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HOME > PAST ISSUE > May-June 1999 > Article Detail

COMPUTING SCIENCE

The Web of Words

Brian Hayes

Black and White

The treelike organization of both nouns and verbs in WordNet leads naturally to the hypothesis that all words are best catalogued in such structures. The treatment of adjectives therefore comes as a surprise. In WordNet adjectives do not grow on trees; instead they come in matched pairs of opposites—black and white, clean and dirty, fast and slow, good and bad. And on reflection, the pairing of antonyms does seem like the natural organizing principle for this class of words. In free-association tests, many adjectives strongly evoke their opposites, suggesting that we file them mentally in symmetrical pairs.

There is something else odd about adjectives. Other relations in WordNet are between meanings or concepts, which are conveniently represented by synsets, but the antonymy of adjectives seems to be a relation between specific words. Hot, sultry, torrid and sweltering may all belong in the same synset, but the antonym cold is strongly associated with only one of these. If you ask people "What's the opposite of hot?" you'll get an immediate answer, but "What's the opposite of torrid?" is a harder question.

The solution adopted in WordNet is to organize adjectives in clusters around focal pairs of antonyms. Thus hot and cold stand face-to-face like gang leaders, each surrounded by a throng of allied words, fiery and blistering on one side, frigid and chilly and frosty on the other. If you ask WordNet for the antonym of torrid, it responds: "indirect (via hot) → cold."

This scheme for organizing adjectives was not planned when the WordNet project began in 1985. The need to provide antonym pointers between specific word forms was something discovered while building the graph, and it was not an altogether welcome discovery. Up to then it had been assumed that all edges of the graph would extend between synsets; the database format had to be altered to accommodate the antonym pointers.

WordNet includes some 3,500 clusters of adjectives arranged in antonymous pairs. Most of them fit neatly into the bipolar plan. Indeed, many of the adjectives can be graded, or arranged along a one-dimensional continuum. Hot and cold, for example, have tepid at the neutral point between them, with warm and cool occupying less extreme positions. A few adjectives, however, refuse to conform. Angry is the chief example mentioned by WordNet's authors. Angry is the focus of a cluster of related words, which can be graded according to intensity from annoyed to furious, but English seems to offer no antonym to angry. The asymmetry is a reminder that although language is a human invention, it is not an engineered product; it doesn't have to be consistent.

WordNet includes adverbs as well as adjectives but finds little to say about them. They are grouped into synsets, and some of them are linked to adjectives from which they are derived, but there are no taxonomic trees or bipolar pairs. Other parts of speech—the prepositions, conjunctions, pronouns and other "little" words—are omitted altogether.




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