Creative Discovery in Lexical Ontologies
Tony VEALE
Dept. of Computer Science 
Belfield, 
Dublin, Ireland, D4
Tony.Veale@UCD.ie
Nuno SECO, Jer HAYES
Dept. of Computer Science 
Belfield, 
Dublin, Ireland, D4
{nuno.seco, jer.hayes}@UCD.ie
Abstract
Compound terms play a surprisingly key role 
in the organization of lexical ontologies. 
However, their inclusion forces one to address 
the issues of completeness and consistency 
that naturally arise from this organizational 
role. In this paper we show how creative 
exploration in the space of literal compounds 
can reveal not only additional compound 
terms to systematically balance an ontology, 
but can also discover new and potentially 
innovative concepts in their own right.
1 Introduction
Broad-coverage lexical knowledge-bases like 
WordNet (Miller et al., 1990) generally contain a 
large number of compound terms, many of which 
are literal in composition. These compounds are 
undoubtedly included for a reason, yet the idea that 
literal compounds might actually be essential to 
WordNet’s usefulness may strike some as heretical 
on at least two fronts: first, the lexicon is a finite 
resource, while the space of compounds is 
potentially infinite; and at any rate, literal 
compounds can be created as needed from purely 
compositional principles (Hanks, 2004). However, 
these retorts are valid only if we view WordNet as 
a dictionary, but of course it is much more than 
this. WordNet is a lexical ontology, and ultimately, 
ontologies derive a large part of their functionality 
from their structure.
So, while the meaning of literal compounds like 
Greek-deity and animal-product may well be 
predictable from compositional principles alone, 
such concepts still serve an important 
organizational role in WordNet by adding much 
needed structure to the middle ontology. Having 
conceded the importance of such compounds, one 
is forced to address the issues of completeness and 
consistency that then arise from their inclusion. 
Completeness suggests that we strive to include as 
many literal compounds as are sensible, if they 
enhance the organization of the ontology or if there 
is evidence that they are in common usage in the 
language. Systematicity is a related issue that 
arises when a group of existing compounds 
suggests that another should also exist for the 
ontology to be consistent. For instance, the 
existence of Greek-deity, Greek-alphabet and 
Hebrew-alphabet leads to the hypothesis that 
Hebrew-deity should also exist if WordNet is to be 
both consistent and symmetric in its treatment of 
different cultural groupings.
Indeed, because literal compounds like these 
arise from the yoking together of two different 
ontological branches into one, compounding 
represents an important contextualization device in 
the design of ontologies, allowing lexical elements 
to be logically grouped into clusters or families 
that share important dimensions of meaning. This 
clustering facilitates both automated reasoning by 
machines (such as the determination of semantic 
similarity based on taxonomic distance) and 
effective browsing by humans. Sometimes this 
yoking results in a compound that, following 
Boden (1990) and Wiggins (2003), deserves to be 
called “creative”, because it exhibits both novelty 
and value. Novelty can be measured along either a 
psychological or a historical dimension, while 
utility is a reflection of the uses to which a 
compound can be put. For instance, a new 
compound may have utility as a clustering node 
when added to the middle ontology if its 
appropriate hyponyms can be identified. 
Alternately, a new compound may represent an 
alternate nominalization of an existing concept 
(e.g., see Vendler’s (1967) insights about 
nominalization, and Lynott and Keane’s (2003) 
application of these insights to compound 
generation). 
In this paper we present a process of ontological 
exploration to identify those areas of the lexicon 
that can contribute to, and may in turn benefit 
from, the invention of new compound terms. Since 
the discovery of new compound terms is 
essentially a process of creative exploration, we 
frame our discussion within the theoretical 
framework of creative computation. Within this 
framework two approaches to validating new 
compounds are presented: internal validation 
determines whether the ontology itself provides 
evidence for the sensibility of a new compound, 
while external validation uses web-search to find 
evidence that the compound already exists outside 
the ontology. We then go on to show how these 
different strategies create a validation gap that can 
be exploited to identify the small number of truly 
creative compounds that arise.
