by L Erlbaum — The third part relates the principles of categorization to the formation of prototypes in those categories that are at the same level of abstraction in a taxonomy. In
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1Principles of Categorization Eleanor Rosch, 1978 University of California, Berkeley First publised in: Rosch, Eleanor and Lloyd, Barbara B. (eds), Cognition and categorization 27-48. Hillsdale, NJ: Lawrence Erlbaum. The following is a taxonomy of the animal king dom. It has been attributed to an ancient Chinese encyclopedia entitled the Celest ial Emporium of Benevolent Knowledge: On those remote pages it is written that an imals are divided into (a) those that belong to the Emperor, (b) embalmed ones, (c) thos e that are trained, (d) suckling pigs, (e) mermaids, (f) fabulous ones, (g) stray dogs, (h) those that are included in this classification, (i) those that tremble as if they were mad, j) innumerable ones, (k) those drawn with a very fine camel’s hair brush, (l) others, (m) those that have just broken a flower vase, (n) those that resemb le flies from a distance (Borges, 1966, p. 108). Conceptually, the most interesting aspect of this cl assification system is th at it does not exist. Certain types of categorizations may appear in the imagination of poets, but they are never found in the practical or linguistic classes of organisms or of man-made objects used by any of the cultures of the world. For some years, I ha ve argued that human categorization should not be considered the arbitrary product of historical accident or of whim but rather the result of psychological principles of categorization, which are subject to investigation. This chapter is a summary and discussion of those principles. The chapter is divided into five parts. The first part presents the two general principles that are proposed to underlie categorization systems. The second part shows the way in which these principles appear to result in a basic and primary level of categorization in the levels of abstraction in a taxonomy: It is essentially a
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2summary of the research already reported on basi c level objects (Rosch et al., 1976). Thus the second section may be omitted by the reader already sufficiently familiar with that material. The third part relates the principles of categorization to the formation of prototypes in those categories that are at the same level of abstra ction in a taxonomy. In particular, this section attempts to clarify the operational concept of prototypicality and to separate that concept from claims concerning the role of prototypes in cognitive processing, representation, and learning for which there is little evidence. The fourth part presents two issues that are problematical for the abstract principles of categorization stated in Part 1: (1) the relation of context to basic level objects and prototypes; and (2) assumptions about the nature of the attributes of real-world objects that underlie the claim that there is structure in the world. The fifth part is a report of initial attempts to base an analysis of the attributes, functions, and contexts of objects on a consideration of objects as props in culturally defined events. It should be noted that the issues in categoriza tion with which we are primarily concerned have to do with explaining the categories found in a culture and coded by the language of that culture at a particular point in time. en we speak of the formation of categories, we mean their formation in the culture. This point is often misunderstood. The principles of categorization proposed are not as such intended to constitute a theory of the development of categories in children born into a culture no r to constitute a model of how categories are processed (how categorizations are made) in the minds of adult speakers of a language. THE PRINCIPLES Two general and basic principles are prop osed for the formation of categories: The first has to do with the func tion of category systems and asserts that the task of category systems is to provide maximum information with the least cognitive effort. The second principle has to do with the structure of the information so provided and asserts that the perceived world comes as structured information rather than as arbitrary or unpredictable attributes. Thus maximum information with least cognitive effort is achieved if categories map the perceived world structure as closely as possible. This condition can be achieved either by the mapping of categories to given attribute st ructures or by the definition or redefinition of attributes to render a give n set of categories appropriately structured.
