Redefining the "Level" of the 
Melissa Macpherson 
EDS Research 
5951 Jefferson NE 
Albuquerque, NM 87109-3432 
melissa @edsr. eds. corn 
"Word" 
Abstract 
Using dictionaries as a model for lexicon development perpetuates the notion that 
the level of "the word", as structurally defined, is the right starting place for semantic 
representation. Difficulties stemming from that assumption are sufficiently serious 
that they may require a re-evaluation of common notions about lexical representa- 
tion. 
1 Introduction 
In a recent paper Boguraev and Levin (1990) point out inadequacies in common concep- 
tions of what a Lexical Knowledge Base (LKB) should be, inadequacies which stem from 
the assumption that a machine-readable dictionary (MRD) is not only the right source for 
acquisition of a lexicon but also the right model for its form. Their points about the kinds 
of generalizations that the standardly conceived LKB does not incorporate, especially if it 
is built from a dictionary source, are well-taken. They cite a need for the representation of 
various kinds of dynamic potential--specifically, the capacity to participate in predictable 
syntactic alternations and regular meaning extensions--which words of particular seman- 
tic classes possess in common, and which constitute the basis for lexical productivity. 
Representation of this kind of dynamic potential is missing from standard dictionaries, 
they suggest, because it is seen as being more predictable to a native speaker than are 
specific -nym relations. The same representation is missing from most LKBs both because 
it was not in the source dictionary and because it cannot be encoded in the usual LKB 
format. Boguraev and Levin conclude that tile conception of the LKB must be extended 
to include both representation of tile semantic classes whose members typically participate 
in these productive alternations and extensions, and sets of inference rules which capture 
the productive potential of members of those classes. They see this as a partial answer 
to the open-endedness problem; when a system of the sort they envision is given a new 
word, we need only specify its semantic class, and we will then have many aspects of both 
its syntactic and its meaning behavior in hand. 
However perhaps a more serious difficulty arising from the adoption of the dictionary as 
model for the lexicon, and one which Boguraev and Levin do not discuss, is the implicit 
assumption that the level of the word, as structurally defined, is a semantically well- 
defined level also. 1 In the sections that follow, I will examine this assumption from 
several angles. Section 2 reviews the semantic behavior of what are structurally words, 
1Boguraev and Levin do mention the possibility that generalizations of the sort they require may 
hold only over partial nodes in a lexicM network; this would seem to indicate that semantic classes may 
be defined in terms of bundles of features, only some of which might be relevant in determining the 
applicability of a given extension or alternation. 
127 
illustrating cases where it seems to take more than one "word" to make a single meaning, 
and where a single "word" is semantically decomposable into several meaning components. 
Section 3 discusses the problem of extricating a functor's argument requirements from 
other aspects of its meaning. In Section 4 the discussion turns to difficulties inherent in 
distinguishing individual word senses. Sections 5 explores some possibilities for escaping 
from the dictionary model and solving some of these problems. 
2 Morphemes and "Semantemes" 
We are led as readers of English, and as dictionary users, to see white space between 
words as indicating a tangible boundary between units of some sort. We base much of 
our linguistic analysis on the assumption that there is such a thing as the "word level". 
If you go "below" this level of structure you are dealing with morphology or with lists 
of idiosyncratic information; "above" it is the level of syntax. Under this assumption 
semantics is also of two sorts: the largely classificatory operations of lexical semantics, 
and the compositional functions of syntactic structure-building. As a working assumption, 
this division of linguistic knowledge has some merit, and has allowed much useful work to 
proceed. 
But in fact, even on the most superficial examination of real language, it is common 
to find gross disruptions of this convenient pattern. Unfortunately, lexicalization is an 
arbitrary process even within a single language, and it is probably only the merest accident 
when it happens that an atomic chunk of meaning is linked bi-uniquely to an invariant 
chunk of phonology. The more common run of structure-meaning correspondences are 
much more complicated. 
