World Knowledge and Word Meaning 
Jerry R. Hobbs 
Artificial Intelligence Center 
SRI International 
We use words to talk about the world. Therefore, to understand what 
words mean, we must have a prior explication of how we view the world. In 
a sense, efforts in the past to decompose words into semantic primitives were 
attempts to link word meaning to a theory of the world, where the set of 
semantic primitives constituted the theory of the world. With the advent of 
naive physics and research programs to formalize commonsense knowledge 
in a number of areas in predicate calculus or some other formal language, we 
now have at our disposal means for building much richer theories of varlous 
aspects of the world, and consequently, we are in a much better position to 
address the problems of le~cal semantics. 
In the TACITUS project for using commonsense.knowledge in the un- 
derstanding of texts about mechanical devices and their failures, we have 
been developing various commonsense theories that are needed to mediate 
between the way we talk about the behavior of such devices and causal 
models of their operation (Hobbs et M., 1986). The theories cover a number 
of areas that figure in virtually every domain of discourse, such as scalar 
notions, granularity, structured systems, time, space, material, physical ob- 
jects, causality, functionality, force, and shape. Our approach has been to 
construct core theories of each of these areas. These core theories may use 
English words as their predicates, but the principal criterion for adequacy 
of the core theory is elegance, whatever that is, and this can usually be 
achieved better using predicates that are not lexically realized. It is easier 
to achieve elegance if one does not have to be held responsible to linguistic 
evidence. Predicates that are lexically realized are then pushed to the pe- 
riphery of the theory. A large number of lexical items can be defined, or at 
least characterized, in terms provided by the core theories. The hypothesis 
is that once these core theories have been formulated in the right way, it 
will be straightforward to explicate the meanings of a great many words. 
The phrase "in the right way" is key in this strategy. The world is 
20 
complex and can be viewed from many different perspectives. Some of these 
will lend themselves well to the investigation of problems of word meaning, 
whereas others will only lead us into difficulties. We could, for example, 
axiomatize space as Euclidean 3-space~ with x, y, and z-coordinates for every 
point. We could then attempt to define what the various prepositions and 
verbs of motion mean in this framework. I am quite sure such an attempt 
would fail. Such a theory of space would be too foreign to the way we talk 
about space in everyday life. Even if we were to succeed in this limited task, 
we would not have advanced at all toward an understanding of metaphorical 
uses of these words. 
In contrast, we view our core theories not so much as theories about 
particular aspects of the world, but rather as abstract frameworks that have 
proven useful in interpreting, generally, a number of different kinds of phe- 
nomena. Thus, at the very center of our knowledge base is an axiomatiza- 
tion of "systems", where a system is a set of elements and a set of relations 
among them. An abstract, "primitive" relation at places entities at loca- 
tions within a system, encoding the basic figure-ground relation. A large 
number of things in the world can be understood as systems, and a large 
number of relations can be understood as at relations. When we apply the 
theory to a particular phenomenon, we buy into a way "of thinking about 
the phenomenon, and, more to the present purposes, of talking about it. It 
is in this way that the metaphorical usages that pervade natural language 
discourse are accommodated. Once we characterize some piece of the world 
as a system, and some relation as an at relation, we have acquired the whole 
locationai way of talking about it. Once this is enriched with a theory of 
time and change, we can import the whole vocabulary of motion. For exam- 
ple, in computer science, a data structure can be viewed as a'system, and 
we can stipulate that if a pointer points to a node in a data structure, then 
the pointer is at that node. We have then acquired a spatial metaphor, and 
we can subsequently talk about, for example, the pointer moving around the 
data structure. Space, of course, is itself a system and can be talked about 
using a locational vocabulary. 
Also central in the knowledge base is an axiomatization of "scales", which 
is a particular kind of system whose relations are a partial ordering and an 
indistinguishability relation (encoding granularity). Once we develop a core 
theory of scales, we can use the predicates it provides to characterize a large 
number of lexical items, such as "range", "limit", and the comparative and 
superlative morphemes. For x to range from y to z, for example, is for y 
and z to be endpoints of a subscale s of a scale, and for x to be a set of 
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entities which are located at elements of s. By choosing different scales, we 
can get such uses as 
The buffalo ranged from northern Texas to southern Saskatchewan. 
The students' SAT scores range from 1100 to 1550. 
The hepatitis cases range from moderate to severe. 
His behavior ranges from sullen to vicious. 
Our desire to optimize the possibilities of using core theories in metaphor- 
ical and analogical contexts leads us to adopt the following methodological 
principle: For any given concept we wish to characterize, we should deter- 
mine the minimal structure necessary for that concept to make sense. In 
efforts to axiomatize some domain, there are two positions one may take, 
one exemplified by set theory and the other by group theory. In axiom- 
atizing set theory, one attempts to capture exactly some concept one has 
strong intuitions about. If the axiomatization turns out to have unexpected 
models, this exposes an inadequacy. In group theory, by contrast, one char- 
acterizes an abstract class of structures. If there turn out to be unexpected 
models, this is a serendipitous discovery of a new phenomenon that we can 
reason about using an old theory. The pervasiveness of metaphor in natural 
language discourse shows that our commonsense theories of the world ought 
to be much more like group theory than set theory. 