2 Exploring the Space of LMH Concepts
Creative discovery requires that we give 
structure to the space of possible concepts that we 
plan to explore. This is made somewhat easier if 
we consider the meaning of conceptual structures 
to be grounded in a semiotic system of meaning-
creating oppositions. Given a starting structure, 
knowledge of allowable oppositions can then be 
used to transform this starting point into a variety 
of different conceivable structures, some of which 
may be novel and possess value on a particular 
utility scale.
The notion of opposition employed here is much 
broader than that of antonymy. For our purposes, 
contextual oppositions exist between terms that 
compete to fill a given dimension of the same 
concept. For instance, Greek
1
 and Hindu can each 
be used to differentiate the concept deity along a 
culture dimension, and so, in the context of deity, 
both are opposed. However, this is a contextual 
opposition that, unlike the role of antonymy, does 
not constitute part of the meaning of either 
concept. WordNet is a rich source of explicit 
antonymous oppositions, but contextual 
oppositions must be inferred from the structure of 
the ontology itself and from existing compounds.
Fortunately, WordNet contains many instances 
of literal modifier-head terms, such as "pastry 
crust" and "Greek alphabet". The concepts denoted 
by these compound terms, or LMH concepts for 
short, have the lexical form M-H (such as pizza-pie
or prairie-dog) and express their literality in two 
ways. First, they must be stored in the WordNet 
ontology under an existing sense of the lexeme H; 
for instance, pizza-pie is actually stored under the 
hypernym pie. Secondly, the gloss for the concept 
M-H should actually contain the lexeme M or 
some synonym of it. Thus, while Greek-alphabet is 
a LMH (it literally is a kind of alphabet, and it is 
literally Greek), neither monkey-bread (which is 
not literally a kind of bread) nor Dutch-courage
(which is not literally Dutch) is a LMH concept.
2.1 A Framework for Creativity
We use the terminology of Wiggins (2003) to 
frame our discussion of creative exploration. 
Wiggins, following earlier work by Boden (1990), 
                                                  
1
 To avoid later confusion with set notion, we denote 
WordNet senses not as synsets but as italicized terms .
formalizes the creative exploration process using 
the following abstractions:
C  -  the realm of concepts that is being explored
R - the set of rules for forming concepts and 
conversely, deconstructing existing ones
T - the transformational rules that generate new 
concepts via R
E -  the evaluation mechanism that ascribes value 
or utility to these new concepts
In applying these terms to creativity in WordNet, 
we introduce the following refinements:
C
w
  - the subset of C described explicitly in 
WordNet as synsets
C* - the set of LMH concepts in C
w
 considered 
as a starting point for creative exploration
R* - the subset of R needed to construct and 
deconstruct LMH compounds in C*
T* - the subset of T needed to hypothesize new 
LMH concepts for R* to construct
So for our current purposes, we define C* as the 
set of LMH concepts in WordNet, and R* as the 
compositional criteria used to identify and 
decompose existing LMH entries and to construct 
new ones by concatenating an appropriate M and H 
term pair. However, to define T*, we first need to 
consider how taxonomic differentiation is used to 
create LMH concepts in the first place.
3 Domain Differentiation
LMH concepts exist in WordNet to differentiate 
more general concepts in meaningful taxonomic 
ways. For instance, the LMH concepts Greek-
alphabet, Hebrew-alphabet and Roman-alphabet
each serve to differentiate the concept alphabet. 
