Nouns and adjectives also differ in the relatedness of one category to another. Common nouns all classify objects at one level [Ros73a]. An object is a dog or a house or a watch or a car or a leaf. Thus the question what is it? is answerable by one basic noun. [Mar89] incorporated this notion in her proposal that children adhere to a mutual exclusivity assumption in early word learning. Although this idea of a one-object, one-name rule is imperfect and complicated by a hierarchical taxonomy and synonyms, it also captures something quite real about the way common nouns are commonly used [Cla73,Mar89,MH84,Mer87,MMJB92,Ros73a]. Dimensional adjectives present a markedly different structure. They are (typically) mutually exclusive within a dimension but overlap completely across dimensions. Objects in the category BIG may also be in the categories WET and FURRY.
An idealization of this difference between common nouns and
dimensional adjectives is depicted in
Figures 
and .
Relatively small noun categories fill all reaches of the space
but rarely overlap with one another. In contrast, the extensions
of dimensional adjectives create a dense grid-work of overlapping
slabs that cut through the space in multiple directions as
illustrated. Again, under the ordinary assumptions of similarity-based
learning, these differences in category structure favors
nouns: between-category similarity among nouns is minimal but
between category similarity among adjectives is great.
: Noun Categories. Only three dimensions from the hyperspace of
possible dimensions are shown. Noun categories tend to
be small and compact and not to overlap with one another.
: Adjective Categories. Only three dimensions are shown.
Adjective categories tend to be large and elongated and to overlap with one
another.