For one thing, combinations of two or more surface words often function synchronically 
as more or less absolute semantic units. 2 These collocations may be able to be taken 
apart for syntactic purposes, but semantically they are not decomposable; they link to 
meanings which are more than, less than, or different from, the sum of the meanings of 
their individual parts. The commonly cited examples are phrasal verbs ('hold up', 'kid 
oneself' 'brim over with') and idiomatic expressions of various sorts ('house of cards', 'lie 
low', 'eat \[someone\] out of house and home'). The difficulties here, including locating the 
boundaries of true idiomaticity, are much discussed, and I will not dwell on them here. 
Second, even what are structurally single words can exhibit the kind of composition- 
ality ordinarily expected in syntactic constructions. The most interesting category here is 
that of lexicalized deverbal compounds, such as clambake, carwash, ill-advised, and time- 
saving. Roeper and Siegel (1978), and Selkirk (1982), among others, have explored the 
compositional regularities found in this highly productive component of the lexicon, and 
found that the construction and interpretation of novel verbal compounds is constrained 
according to criteria whose canonical field of application is syntax. 
Both these cross-over phenomena carl be accommodated structurally by means of an- 
alytical tricks of one sort or another. We have ways of representing verb-particle combi- 
nations, for example, so that they are looked up in the same fashion as structurally single 
words, even while their parts can be separated by theoretically indefinite stretches of the 
sentence. It is also possible to install vestiges of verbal structure into the representation of 
2This is true even if we leave aside such trouhlesome orthographic conventions as the fact that fully 
lexicalized compounds are often written as separate words. 
128 
derivationally deverbal items, so that the regularities which they exhibit in compounding 
are predicted. 
A more severe problem than either of these for locating the "level" of the "word" is 
exemplified by these examples of perfectly legitimate paraphrase (from Talmy, 1985): 
be of interest to somebody = interest somebody 
carry out an investigation into something = investigate something 
Certainly we would want a natural language understanding system to arrive at the same 
conclusions from either variant in each pair. It is not at all clear, however, whether we 
should cause the single-word versions to be represented as decomposable into the phrasal 
expressions, or analyze the phrasal expressions as stylistically or pragmatically inspired 
expansions of the more succinct variants, or take some other other tack, such as just 
finding a way to list both variants as meaning the same thing. 3 
Even when they are not so naturally expanded to phrasal versions, many--maybe most, 
depending on the granularity of the analysis-- of what we consider to be monomorphemic 
words have flagrantly compositional meanings; the most timeworn examples are kill (= 
'cause-to-die') and other verbs in which a change-of-state (one meaning component) is 
brought about by a cause (the other meaning component). Accurate analysis of event 
descriptions containing such words would seem to require such sub-word semantic analysis, 
whether it is done as such or as an external listing of presuppositions and entailments of the 
description as a whole. The trouble, however, is sorting out some set of possible meaning 
components which is adequate to the compositional task and yet smaller (preferably both 
in cardinality and in size required for storage) than the set of English words. 
Compare, for example, the pairs kill / murder and send_away / banish. Both pairs in- 
corporate an additional meaning element in the second word of each pair which is at least 
somewhat similar in both cases; it would mean something like 'with malice aforethought 
and with the intention of instituting a permanent solution to a perceived problem and 
probably done with heightened actional intensity'. It is possible that we could find other 
pairs of lexical expressions whose only difference in meaning is the presence of this addi- 
tional element in one of them; this would be a good indication that a productive meaning 
component was at work. But if this component itself can only be expressed in English by 
means of a very long phrase, we have a considerable difficulty of representation. 
It is not that the notions of "word" and "morpheme" make no sense, where the latter 
is defined as "the minimal meaningful unit of language" (Pei and Gaynor, 1980:140); 
rather, both are invaluable from the vantagepoint of surface structure. But when we use 
semantics instead as the starting point, and we define something called a "semanteme",4 
say, as "the minimal unit of meaning that has a sound", then it is not at all clear that the 
two constructs would meet in the middle and we would arrive at the same set of words 
again. 
Nor is this to say that the expanse of meaning encompassed by a structural word 
cannot be a unit. It must be true, as Jackendoff (1983, 1990) claims, that linguistic 
categories correspond fairly directly to conceptual categories. However his formulation of 
the mapping leaves the status of the structural word itself somewhat indeterminate: 
3One of the pervasive fallacies inherent in the notion of the "level" of the "word" is that below this 
everything must be listed, while above this level everything must be representable as sets of rules. 