This is a useful ontological distinction that 
contributes to the definition of individual letter 
concepts like Alpha, Beta and Gimel. Since we can 
represent this specialization pattern via a 
differentiation set D
alphabet
 as follows:
   D
alphabet
  = {Greek, Hebrew, Roman}
More generally, the differentiation set of a concept 
H comprises the set of all concepts M such that the 
LMH concept M-H is in C*. Thus we have:
   D
deity
        = {Hindu, Roman, Greek,  ...}
   D
architecture
=  {Greek, Roman, ...}
   D
calendar
=  {Muslim, Jewish, Hebrew, ...}
We use D to denote the set of all differentiation 
sets that are implied by C*, allowing us to define 
the absolute affinity between two modifier terms c
1
and c
2
 in terms of differentiation as follows:
 A
abs
(c
1
, c
2
) =   |{x  D: c
1
 x  c
2
 x}|       (1) (1)
This simply counts the number of base concepts 
that c
1
 and c
2
 can both differentiate. We thus 
define the relative affinity between two modifier 
terms c
1
 and c
2
 as follows:
A
rel
(c
1
, c
2
) =     |{x  D : c
1
 x  c
2
 x}|      (2) 
        /  |{x  D : c
1
 x  c
2
 x}| 
A relative affinity of 1.0 means that both terms 
differentiate exactly the same concepts in 
WordNet. It follows that the higher the relative 
affinity between c
1
 and c
2
, then the greater the 
likelihood that a concept differentiated by c
1
 can 
also be differentiated by c
2
, while the higher the 
absolute affinity, the more reliable this likelihood 
estimate becomes. Affinity thus provides an 
effective basis for formulating the transformation 
rules in T*. 
We should naturally expect near-synonymous 
modifiers to have a strong affinity for each other. 
For instance, Jewish and Hebrew are near-
synonyms because WordNet compounds Jewish-
Calendar and Hebrew-Calendar are themselves 
synonymous. This is clearly a form of contextual 
synonymy, since Jewish and Hebrew do not mean 
the same thing. Nonetheless, their affinity can be 
used to generate new compounds that add value to 
WordNet as synonyms of existing terms, such as 
Jewish-alphabet, Hebrew-Religion, and so on.
Recall that literal compounds represent a yoking 
together of two or more ontological branches. In 
exploring the space of novel compounds, it will be 
important to recognize which branches most 
naturally form the strongest bonds. Another variant 
of affinity can be formulated for this purpose:
A
domain
(x, y) =     |D
x
 D
x
|                   (3)
For instance,  A
domain
(sauce, pizza) = 2, since in 
WordNet the modifier overlap between the pizza
and sauce domains is {anchovy, cheese}.
4 Creative Exploration in the LMH Space
We consider as an exploratory starting point any 
LMH concept M-H in C*. We can transform this 
into another concept M'-H by replacing M with 
any M' for which:
M'  {x | x  D - {D
H
}  M  x}             (4)
This formulation may suggest a large range of 
values of M'. However, these candidates can be 
sorted by A
rel
(M, M'), which estimates the 
probability that a given M'-H will later be 
validated as useful. One rule in T* can now be 
formulated for our further consideration:
T*:   M
1
-H
1
 M
1
-H
2
 M
2
-H
1
      M
2
-H
2     
(5)
This rule allows the LMH space to be explored 
via a process of modifier modulation. Suppose we 
choose Greek-deity as a starting point. Since M = 
Greek and H = Deity, we can choose M' from any 
set other than D
deity
 that contains Greek:
D
alphabet
      = {Hebrew, Greek, Roman}
D
deity
          = {Greek, Roman, Norse, Hindu, …}
These differentiation patterns suggest that new 
compounds can meaningfully be created by yoking 
the ontological branches of alphabet and deity
together. Thus, from D
alphabet
 we can choose M' 
to be either Hebrew or Roman, leading to the 
creation of the LMH concepts Hebrew-deity and 
Roman-deity. One of these, Roman-deity, already 
exists in C*, but another, Hebrew-deity is novel in 
a way that Boden terms psychologically or P-
Creative, inasmuch as it is neither in C
w
 nor C*. It 
may thus be of some value as a hypernym for 
existing WordNet concepts like Yahwe and 
Jehovah.
Rule (5) is a general principle for ontological 
exploration in the space of compound terms. 
Consider the compound software-engineering, 
which, following (5), is suggested by the joint 
existence in WordNet of the concepts software-
engineer, automotive-engineer and automotive-
engineering. While this particular addition could 
be predicted from the application of simple 
morphology rules, the point here is that a single 
exploration principle like (5) can obviate the need 
for a patchwork of such simple rules.