Our approach to space and dimensionality illustrates this. Rather than 
defining dimension in the classical manner of linear algebra, in a way that 
requires a measure and arithmetic operations, we have sought to be able to 
build spaces out of less structured components. Thus, we have defined a two- 
dimensional space as a set of elements that can be located on two different 
scales that are independent in the sense that the order of two elements on one 
scale cannot be predicted from their order onthe other. A space can then be 
defined corresponding to any set of scales, l~eal space is an instantiation of 
this theory, and so are various idealizations of it. But metaphorical spaces 
are also instantiations. We can, for example, talk about salary and quality 
of life as different dimensions relevant to job choice. 
We have concentrated more on specifying axioms than on constructing 
models. Thus, our approach is more syntactic than semantic, in the logical 
sense. Our view is that the chief role of models in our effort is for proving 
the consistency and independence of sets of axioms, and for showing their 
adequacy. Many of the spatial and temporal theories we construct are in- 
tended at least to have Euclidean space or the real numbers as one model, 
22 
but they are also intended to have discrete, finite, and less highly structured 
models as well. 
Not only do people seem to have single theories for multiple phenomena, 
they also seem to have multiple theories for single phenomena. Where this 
is so, where for example several competing ontologies suggest themselves, 
we attempt to construct a theory that accommodates both. R.ather than 
commit ourselves to adopting one set of primitives rather than another, we 
try to show how each set of primitives can be characterized in terms of the 
other. Then one need not make claims of primacy for either. Generally, 
each of the ontologies is useful for different purposes, and it is convenient 
to be able to appeal to both. Our treatment of time illustrates this. One 
possible approach is to take the time line as basic, and to say that events 
and conditions have associated time instants or intervals. In this view, 
there is a change in the world if an entity is in one state at one point in 
time and in another state at another point in time. This view is reflected 
in language in the clock and calendar vocabulary. Another approach, one I 
think corresponds better with the way we really view the world most of the 
time, is to say that there isa primitive relation change between conditions 
or situations, that these conditions and. changes can co-occur, and that the 
time line is just an idealized sequence of changes that many other events 
co-occur .with. This view seems to be deeply embedded in language, in, for 
example, verbs describing changes of state. Rather than be forced into one 
ontology or the other, we have shown how each can be defined in terms of 
the other. 
In addition to being cavalier about the match between the core theories 
and the way the world really is, we are being cavalier about whether the 
axiomatizations fit into the classical mold of a few undefined, "primitive" 
predicates and a large number of predicates defined in terms of these primi- 
tives. We take it that one can rarely expect to find necessary and sufficient 
conditions for some concept p. There will be few axioms of the form 
_=_ Q 
The most we can hope for is to find a number of necessary conditions and a 
number of sufficient conditions, that is, a number of axioms of the form 
(w)p(x) o Q 
and a number of axioms of the form 
(Vx)R 
23 
It is generally hopeless to aim for definitions; the most we can expect is 
characterizations. This amounts to saying that virtually every predicate is 
a primitive, but a primitive that is highly interrelated with the rest of the 
knowledge base. 
One way this can happen is illustrated by the predicate at. There are 
very few facts that one can conclude from the fact that one entity is at 
another in an arbitrary system. The predicate is used first as a way of 
relating many other concepts, especially concepts involving change, with 
each other. So there are axioms that say that when something moves from 
one point to another, it is no longer at the first and is now at the second. Its 
second use is as an entry point into spatial metaphors. There are a number 
of axioms of the form 
^ q(y, s) at(x,y,s) 
when we see a spatial metaphor and ask what would imply such a usage, 
axioms like these enable an interpretation. 
The predicate cause is another illustration of the roles of primitive pred- 
icates in thh knowledge base. We do not attempt to define causality in 
terms of other, more basic concepts. There are a few things we know about 
causality in general, such as the transitivity, of cause and the relation be- 
tween cause and temporal order. But otherwise almost all we know about 
causality is particular facts about what kinds of particular events cause what 
other kinds of particular events. We should not expect to have a highly de- 
veloped theory of causality per se. Rather we should expect to see causal 
information distributed throughout the knowledge base. 
Another example of characterization rather than definition is provided 
by natural kind terms, like "metal". We all know from Putnam that we can't 
hope to define such terms in ways that will survive future scientific discovery. 
Even if we were able to define them in ways consistent with current science, 
the definitions would be very distant from common sense. Nevertheless, we 
know a great many properties of metals, and this knowledge plays a role in 
the interpretation of many texts we encounter. Therefore, the knowledge 
base contains a number of axioms encoding things like the fact that metals 
behave in a certain way when subjected to increasing forces. 