4 My apologies to anyone who has used this made-up term previously for a different purpose. 
129 
Every content-bearing major phrasal constituent of a sentence (S, NP, AP, 
PP, etc.) corresponds to a conceptual constituent of some major conceptual 
category. (1990:44) \[emphasis added\] 
We are still left with the question of the proper representation of lexical semantics in a 
lexicon when the borders of what is "lexical" are so fuzzy. 
3 Argument Structure and Valence 
The problem of the imperfect mapping between structural words and atomic or even 
bounded meanings extends into what might have been expected to be one of the simpler 
aspects of representing lexical semantics, that of the exposition of argument structure. 
Attempts to assign thematic role structure to a wide range of verbs run immediately into 
two major problems. One is that, as Jackendoff (1990) points out, it may be only part 
of the meaning of a verb which is responsible for assigning a particular thematic role to 
a given argument. For instance, the verb pass (in the construction pass the house) is 
analyzed as incorporating a Path functor, similar in its function to a surface preposition 
by, which is part of the meaning of the verb. What appears in surface structure therefore 
as the object of pass is better analyzed semantically as the object of the underlying by; 
this is why there is no very satisfactory thematic role label for the object of pass. It is 
also possible for a verb to assign a role to only part of the referent of an argument. For 
example, the verb climb, in climb the mountain, has as its Goal not the mountain but only 
the top of the mountain. In general, Jackendoff's arguments that the standard conception 
of thematic roles requires an accompanying commitment to lexicai decomposition are 
difficult to refute. 
At the same time, thematic role labels, or something like them, are necessary for the 
description of events whether we are willing to commit to lexical decomposition or not. 
The fact that the set of labels does not seem to be able to be cleanly inventoried at 
the level of the structural word does not change the fact that some such set of labels is 
necessary to let us individuate event types, that is, to distinguish events in which 'X verbs 
Y' from ones in which 'Y verbs X' (Carlson 1984). While we may not be able to arrive at 
a satisfactorily comprehensive set of labels without complete semantic decomposition, we 
must have a sufficient set to be able to extract the basic 'who did what to whom' structure 
from a sentence. 
However, finding a boundary between the semantic contribution of the verb itself and 
that of its expected arguments is often practically impossible. Boguraev and Levin's 
contention that semantic class determines syntactic behavior, while arguable on other 
grounds, surely stems from the accurate observation that verbs which describe similar 
situations naturally tend to govern similar numbers of arguments in similar ways. 5 Par- 
SUnfortunately, the arbitrariness of lexicalization rears its ugly head here also, and the notion of 
"semantic class" necessary to make this linkage between syntactic behavior and meaning may be somewhat 
circular; that is, in doubtful cases will we decide whether an item is a change-of-state verb on the basis 
of whether it undergoes the causative/inchoative alternation? Carlson and Tanenhaus (1988) would not 
have been able to construct their experiments comparing the effects of thematic structure ambiguity with 
those of semantic ambiguity, if there did not exist verbs of the same semantic class which do not share 
the same possibilities for thematic structure alternation. An example pair from their study is load / filh 
load / fill the truck (with bricks) 
load / *fill the bricks 
130 
ticularly where it is true that a whole semantic class of verbs exhibits similar syntactic 
behavior, it becomes extremely difficult to separate argument structure as an independent 
phenomenon from the core meaning,Sf there is such a thing, of the assigning verb. What 
would it mean to 'eat' without 'eating something' ?--it seems pointless even to consider 
such a question. 
Even the definition of individual thematic roles is susceptible to influence from the 
meanings of expected fillers for those roles; this fact, along with the the difficulty of 
discovering how these roles are assigned to arguments by word-level functors, may help to 
explain the persistent lack of consensus on the proper set of thematic roles, as exemplified 
in Wilkins (1988) and elsewhere. Jackendoff faces one aspect of this problem by removing 
Agent from the set of primary thematic roles; rather, Agent and Patient are derived role 
types, designated on a separate Action Tier and superimposed on basic thematic structure 
through an interaction of the semantics of the verb and the nature of the entities filling 
potential Agent and Patient role slots. That is, a sentence with kill always implies a killer, 
but it has an Agent just if the killer is an entity capable of Agent-like activity, for instance 
a person. 