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3These principles are elaborated in the following. Cognitive Economy . The first principle contains the almost common-s ense notion that, as an organism, what one wishes to gain from one’s categories is a grea t deal of information about the environment while conserving finite resources as mu ch as possible. To ca tegorize a stimulus me ans to consider it, for purposes Purposes of that categorization, not only equivale nt to other stimuli in the same category but, also different from stimuli not in that catego ry, On the one hand, it would appear to the organism’s advantage to have as many properties as possible predictable from Knowing any one property, a principle that would lead to formatio n of large numbers of categories with as fine discriminations between categories as possible. On the other hand , one purpose of categorization is to reduce the infinite differences among stimuli to behaviorally and cognitively usable proportions. It is to the organism’s advantage not to differentiate one stimulus from others when that differentiatio n is irrelevant to the purposes at hand. Perceived World Structure. The second principle of categorization asserts that unlike the sets of stimuli used in traditional laboratory concept attainment tasks, the percei ved world is not an uns tructured total set of equiprobable co-occurring attributes Rather, the material objects of the world are perceive to possess (in Garner’s, 1974, sense) high correlational structure. That is, given a knower who perceives the complex attributes of feathers fur an d wings, it is an empirical fact provided by the perceived world that wings co-occur with feat hers more than with fur. And given an actor with the motor programs for sitting, it is a fact of the perceived 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 an other attribute; others are rare; others logically cannot or empi rically 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 perceiving. Furthermore, because a dog’s body is constructed differently from a
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4human’s, its motor interactions with objects are necessarily differently structured. 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 functional n eeds of the knower interacting with the physical and social environment. One influence on how a ttributes will be defined by humans is clearly the category system already existent in the cult ure at a given time. Thus, our segmentation of a bird’s body such that there is an attribut e called “wings” may be influenced not only by perceptual factors such as the gestalt laws of fo rm that would lead us to consider the wings as a separate part (Palmer, in press) but also by the fact that at present we already have a cultural and linguistic category called “birds.” Vi ewing 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 perceived world. These two basic principles of categorization, a drive toward cognitive economy combined with structure in the perceived world, have implic ations both for the level of abstraction of categories formed in a culture and for the, inte rnal structure of those categories once formed. For purposes of explication, we may conceive of category systems as having both a verbal 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 segm entation 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; ra ther, 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 the attributes most representative of items inside and least representative of items outside the category. THE VERTICAL DIMENSION OF CATEGORIES: BASIC-LEVEL OBJECTS In a programmatic series of experiments, we ha ve attempted to argue that categories within taxonomies of concrete objects are structured such that there is generally one level of abstraction at which the most basic category cuts can be made (Rosch et al., 1976a). By category is meant a number objects that are co nsidered equivalent. Categories are generally designated by names (e.g., dog, animal) A ta xonomy is a system by which categories are
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5related to one another by means of class inclusio n, The greater the inclusiveness of a category within a taxonomy, the higher the level of abstraction., Each ca tegory within a taxonomy is entirely included within one other category (unle ss it is the highest level category) but is not exhaustive of that more inclusive cate gory (see Kay, 1971). Thus the term level of abstraction within a taxonomy refers to a particular level of inclusiveness. A familiar taxonomy is the Linnean system for the classification of animals. Our claims concerning a basic le vel of abstraction can be formalized in terms of cue validity (Rosch et al., 1976a) or in terms of the set th eoretic representation of similarity provided by Tversky (1977, and Chapter 4 in this volume). Cue validity is a probabilistic concept; the validity of a given cue x as a predictor of a give n category y (the conditio nal probability of y/x) increases as the frequency with which cue x is associated with category y increases and decreases as the frequency with which cue x is associated with categories other than y increases (Beach, 1964a, 1964b; Reed, 1972). The cue validity of an entire category may be defined as the summation of the cue validities for that category of each of the attributes of the category. A category with high cue validity is, by definition, more differentiated from other categories than one of lower cue validity. The elegant formulization that Tversky provides in Chapter 4 is in terms of the variable “category resemblance,” which is defined as the weighted sum of the measures of all of the common features within a category minus the sum of .the measures of all of the distinctive features., Dist inctive features include those that belong to only some members of a given category as well as those belonging to contrasting categories. Thus Tversky’s formalization does not weight the ef fect of contrast catego ries as much as does the cue validity formulation. Tversky suggests th at two disjoint classes tend to be combined whenever the weight of the added common feat ures exceeds the weight of the distinctive features. A working assumption of the research on basic objects that (1) in the perceived world, information-rich bundles of perceptual and functional attributes occur that form natural discontinuities, and that (2),basic cuts in categorization are made at these discontinuities. Suppose that basic objects, (e.g., chair, car), ar e at the most inclusive level at which there are attributes common to all or most members of th e category. Then both total cue validities and category resemblance are maximized at that le vel of abstraction at which basic objects are categorized. This is, categories one level more abstract will be superordinate categories (e.g., furniture, vehicle) whose members share only a few attributes among each other. Categories below the basic level will be bundles of common and, thus, predictable attributes and functions but contain many attributes that overla p with other categories (for example, kitchen chair shares most of its attributes with other kinds of chairs).