Of course, one can imagine rules other than (5) 
to exploit the regularities inherent in WordNet 
definitions. For instance, consider the sense 
gasoline-bomb, which WordNet glosses as: “a 
crude incendiary bomb made of a bottle filled with 
flammable liquid and fitted with a rag wick”. By 
determining which definite description in the gloss 
conforms to the modifier – in this case it is 
“flammable liquid” – other modifiers can be found 
that also match this description. Thus, the new 
concepts methanol-bomb and butanol-bomb can be 
generated, and from this the creative concept 
alcohol-bomb can be generalized. However, each 
strategy raises its own unique issues, so for now 
we consider a T* comprising (5) only.
4.1 The Evaluation Mechanism E
For purposes of ascribing value or ontological 
utility to a new LMH concept M'-H, the concept 
must first be placed into one of the following 
categories:
a) M'-H already exists in C* is thus ascribed zero 
value as an addition to C*.
b)  M'-H does not exist in C* but does exist in C
w
, 
and thus corresponds to an existing non-literal 
concept (such as monkey-bread). While it may 
have value if given a purely literal reading, it 
cannot be added to C
w
 without creating 
ambiguity, and so has zero value.
c)  Using R*, M'-H can be seen to describe a non-
empty class of existing concepts in C
w
, and 
would thus have value as either a synonym 
(when this set is a singleton) or as a new 
organizing super-type (when this set is a 
severalton). In this case, we say that M'-H has 
been internally validated against C
w
.
d) Using a textual analysis of a large corpus such 
as the World-Wide-Web, M'-H is recognized 
to have a conventional meaning in C even if it 
is not described in C
w
. In this case, we say that 
M'-H has been externally validated for 
inclusion in C
w
. The fact that M'-H is novel to 
the system but not to the historical context of 
the web suggests that it is merely a 
psychologically or P-Creative invention in the 
sense of Boden (1990). 
e) M'-H is recognized to have a hypothetical or 
metaphoric value within a comprehension 
framework such as conceptual blending theory 
(e.g., see Veale et al. 2000), mental space 
theory, etc. In this case, M'-H may truly be a 
historically or H-Creative invention in the 
sense of Boden (1990).
In general, a new compound has value if its 
existence is suggested by, but not recognized by, 
the lexical ontology. As noted in the introduction, 
this value can be realized in a variety of ways, e.g., 
by automatically suggesting new knowledge-base 
additions to the lexical ontologist, or by providing 
potentially creative expansions for a user query in 
an information retrieval system (see Veale, 2004). 
4.2 Validating New Concepts
The evaluation strategies (c) and (d) above 
suggest two ways of validating the results of new 
compound creation: a WordNet-internal approach 
that uses the structure of the ontology itself to 
provide evidence for a compound’s utility, and a 
WordNet-external approach that instead looks to 
an unstructured archive like the web. In both cases, 
a new compound is validated by assembling a 
support set of precedent terms that argue for its 
meaningfulness.
4.2.1 Internal Validation
The internal support-set for a new compound M-H 
is the set of all WordNet words w that have: (i) at 
least one sense that is a hyponym of a sense of H; 
and (ii) a sense that contains M or some variant of 
it in its gloss. For instance, the novel compound 
“rain god” is internally validated by the word set 
{“Thor”, “Parjanya”, “Rain giver”}. 
When w is polysemous, two distinct senses may 
be used, reflecting the fact that M-H may be 
metonymic in construction. For instance, the 
compound “raisin-wine” can be validated 
internally by the polysemous word “muscatel”, 
since one sense of “muscatel” is a kind of wine, 
and another, a kind of grape, has a WordNet gloss 
containing the word “raisin”. From this 
perspective, a “raisin wine” can be a wine made 
from the same grapes that raisins are made from. 
Likewise, the compound “Jewish robot” can be 
validated by simultaneously employing both senses 
of “Golem” in WordNet, which defines “Golem” 
as either a Jewish mythical being or as a robotic 
automaton.