The TACITUS project is fairly new, and we have not yet characterized 
a large number of words or axiomatized very many core theories. But a~- 
ready the range of words we have been able to handle indicates the promise 
of our approach. Here are some examples. The word "range" has already 
been discussed. Assemblies and environments are both systems of particular 
24 
kinds, and we can say that an assembly "operates" if it engages i n its norma- 
tive behavior in an environment. The word "immediately", etymologically, 
predicates of an ordering relation between two events that a third relevant 
event does not occur between them. This fact can be expressed in terms 
provided by the core theories of scales and time. The word "brittle" can be 
characterized within the same theory of materials acted upon by forces that 
was useful in specifying some properties of metals, mentioned, above. The 
concept "wear", as in "worn bearings" or "a worn-out shirt", was one of the 
original targets of our research effort. Wear is the cumulative small-scale 
loss of material from the surface of an object due to the abrasive action of 
some external material. We have been able to state this formally in terms of 
predicates from core theories of granularity, change, force, and the topology 
and cohesion of pieces of material. The diversity and complexity of the set 
of words we have been able to handle encourages us in the belief that lex- 
ical semantics should be integrated with efforts to formalize commonsense 
knowledge. 
An old favorite question for lexical theorists is whether one can make a 
useful distinction between linguistic knowledge and world knowledge. The 
position I have articulated leads one to an answer that can be stated briefly. 
There is no useful distinction. In discourse comprehension and generation, 
both kinds of knowledge are required a.nd, in our work so far on interpre- 
tation, both are handled in the same way. Defining or characterizing words 
can only be done as an adjunct to an effort to build theories useful for un- 
derstanding phenomena in the world. In fact, the only reason I can imagine 
for maintaining such a distinction is for preserving discipline boundaries. 
There is, however, a useful, related distinction in kinds of knowledge 
bases one might build. The knowledge base we are building is geared toward 
communication. There are other efforts, such as those in qualitative physics 
.(e.g., DeKleer and Brown, 1985), which are geared toward the prediction 
of physical events in the absence of complete information. In such efforts, 
one is less concerned about metaphor and more concerned about detailed 
correspondence with the world. It wouldn't disturb me if with our knowledge 
base we failed to predict when a valve would close, but I would be disturbed 
if we could not cope with spatial metaphors for, say, economic information. 
So far we have spent more time developing the core theories than in 
characterizing words in terms of them. What we have done in the latter area 
has primarily been for exploratory and illustrative purposes. Moreover, the 
entire effort is so new that frequently when we try to characterize a word we 
discover another core theory or two that needs to be axiomatized first. So 
25 
we have barely scratched the surface in constructing the kind of knowledge 
base required for genuine text processing. What hope is there for scaling 
up? There are two points to make here. First of all, Maurice Gross is fond 
of pointing out that other fields, such as astronomy and botany, have faced 
just as formidable a task of classification and cataloguing as we face, and 
have thrived on it. When we have a better idea of what we want to do, there 
will be people enough to do it. 
Secondly, there is promise in the recent attention given to automatic pro- 
cessing of already existing on-line dictionaries and other knowledge sources. 
I can imagine that work eventually converging in a fruitful way with our 
research. I like to characterize the difference between the TACITUS project 
and recent projects aimed at encoding all the knowledge in an encyclope- 
dia by saying that rather than encoding the knowledge in the encyclopedia, 
we are trying to encode the knowledge required by someone before he even 
opens the encyclopedia, just to be able to read it. The same holds true 
of a dictionary. As we build up a larger and larger knowledge base and 
further implement the procedures that will use this knowledge in text com- 
prehension, we will be more and more in the position of being able to use 
the information in large, on-line dictionaries. Work on extracting semantic 
hierarchies from on-line dictionaries (Amsler, 1980; Chodorow, Byrd, and 
Heidorn, 1985) will not merely reveal a set of semantic primitives for some 
domain. These semantic primitives will be concepts that have already been 
explicated in core theories in the knowledge base, so that this automatic 
analysis will have in turn yielded more valuable results. We will have ex- 
tended the knowledge base itself using these on-line resources. 
Acknowledgements 
The research described here is a joint effort with William Croft, Todd Davies, 
Douglas Edwards, and Kenneth Laws. The opinions expressed here are, 
however, my own. The research is funded by the Defense Advanced Research 
Projects Agency under Office of Naval Research contract N00014-85-C-0013. 
References 
\[1\] Amsler, Robert A., 1980. The Structure of the Merriam-Webster Pocket 
Dictionary, doctoral dissertation, TP~-164, University of Texas, Austin. 
26 
\[2\] Chodorow, Martin S., Roy J. Byrd, and George E. Heidorn, 1985. "Ex- 
tracting Semantic Hierarchies from a Large On-line Dictionary", Proceed- 
ings, 23rd Annum Meeting of the Association for Computational Linguis- 
tics, Chicago, Illinois, pp. 299-304. 
\[3\] De Kleer, Johann, and John Seely Brown. 1985. "A Qualitative Physics 
Based on Confluences." Formal Theories of the Commonsense World, ed. 
by Jerry R. Hobbs and Robert C. Moore, Ablex Publishing Corp., pp. 
109-184. 
\[4\] Hobbs, Jerry R., William Croft, Todd Davies, Douglas Edwards, and 
Kenneth Laws, 1986. "Commonsense Metaphysics and Lexical Seman- 
tics", Proceedings, 24th Annual Meeting of the Association for Compu- 
tational Linguistics, New York, pp. 231-240. 
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