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6Superordinate categories have lower total cue va lidity and lower category resemblance than do basic-level categories, because they have fewer common a ttributes; in fact, the category resemblance measure of items within the superord inate can even be negative due to the high ratio of distinctive to common features. Subordinate categories have lower total cue validity than do basic categories, because they also shar e most attributes with contrasting subordinate categories; in Tversky’s terms, they tend to be combined because the weight of the added common features tend to exceed the weight of the distinctive features, That basic objects are categories at the level of abstraction that maximizes cue validity and maximizes category resemblance is another way of asserting that basic objects are the categories that best mirror the correlational structure of the environment. We chose to look at concrete objects because th ey appeared to be a domain that was at once an indisputable aspect of comp lex natural language cl assifications yet at the same time were amenable to methods of empirical analysis. In our investigations of basic categories, the correlational structure of concrete objects was considered to consist of a number of inseparable aspects of form and function, any one of which could serve as the starting point for analysis. Four investigations provided converging operational definitions of the basic level of abstraction: attributes in common, motor movements in common, objective similarity in shape, and identifiability of averaged shapes. Common Attributes. Ethnobiologists had suggested on the basis of linguistic criteria and field observation that the fobs was the level of classification at which organisms had bundles of attributes in common and maximum discontinuity between classes (see Chapter 1). The purpose of our research was to provide a systematic empirical study of the co-o ccurrence of attributes in the most common taxonomies of biological and man-made objects in our own culture. The hypothesis that basic level objects are the mo st inclusive level of classification at which objects have numbers of attributes in common was tested for categories at three levels of abstraction for nine taxonomies: tree, fish, fruit, musical instruments, tool, clothing furniture and vehicle. Examples of the three levels for one biological and one nonbiological taxonomy are shown in Table 2.1. Criteria for choice of these specific items were that the taxonomies contain the most common (defined by word freque ncy) categories of conc rete nouns in English, that the levels of abstraction bear simple class- inclusion relations to each other, and that those class-inclusion relations be generally known to our subjects (be agreed upon by a sample of
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8Inseparable from the perceived attributes of objects are the ways in which humans habitually use or interact with those objects. For concrete objects, such interactions take the form of motor movements. For example, when performing the action of sitting down on a chair, a sequence of body and muscle movements are typically made that are inseparable from the nature of the attributes of chairs – legs, seat, back, etc. This aspect of objects is particularly important in light of the role that sensory-moto r interaction with the world appears to play in the development of thought (Brunei, Olver, & Greenfield, I966; Nelson , I974; Piaget, I952). In our study of motor movements, each of the sets of words used in the previous experiment was administered to new subjects. A subject was as ked to describe, in as much finely analyzed detail as possible, the sequences of motor movements he made when using or interacting with the object. Tallies of agreed upon listings of the same movements of the same body part in the same part of the movement sequence formed the unit of analysis. Results were identical to those of the attribute listings; basic objects were the most general classes to have motor sequences in common. For example, there are few motor programs we irry out to items of furniture in general and several specific motor programs +cried out in regard to sitting down on chairs, but we sit on kitchen and livingroom ch airs using essentially the same motor programs. Similarity in Shapes. Another aspect of the meaning of a class of ob jects is the appearance of the objects in the class. In order to be able to analyze correl ational structures by different but converging methods, it was necessary to find a method of analyzing similarity in the visual aspects of the objects that was not dependent on subjects’ de scriptions, that was free from effects of the object’s name (which would not have been the ca se for subjects’ ratings of similarity), and that went beyond similarity of _analyzable, listable attributes that had already been used in the first study described. For this purpose, outlines of the shape of two-dimensional representations of objects were used, an integral aspect of natural forms. Similarity in shape was measured by the amount of overlap of the two outlines when the outlines (normalized for size and orientation) were juxtaposed. Results showed that the ratio of overlapped to nonoverlapped area when two objects from the same basic-level category (e.g., two cars) were superimposed was far greater than when two objects from the same superordinate category were superimposed (e-g., a car an d a motorcycle). Although some gain in ratio of overlap to nonoverlap also occurred for subordinate catego ry objects (e.g., two sports cars), the gain
