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Principles of Categorization 259

it was the basic-level categories that were most often coded by single signs and
super- and subordinate categories that were likely to be missing. Thus a wide
range of converging operations verify as basic the same levels of abstraction.

The Horizontal Dimension: Internal Structure of Categories: Prototypes
Most, if not all, categories do not have clear-cut boundaries. To argue that basic
object categories follow clusters of perceived attributes is not to say that such
attribute clusters are necessarily discontinuous.
In terms of the principles of categorization proposed earlier, cognitive econ-
omy dictatesthatcategoriestendtobe viewedasbeing as separate from each
other and as clear-cut as possible. One way to achieve this is by means of for-
mal, necessary and sufficient criteria for category membership. The attempt
to impose such criteria on categories marks virtually all definitions in the tra-
dition of Western reason. The psychological treatment of categories in the
standard concept-identification paradigm lies within this tradition. Another
way to achieve separateness and clarity of actually continuous categories is by
conceiving of each category in terms of its clear cases rather than its bound-
aries. As Wittgenstein (1953) has pointed out, categorical judgments become a
problem only if one is concerned with boundaries—in the normal course of life,
two neighbors know on whose property they are standing without exact de-
marcation of the boundary line. Categories can be viewed in terms of their clear
casesifthe perceiverplacesemphasisonthe correlationalstructure of per-
ceived attributes such that the categories are represented by their most struc-
tured portions.
By prototypes of categories we have generally meant the clearest cases of
category membership defined operationally by people’s judgments of goodness
of membership in the category. A great deal of confusion in the discussion of
prototypes has arisen from two sources. First, the notion of prototypes has
tended to become reified as though it meant a specific category member or
mental structure. Questions are then asked in an either-or fashion about whether
something is or is not the prototype or part of the prototype in exactly the same
way in which the question would previously have been asked about the cate-
gory boundary. Such thinking precisely violates the Wittgensteinian insight
that we can judge how clear a case something is and deal with categories on the
basis of clear cases in the total absence of information about boundaries. Second,
the empirical findings about prototypicality have been confused with theories
of processing—that is, there has been a failure to distinguish the structure of
categories from theories concerning the use of that structure in processing.
Therefore, let us first attempt to look at prototypes in as purely structural a
fashion as possible. We will focus on what may be said about prototypes based
on operational definitions and empirical findings alone without the addition of
processing assumptions.
Perception of typicality differences is, in the first place, an empirical fact
of people’s judgments about category membership. It is by now a well-
documented finding that subjects overwhelmingly agree in their judgments
of how good an example or clear a case members are of a category, even for
categories about whose boundaries they disagree (Rosch 1974, 1975b). Such