What is the difference between common nouns and dimensional adjectives that allows children to acquire nouns more rapidly than adjectives? We could distinguish the two categories in purely syntactic terms, with respect to the other categories with which they co-occur. We could also distinguish them in terms of their function, as [Mar89] does; we carve up the world in useful ways with nouns and then resort to adjectives when we need to distinguish members of the same nominal category along arbitrary dimensions. But underneath all this might be a more mundane distinction, one based on the tendencies of nouns and adjectives to delineate particular sorts of regions in multi-dimensional perceptual space. Unlike the first two kinds of distinctions, this third is one which is directly available to a relatively simple learning device, as we have shown in this paper. Of course a child must eventually learn about more abstract functions and about syntactic categories much richer than those examined here, but the distinction based on the most accessible sort of information could provide a foundation for this later learning.