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9obtained by shifting from basic-level to subord inate objects was significantly less than the gain obtained by shifting from superordinate to basic-level objects. Identifiability of Averaged Shapes. If the basic level is the most inclusive level at which shapes of objects of a class are similar, a possible result of such similarity may be that the basic level is also the most inclusive level at which an averaged shape of an object can be recognized. To test this hypothesis, the same normalized superimposed shapes used in the previous experiment were used to draw an average outline of the overlapped figures. Subj ects were then asked to identify both the superordinate category and the specific object depicted. Results showed that basic objects were the most general and inclusive categori es at which the objects depicted could be identified. Furthermore, overlaps of subordinate objects were no more identifiable than objects at the basic level. In summary, our four converging operational definitions of basic objects all indicated the same level of abstraction to be basic in our taxonomies. Admittedly, the basic level for biological objects was not that predicted by the folk genus ; however, this fact appeared to be simply accounted for by our subjects’ lack of knowledge of the additional depth of real-world attribute structure available at the level of the folk generic (see Rosch et al., 1976a). Implications for Other Fields The foregoing theory of categorization and basic objects has implications for several traditional areas of study in psychology; some of these have been tested. Imagery . The fact that basic-level objects were the mo st inclusive categories which an averaged member of the category could be identified suggested at basic objects might be the most inclusive categories for which it was possible form a mental image isomorphic to the appearance of members of the class as whole. Experiments using a signal-detection paradigm and a priming paradigm, both of which have been previously argu ed to be measures of imagery (Peterson Graham, I974; Rosch, 1 975c), verified that, in so far as it was meaningful to e the term imagery, basic objects appeared to-be the most abstract categories for which an image could be reasonably representative of the class as a whole. Perception.
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10From all that has been said of the nature of basi c classifications, would hardly be reasonable to suppose that in perception of the world, obje cts were first categorized either at the most abstract or at the most concrete level possible Two separate studies of picture verification (Rosch et al., 1976a; Smith, Balzano, & Walker , I978) indicate that, in fact, objects may be first :en or recognized as members of their ba sic category, and that only with the ad of additional processing can they be identified as members of their superordinate or subordinate category. Development. We have argued that classification into categori es at the basic level is overdetermined because perception, motor movements, functions, and iconic images would all lead to the same level of categorization. Thus basic object s should be the first categorizations of concrete objects made by children. In fact, for our nine taxonomies, the basic level was the first named. And even when naming was controlled, pictur es of several basic-level objects were sorted into groups “because they were the same type of thing” long before such a technique of sorting has become general in children. Language. From all that has been said, we would expect th e most useful and, thus, most used name for an item to be the basic-level name. In fact, we fo und that adults almost invariably named pictures of the subordinate items of the nine taxonomies at the basic level, although they knew the correct superordinate and subordinate names for the objects. On a more speculative level, in the evolution of languages, one would expect names to evolve first for basic level objects, spreading both upward and downwa rd as taxonomies increased in depth. Of great relevance for this hypothesis are Berlin’s (1972) claims for such a pattern for the evolution of plant names, and our own (Rosch et al., 1976a) and Newport and Bellugi’s (Chapter 3, this volume) finding for American Sign Language of the Deaf, that 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 oper ations 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 c lusters of perceived attributes is not to say that such attribute clusters are necessarily discontinuous.
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11In terms of the principles of categorization proposed earlier, cognitive economy dictates that categories tend to be viewed as being as separate from each other and as clear-cut as possible. One way to achieve this is by means of formal, necessary and sufficient criteria for category membership. The attempt to impose such-criteria on categories marks virtual definitions in the tradition of Western reason. The psychologica l treatment of categories in the standard concept-identification paradigm lies within th is 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 boundaries. 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 demarcation of the boundary line. Categories can be viewed in terms of their clear cases if the perceiver places emphasis on the correlational structure of perceived attributes su ch that the categories are represented by their most structured portions. By prototypes of categories we have generally meant the clearest cases of pry 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 aske d about the category 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 concerni ng 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 th e 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 whos e boundaries they disagree (Rosch, 1974, 1975b). Such judgments are reliable even under changes of instructio ns and items (Rips, Shoben, & Smith, 1973; Rosch, 1975b, 197 5c; Rosch & Mervis, 1975). Were such agreement and reliability in judgment not to have been obtained, there would be no further point in discussion or
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