Syntax too has an effect on the definition of thematic roles, and that in turn has a 
pervasive effect on verbal semantics. Talmy (1985) discusses the fact that subjecthood, 
because of its frequent association with Agenthood, "may tend to confer upon any seman- 
tic category expressed in it some initiatory or instigative characteristics." Consequently a 
Stimulus expressed as Subject may be seen to be acting purposefully or at least actively 
upon the Experiencer, while where tile Experiencer is Subject, "the mental event may be 
felt to arise autonomously and to direct itself outward toward a selected object." Verbs 
of experience whose canonical syntactic pattern includes either Stimulus or Experiencer 
as Subject (for example, please and admire, respectively) are thus under the influence of 
this effect in one direction or the other; this may be one source of the ambiguity in the 
sentence 'He pleases me' and hence in the verb please. 
4 Senses of words 
This blurring of what we would rather see as a clear division between the semantic con- 
tributions of functor and argument also complicates the proper treatment of word senses. 
Compare the following uses of the verb introduce: 
1. We then introduced a catalyst into the solution. 
2. She wants to introduce a little culture into his life. 
3. I introduced Pete to green chili / green chili to Pete. 6 
4. I introduced Joe to Pete / Joe and Pete. 
Only load undergoes the locative alternation. 
Talmy (1985) also gives numerous examples of verbs in which shared semantic class does not guarantee 
identity of valence. An example is the triple emanate, emit, and radiate. Emanate must have Figure as 
subject, emit must have Ground as subject, and radiate can have either. Likewise steal, rob, and rip off, 
which all have Agent prominently in focus as subject, differ in their treatment of Figure and Ground. 
Steal makes Figure the object, rob makes Ground the object, and rip off can put either one in that 
position. 
6I do not know why the second version of this sentence is so much less acceptable than the first; I 
assume it has to do with a violation of focusing constraints. 
131 
5. Ronco has introduced several fascinating new products. 
6. Joe introduced the coyote into Santa Fe stores. 
As the definitions and examples in the ~llowing (partial) dictionary entries suggest, 
it is common to think of each of these sentences as utilizing a different sense of the verb 
introduce. 
(~om OALDgE) 
4883820, ..<ent h=introduce> 
<def>bring in</def> 
<def>bring forward</def> 
<ip>introduce into / to</ip> 
<ex>introduce a Bill before Parliament</ex> 
<def>bring (sth) into use or into operation for the 
first time</def> 
<def>cause (sb) to be acquainted with (sth)</def> 
<ex>introduce nee ideas into a business</ex> 
<ex>Tobacco was introduced into Europe from America</ex> 
<ex>The teacher introduced his young pupils to the 
intricacies of geometry</ex> 
<ip>introduce sb (to sb)</ip> 
<def>make (persons) known by name (to one another), 
esp in the usual formal way</def> 
<ex>introduce two friends</ex> 
<ex>He introduced me to his parents</ex> 
<ex>The chairman introduced the lecturer to the 
audience</ex> 
<ip>introduce (into)</ip> 
<def>insert</def> 
<ex>introduce a tube into a wound</ex> 
<ex>introduce a subject into a conversation</ex> 
°°.° °° 
(from CED Fact Base) 
c_DEF( \[ "introduce", 1,1, I \], 
c DEF(\[ "introduce",1,1,2 \], 
c_SAMP(\[ "introduce", 1,1,2 \] , 
c_DEF(\[ "introduce",l,l,3 \], 
c_SAMP(\[ "introduce",1,1,3 \], 
c_DEF(\[ "introduce",1,1,4 \], 
c_SAMP(\[ "introduce",l,l,4 \], 
c_DEF(\[ "introduce",1,1,5 \], 
"(often fo111 by to) to present 
(someone) by name (to another person) 
or (two or more people to each other)" ,2 ). 
"(follJ by to) to cause to experience 
for the first time" ,1 ). 