Creative products arise when conceptual 
ingredients from different domains are effectively 
blended (see Veale and O’Donoghue, 2000). It 
follows that a creative product can be validated by 
deblending it into its constituent parts and 
determining whether there is a precedent for 
combining elements of these types, if not these 
specific elements. We can thus exploit this notion 
of deblending to provide internal validation for 
new compounds. For instance the WordNet gloss 
for pizza lists “tomato sauce” as an ingredient. 
This suggests we can meaningfully understand a 
compound of the form  M-pizza if there exists a 
compound M-sauce that can be viewed as a 
replacement for this ingredient. Generalizing from 
this, we can consider a new compound M
1
-H
1
 to 
be internally validated if H has a sense whose gloss 
contains the compound M
2
-H
2
, and if the ontology 
additionally contains the concept M
1
-H
2
. It follows 
then that the novel compounds apple-pizza, 
chocolate-pizza, taco-pizza, and curry-pizza will 
all  be internally validated as meaningful (if not 
necessarily enjoyable) varieties of pizza.
4.2.2 External Validation
In contrast, the external validation set for a 
compound M-H is the set of distinct documents 
that contain the compound term “M H”, as 
acquired using a web search engine. For instance, 
given the WordNet concepts naval-engineer, 
software-engineer and naval-academy, rule (5) 
generates the hypothesis software-academy, which 
cannot be validated internally yet which retrieves 
over 1000 web documents to atest to its validity.
This web strategy is motivated by Keller and 
Lapata’s (2003) finding that the number of 
documents containing a novel compound reliably 
predicts the human plausibility scores for the 
compound. 
Nonetheless, external validation in this way is 
by no means a robust process. Since web 
documents are not sense tagged, one cannot be 
sure that a compound occurs with the sense that it 
is hypothesized to have. Indeed, it may not even 
occur as a compound at all, but as a coincidental 
juxtaposition of terms from different phrases or 
sentences. Finally, even if found with the correct 
syntactic and semantic form, one cannot be sure 
that the usage is not that of a non-native, second 
language learner.
These possibilities can be diminished by 
seeking a large enough sample set, but this has the 
effect of setting the evidential bar too high for truly 
creative compounds.  However, another solution 
lies in the way that the results of external 
validation are actually used, as we shall later see.
4.2.3 Validating New Synonyms
Many of the compounds that are validated either 
by internal or external means will be synonyms of 
existing WordNet terms. As such, their creative 
value will not represent an innovative combination 
of ideas, but rather a creative use of paraphrasing. 
The nature of (5) makes it straightforward to 
determine which is the case.
In general, when M
1
-H
1
 and M
2
-H
1
 are 
themselves synonyms, then M
2
-H
2
 will be a 
synonym of M
1
-H
2
. For instance, from the 
combination of applied-science, engineering-
science and applied-mathematics, we can generate 
from (5) the new compound engineering-
mathematics. This compound cannot be validated 
internally, but since it retrieves more than 300,000 
documents from the web, this is enough to 
adequately atest to its meaningfulness. Now, since 
applied-science and engineering-science are 
synonymous in WordNet, we can conclude that 
engineering-mathematics and applied-mathematics
are themselves synonymous also.
         
4.3 Creativity in the Validation Gap
The difference between internal and external 
validation strategies can be illuminating. Internal 
validation verifies a compound on the basis that it 
could meaningfully exist, while external validation 
verifies it on the basis that it does actually exist in 
a large corpus. Therefore, if a compound can be 
validated externally but not internally, it suggests 
that the concept may by P-Creative. In contrast, if 
the compound can be validated internally but not 
externally, it suggests that the compound may be 
H-Creative and represent a genuine historical 
innovation (if only a lexical one, and of minor 
proportions).
For instance, the new compound “sea dance”
(analogous to “rain dance”) cannot be validated 
internally, yet can be found in over 700 internet 
documents. It thus denotes a P-Creative concept. In 
contrast, the compound “cranial vein” yields no 
documents from a web query (on AltaVista), yet 
can be internally validated by WordNet via the 
word-concept Diploic-Vein, a blood vessel that 
serves the soft tissue of the cranial bones. 
