, 
similar to the favor giving verb ``kureru,''
``wa'' works to change the circumstance component such that $x_{\cst{FOA}}$
is equated with the discourse referent for
the NP introduced by ``wa'' in the added utterance event $u$ in all
resulting information states.
\[\forall\tuple{w,f,c.u}\in{s\trns{\cst{NP}_{x}-``wa''\ \cst{Pred}}}\ 
[u(x_{\cst{FOA}}) = f(x)]\]

\section{Comparison with the centering account}

Centering has been one of the most prominent mechanism proposed in the
computational discourse modeling literature, especially for the
anaphoric pronoun resolution problem.  Even though we still lack an
extensive model in which to describe the dynamics of the circumstance,
we give in this section some of the main characteristics of our
implicit argument analysis by contrasting them to the basic
assumptions of the centering account.

\subsubsection*{Scope of phenomena}
Centering restricts its scope to the discourse anaphoric pronouns,
whereas implicit argument analysis accommodates both discourse
anaphoric and indexical/demonstrative uses of zero's.  We've already
shown that removing the restriction enables to capture the generality
among various uses of zero's, including the interaction of zero's and
other non-subcategorized implicit arguments.

\subsubsection*{Strategies for establishing salience}
Since centering mechanism relies on the hierarchy of grammatical
relations, choice of surface syntactic structure is the primary means
of maintaining the salience of objects in discourse.  Furthermore,
center establishment and anaphor-antecedent relation determination
takes place simultaneously in the centering.  On the other hand, in
our implicit argument analysis, we distinguish
$x_{\cst{POV}}/x_{\cst{FOA}}$ establishment in the circumstance and
reference with zero's as two different operations.  The
former can be realized either by linguistic means such as the use of
specific expressions including favor giving verbs and the topic marker
``wa,'' or by non-linguistic means such as pointing and presenting an
object to the hearer.  Coreference with zero's is then achieved
through role merging between successive utterances.

\subsubsection*{Domain knowledge intervention}
Centering is assumed to operate with lower priority than domain
knowledge.  An anaphor-antecedent relation proposed by the centering
mechanism has to be filtered out when it doesn't conform to what
domain knowledge dictates.  But in certain uses of zero's, this
clearly does not hold.  Think of our example (4).  Even though (4) is
uttered with the speaker presenting an object not at all to be
considered pretty by anyone, rather than suddenly switch the
denotation of $\phi_{\cst{GA}}$ to other pretty object, the hearer
should interpret the utterance as false.

Similar phenomena can be found with respect to ``wa'' introduced
topics.  In the example below, a preliminary interview with several
native Japanese speakers confirmed that there is a difference in
acceptability of the second sentence with the interpretation
$\phi_{\cst{WO}}=\cst{Taro}$.  In (7), it is impossible to get the
interpretation, since the interpretation does not conform to our
common sensical knowledge that people usually look after those who got
hurt rather than those who hurt others, where as in (8), the
interpretation can be possible even though what it says sounds
unreasonable.  These contextual role supremacy in interpretation of
zero's can be captured only by our implicit argument analysis of zero's.
\begin{enumerate}\itemsep0pt
\item[(7)]
\begin{tabular}[t]{l@{--}lr@{--}ll}
Taro & ga & Jiro & wo & butta.\\
& \cst{SBJ} & & \cst{OBJ} & hit\\
\multicolumn{5}{l}{(Taro hit Jiro.)}
\end{tabular}\\
\begin{tabular}[t]{lr@{--}lll}
Sorede, & Hanako & ga & $\phi_{\cst{WO}}$ & kaihoo-sita.\\
and & & \cst{SBJ} & & look-after\\
\multicolumn{5}{l}{(And Hanako looked after him.)}
\end{tabular}
\item[(8)]
\begin{tabular}[t]{l@{--}lr@{--}ll}
Taro & wa & Jiro & wo & butta.\\
& \cst{SBJ} & & \cst{OBJ} & hit\\
\multicolumn{5}{l}{(Taro hit Jiro.)}
\end{tabular}\\
\begin{tabular}[t]{lr@{--}lll}
Sorede, & Hanako & ga & $\phi_{\cst{WO}}$ & kaihoo-sita.\\
and & & \cst{SBJ} & & look-after\\
\multicolumn{5}{l}{(And Hanako looked after him.)}
\end{tabular}
\end{enumerate}

\section{Concluding remarks}

We proposed a dynamic semantical analysis of Japanese zero's based on
the assumption that zero's are implicit arguments of underlying
semantic predicates.  We argued that the analysis captures the
generality among various types of uses of zero's including discourse
anaphoric, indexical, and demonstrative uses.  We compared our
analysis with the centering account and pointed out that even in
discourse anaphoric uses of zero's our analysis does seem to be
superior in regard to the topic domain knowledge interaction.  

We have identified several methods to control the dynamics of the
circumstance.  We also noted circumstance persistence as a
defeasible default.  We believe that ultimately these circumstantial
control should be integrated together with the centering mechanism
within a framework of competing discourse management heuristics
envisioned in \cite{kame:inde94}, and our dynamic semantical analysis
provides a basic framework on which to state these control heuristics.

% \bibliographystyle{named}
% \bibliography{discourse,pragmatics}

{\small
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}
\end{document}