"to introduce a visitor to beer" ). 
"to present for consideration or 
approval, espl before a legislative 
body" ,I ). 
"to introduce a draft bill in Congress" ). 
"to bring in; establish" ,1 ). 
"to introduce decimal currency" ). 
"to present (a radio or television 
132 
programme, etcl)verbally" ,1 ). 
c_DEF(\[ "introduce",1,1,6 \], "(folll by with) to start" ,I ). 
c_SAMP(\[ "introduce",l,l,6 \], "he introduced his talk with some music" ). 
c_DEF(\[ "introduce",l,l,7 \], "(often fo111 by into) to insert or 
inject" .I ). 
c_SAMP(\[ "introduce",1,1,7 \], "he introduced the needle into his arm" ). 
oo • ooo 
However the example sentences (1)-(6) seem to illustrate not separate senses but a 
natural continuum of meanings, in which the contribution of introduce itself does not 
perceptibly change. In sentence (1), introduce comes closest to what we would probably 
call its core meaning, something like 'bring / put something into a new place'. Sentence 
(2) relocates this action in a different kind of space; both the Theme 'something' and the 
Goal 'new place' are abstract in this case. 7 In Sentence (3), the Goal location is some sort 
of recognition space, associated either with the recognizer or the subject matter; the effect 
is that Pete now knows about green chili. Obviously this sense extension for introduce 
only works if at least one of its arguments is an entity which is capable of recognizing, i.e. 
a person. Moreover, where the introduce-ee is also a person as in (4), (or where the single 
Object of introduce refers to more than one person) the introduction becomes reciprocal; 
the "actional content" of the verb, in Talmy's words, has been doubled. 
In (5), the meaning extension present in (2), (3), and (4) is further specialized so 
that the implicit Goal location is not only a "recognition space", but is understood to be 
the recognition space belonging collectively to the individuals making up a market. We 
make this additional extension, however, not because we are now using a different sense 
of the verb introduce, but simply because of what we know about products. That is, our 
understanding of the type of space in which the introduction is effected is governed by both 
the core meaning of the verb--we know introduce means putting something into some 
space just as we know that eating means eating something--and the semantic content of 
its first argument. 
In (6) the inference from 'product' to 'market' is reversed. In the most plausible 
reading of the sentence, the coyote refers generically to a decorative motif or to some kind 
of typically salable item, not to an actual animal. This is true because we know that 
Santa Fe stores represents a particular kind of market, and so we prefer an interpretation 
in which the thing introduced is something which can typically be marketed. 
If we try to decide then what the senses of introduce are, as the dictionary model would 
suggest that we should do, we can take several tacks. We could just take a dictionary's 
word for the number and definitions of separate senses (leaving aside for the moment the 
problems of mapping between the different sense divisions of different dictionaries), but 
then we have clearly missed a generalization about the extensions of meaning possible for 
the verb introduce and others like it. We could distinguish separate senses on the basis of 
the use in some of the examples of into and in others of to, but that distinction cross-cuts 
what are clearly more important semantic distinctions. 
On the basis of the thematic structure that the verb introduce instantiates, which is 
more or less the same for all six examples, we might say that it has only one sense, s 
7This is a legitimate and extremely common meaning extension, from physical space to abstract space; 
in fact it is exactly the sort of regularity that a LKB should include among its lexical rules. 
SThe one problematical construction here would be the one with a plural Theme and no explicit Goal, 
as in 'I introduced Joe and Pete.' In this case, as noted above, the introduction is reciprocal and therefore 
the thematic structure would apparently have to be copied over into another, simultaneous action. 
133 
Likewise it would decompose in every case to something like 
CAUSE (GO (X (FROM (outside-some-space))(TO (inside-some-space)))) 
Under this analysis it is the characteristics of 'X', the locatum, and of the respective 
entities which serve as Goal that differentiate senses; sentence (1) makes reference to a real 
thing and a physical space, (2), (3), and (4) refer to recognizable entities and individual 
cognitive space, and (5) and (6) refer to something which can be sold and a particular 
kind of collective cognitive space. In (5), naming a product allows us to infer a market as 
the implicit Goal, while in (6) knowing that the Goal is a market lets us know that the 
coyote is to be construed as a product. What seems to be happening is what Cruse (1986) 
calls "modulation" of a word sense by context, except that in this case the "context" is 
an integral part of the argument structure of the verb. 