Likewise, the compounds “chocolate pizza”, “taco 
pizza” and many more from the yoking of D
pizza
and D
sauce
 can all be validated externally via 
hundreds of different web occurrences, and so 
represent P-Creative varieties of pizza. However, 
compounds like “Newburg pizza” (a pizza made 
with lobster sauce) and “wine pizza” (a pizza made 
with wine sauce) can only be validated internally 
and are thus candidates for H-Creative innovation.
5 Large-Scale Evaluation
A large scale evaluation of these ideas was 
conducted by exhaustively applying the T* rule of 
(5) to the noun taxonomy of WordNet 1.7. To 
better see the effect of affinity between modifiers, 
Table 1 ranks the results according to the measure 
A
abs
 from (1).
A
abs
1 2 3
No. compounds 
generated 
941,841 22,727 2,175
% H-Creative 0.49% 0.63% 1.38%
% P-Creative 35.65% 33.77% 34.57%
% Conflations 0.10% 0.10% 0.05%
%Indeterminate 63.76% 65.49% 64.00%
Table 1: Number of compounds created, and their 
assessment, for each affinity level.
Conflations are terms that exist both as compounds 
and as conflated lexical atoms. For instance, while 
the compound “bull dog” may not exist in 
WordNet, its conflation “bulldog” does. 
Compound discovery is thus a useful means of re-
expanding these conflations when it is meaningful 
to do so. 
As one might expect, lower affinity levels allow 
greater numbers of new compounds to be created. 
Interestingly, however, Table 1 suggests that as the 
affinity threshold is raised and the number of 
compounds lowered, the creativity of these 
compounds increases, as measured by the relative 
proportion of H-Creative terms that are generated.
Generating compound terms in a lexical 
ontology is a creative process that demands 
rigorous validation if the ontology is not to be 
corrupted. Of the two strategies discussed here, 
external validation is undoubtedly the weaker of 
the two, as one should be loathe to add new 
compounds to WordNet on the basis of web 
evidence alone. However,  external validation does 
serve to illustrate the soundness of internal 
validation, since 99.51% of internally validated 
concepts (at A
abs
 = 1) are shown to exist on the 
web. It follows then that the absence of external 
validation yields a very conservative basis for 
assessing H-Creativity. Web validation is perhaps 
better used therefore as a means of rejecting 
creative products than as a means of discovering 
them. In fact, when used as a reverse barometer in 
this way, the inevitable errors that arise from web-
based validation serve only to make the creative 
process more selective.
6 Conclusions and Future Work
We are currently considering ways of 
broadening the scope of internal validation while 
maintaining its conceptual rigour. This should 
counter-balance the high rejection rate caused  by 
an overly conservative external validation process, 
and thereby allow us to identify a higher 
percentage of H-creative products. As shown with 
the “pizza” examples of section 4.3, we have 
already begun to explore the possibilities of 
validation latent in the WordNet ontology itself. So 
while the use of web content for external validation 
suggests that creative discovery has a role to play 
in producing and expanding web queries, internal 
validation remains our central thrust, leading to 
what we hope will be a new, more creative model 
of the thesaurus. 
In grounding our discussion in the creative 
framework of Boden (1990) and its formalization 
by Wiggins (2003), we have placed particular 
emphasis on the labels P-Creative and H-Creative. 
However, the empirical results of section 5 suggest 
that this binary categorization may be overly 
reductive, and that a more gradated system of 
labels is needed. For instance, the novel 
compounds computer-consultant and handwriting-
consultant are both created from a yoking of the 
domains expert and consultant, and because each is 
externally validated, each is considered P-Creative. 
However, while only a handful of documents can 
be marshalled to support handwriting-consultant,
the amount of web evidence available to support 
computer-consultant is vast. So it is wrongheaded 
to consider both as equally P-Creative and lacking 
in H-Creativity, since the dearth of existing uses 
suggests handwriting-consultant has far greater 
novelty. Perhaps what is needed then is not a 
binary categorization but a continuous one, a 
numeric scale with P- and H-Creativity as its poles. 
This scale would function much like the continuum 
used by (MacCormac, 1985) to separate banal 
metaphors (which he dubbed epiphors) from 
creative ones (or diaphors).

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