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Principles of Categorization 253

sense) high correlational structure. That is, given a knower who perceives the
complex attributes of feathers, fur, and wings, it is an empirical fact provided
by the perceived world that wings co-occur with feathers more than with fur.
And given an actor with the motor programs for sitting, it is a fact of the per-
ceived world that objects with the perceptual attributes of chairs are more
likely to have functional sit-on-able-ness than objects with the appearance of
cats. In short, combinations of what we perceive as the attributes of real objects
do not occur uniformly. Some pairs, triples, etc., are quite probable, appearing
in combination sometimes with one, sometimes another attribute; others are
rare; others logically cannot or empirically do not occur.
It should be emphasized that we are talking about the perceived world and
not a metaphysical world without a knower. What kinds of attributes can be
perceived are, of course, species-specific. A dog’s sense of smell is more highly
differentiated than a human’s, and the structure of the world for a dog must
surely include attributes of smell that we, as a species, are incapable of per-
ceiving. Furthermore, because a dog’s body is constructed differently from a
human’s, its motor interactions with objects are necessarily differently struc-
tured. The ‘‘out there’’ of a bat, a frog, or a bee is surely more different still from
that of a human. What attributes will be perceived given the ability to perceive
them is undoubtedly determined by many factors having to do with the func-
tional needs of the knower interacting with the physical and social environ-
ment. One influence on how attributes will be defined by humans is clearly the
category system already existent in the culture at a given time. Thus, our seg-
mentation of a bird’s body such that there is an attribute called ‘‘wings’’ may be
influenced not only by perceptual factors such as the gestalt laws of form that
would lead us to consider the wings as a separate part (Palmer 1977) but also
by the fact that at present we already have a cultural and linguistic category
called ‘‘birds.’’ Viewing attributes as, at least in part, constructs of the perceiver
does not negate the higher-order structural fact about attributes at issue,
namely that the attributes of wings and that of feathers do co-occur in the per-
ceived world.
These two basic principles of categorization, a drive toward cognitive econ-
omy combined with structure in the perceived world, have implications both
for the level of abstraction of categories formed in a culture and for the internal
structure of those categories once formed.
For purposes of explication, we may conceive of category systems as having
both a vertical and horizontal dimension. The vertical dimension concerns the
level of inclusiveness of the category—the dimension along which the terms
collie, dog, mammal, animal, and living thing vary. The horizontal dimension
concerns the segmentation of categories at the same level of inclusiveness—the
dimension on which dog, cat, car, bus, chair, and sofa vary. The implication of
the two principles of categorization for the vertical dimension is that not all
possible levels of categorization are equally good or useful; rather, the most
basic level of categorization will be the most inclusive (abstract) level at which
the categories can mirror the structure of attributes perceived in the world. The
implication of the principles of categorization for the horizontal dimension is
that to increase the distinctiveness and flexibility of categories, categories tend
to become defined in terms of prototypes or prototypical instances that contain