In any case, it is clear that splitting the verb introduce into a set of separate senses at 
the word level will be arbitrary in one way or another. 
5 Words as Worlds 
The ideal, but computationally outlandish, solution to this problem would be to represent 
every word in English (not just each functor, but every word) as the union of all the 
situations in which it could potentially participate, so that all the combinatorial potential 
of each word would be an inherent part of its inventory of meaning(s). Under this system, 
a structural word would just be the entry point into a rich representation which might or 
might not observe that bit of structure further, being built instead around the way the 
word works in combination with others. All lexical categories, not just the prototypical 
functors, would have combinatorial preferences, as McCawley (1986) suggests. Semantic 
composition of functor and arguments in a sentence then would consist of unifying these 
representations in the most cognitively perspicuous way, in accordance with the bounds 
imposed by syntax, to create a single coherent scene. In the process of unification, gaps 
would be filled, construals or views chosen in accordance with constraints instantiated by 
specific lexical interactions and general cognitive conventions, and aspects of the unifying 
representations which are contradictory or not currently in focus would fall away. A new 
semantic combination constructed in this way would be acceptable in inverse proportion 
to the strain which constructing it placed on these general conventions, and selectional 
restrictions would simply be statistically based thresholds of strain. 
Various efforts have been made toward constructing a realistic version of this ambitious 
model. Raskin (1986) discusses a system he calls Script-based Semantics, which captures 
many of the ideas described above. In his system, the appearance of a word in a sentence 
being analyzed evokes both a set of syntactic senses and one or more scripts, where a script 
is defined as "a large chunk of semantic information surrounding the word or evoked by 
it ... a cognitive structure internalized by the native speaker, \[representing\] knowledge of 
a small part of the world. ~ These scripts are represented as graphs with lexical nodes and 
semantic relation arcs, thus bearing a strong resemblance to tile Conceptual Structures of 
Sowa (1984). Under both schemes a set of combinatorial rules then unifies graphs in such 
a way that ambiguity between word senses disappears and a complete representation for 
a sentence is constructed. 
What is not clear in either system is where the graphs for individual word senses come 
from. If we were to use the dictionary definitions given earlier as a source of the script(s) 
134 
or graph(s) for the verb introduce, in combination with the following entries for product, 
we would still have absolutely no basis for combining them to mean what we want them 
to mean in examples (5) and (6). , 
(from OALD3E) 
7296955, . .<ent h=product> 
<def>sth produced (by nature or by man)</def> 
<ex>farm produces</ex> 
<ex>the chief products of Scotland</ex> 
<def>(maths) quantity obtained by multiplication</def> 
<def>(chem) substance obtained by chemical reaction</def> 
• °°, °° 
(from CED Fact Base) 
c_DEF(\[ "product", 1, I, 1 \] 
c_DEF(\[ "product",l,l,2 \] 
c_DEF( \[ "product", 1,1,3 \] 
c_DEF( \[ "product", 1,1,4 \] 
c_DEF( \[ "product", 1,1,4, 1 
c_DEF ( \[ "product", 1,1,4,2 
.,.,, . 
, "something produced by effort, or 
some mechanical or industrial process" , 1 ). 
, "the result of some natural process" ,I ). 
, "a result or consequence" ,I ). 
, "a substance formed in a chemical 
reaction" ,1 ). 
\], "the result of the multiplication of 
¢.o or more numbers, quantities, etc" ,I ). 
\], "another name for intersection ,1,3" ,I ). 
Hand-building such scripts on a sense-by-sense basis for large-scale efforts, on the 
other hand, would require not only massive redundancy but also an enormous amount of 
sophisticated-yet-tedious lexical analysis. Not only that, but by pegging these scripts or 
conceptual graphs to individual word senses, we return to the problem of knowing how to 
arbitrarily distinguish those senses. 
The most promising answer to many of these problems can be found in the approach 
of collocational semantics, in which the "meaning" of a word consists of sets of observed 
uses of that word. Practitioners of this approach have sometimes apologized for it (see for 
instance Pazienza and Velardi, 1989) as a practical but unprincipled substitute for "real" 
semantics. But in fact the collection, classification, and normalization of collocational pat- 
terns may constitute the most realistic way to sidestep various serious difficulties involved 
in other methods of building a large inventory of lexical knowledge; it might be the way to 
discover what it "means" for a text to talk about the introducing a product. Collocational 
semantics provides an objective methodology for detecting functional equivalence between 
expressions, which is really what we want when we attempt to encode synonymy, and for 
pinpointing functional distinctions, which is what we are trying to do when we attempt 
to distinguish word sense meanings. Above all, this approach offers promise precisely 
because it does not depend crucially on the level of the structural word for the definition 
of semantic units. 
As for the operation of a language-understanding system including such a lexical store, 
obviously allowing the appearance of each word in a sentence to immediately evoke ev- 
erything we know about its uses would be a computational nightmare. Instead, what is 
required is a mechanism for incrementally introducing lexical information into sentence 
135 
analysis in such a way that just enough, and no more, is present at any given stage, after 
the fashion of ltirst (1987) with his "Polaroid" words. Under such a methodology several 
stages of lexical lookup would be necessary. Syntactic patterns, morphologically-derived 
categorizations, and minimal thematic structure, as discussed above, might be available 
in the usual way at the first level. The matching of partially disambiguated, intermediate 
sentence graphs constructed at the first level against an inventory of highly schematic 
"entry" graphs, and then against successive layers of more completely specified model 
graphs, would funnel the analysis through the next levels of lookup. 9 A system of seman- 
tic classes, along with a set of redundancy rules encoding systematic relations between 
and alternations possible for members of those classes, would constitute yet another level 
of lexical knowledge; this level would be important in the organization of the lexicon, but 
might be accessed directly during lookup only ill the case of the appearance of previously 
unknown words. 
How would we construct this kind of multi-level lexical knowledge base? The first level 
described above can and should be constructed on the basis of information from machine- 
readable dictionaries. The next set of lookup levels described, in which the structural word 
is no longer the primary entity but just a handle for indexing graphs, can be built on the 
basis of knowledge assembled from large volumes of text; the methodology employed by 
Smadja and McKeown (1990) would be one of many techniques possible for obtaining 
and organizing the various levels of model graphs. Our idea is that the graded levels of 
specificity would be constructed by means of successive generalization operations over the 
most specific set of graphs, extracted more or less directly from text. The final level of 
knowledge--that is, what Boguraev and Levin have found missing from standard LKBs-- 
must still be installed by linguists. Dictionaries do contain some clues, and certainly the 
ability to systematically and automatically pull together usage instances can help; but 
the real work must be done by people. 
6 Conclusion 
The notion that the level of "the word", as structurally defined, is the appropriate starting 
place for semantic representation, has been implicit in the design of most lexical knowledge 
bases. The use of machine-readable dictionaries as a source of lexical knowledge reinforces 
this notion, at the cost of considerable descriptive loss. This discussion has also revealed a 
further detrimental assumption fostered by the use of the dictionary as the model for the 
lexicon: the idea that a single store of lexical knowledge, with a single lookup function and 
a unified structure, is the necessary mechanism for bringing word meanings into sentence 
analysis. On the contrary, the multi-faceted semantic behavior of the structural units we 
call "words" requires a base of knowledge consisting of multiple knowledge stores, each 
organized in a way that is appropriate to the knowledge it contains and the stage of 
processing to which it must contribute. The structural word can be used as the indexing 
key which relates the separate, differently structured stores, but it need not be the basic 
currency of all of "lexical" semantics. 
9 One or more of these levels of distillation would presumably be equivalent to the single set of canonical 
graphs envisioned by Sowa (1984). 
136 

References 
\[1\] Boguraev, Bran, and Beth Levin (1990). "Models for lexical knowledge bases", in 
Electronic Text Research: Proceedings of the Sixth Annual Conference of the UW 
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