Which features make up the prototype?

One is tempted to answer: the typical features. Some features of birds are intuitively considered essential and typical features of birds, e.g. having wings and feathers and being able to fly as well as the properties of singing, having a beak, laying eggs and hatching them in a nest. Others, like having a particular colour, a certain weight and size may be characteristic for certain kinds of birds but not for birds in general. Having feathers is a ‘good’ feature because it well distinguishes birds from non-birds. The feature is therefore said to have a high cue validity for the category BIRD which means that it applies to a high proportion of members and to a low proportion of non-members. The features of having wings, being able to fly and laying eggs have a lower cue validity because they are shared by other kinds of animals, e.g. most insects. They are, however, of higher cue validity within the narrower domain of vertebrates. Here, the feature of having wings and being able to fly is very distinctive, since except for birds only bats share it. Thus, a prototype must be defined by a combination of features that together maximize the cue validity. Features of low cue validity that distinguish birds and other vertebrates from insects have to be included in order to enhance the cue-validity of the features such as having wings and being able to fly.

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9.2.9 Similarity

Prototype theory claims that membership in a given category is a matter of similarity to the prototype. While this seems a clear and simple criterion, a closer look shows that it is anything but that. When thinking of applica- tions, we will think of comparing, for example, various kinds of birds to a prototypical bird with respect to physical appearance, primarily shape, size and colour. However, similarity in this concrete sense of resemblance is not relevant in most cases. Consider, for example, a wolf, a husky and a poodle. Wolves and huskies are physically very similar and both very different from poodles. Yet the husky and the poodle belong to the same category DOG and the wolf does not. The categorization of dogs and wolves is not only a matter of physical similarity. Membership in the category DOGmust be a matter of similarity to the prototypical dog in some other respects. The question, then, arises: in which regard is a would-be member of a given cate- gory to be similar to the prototype? Which of its properties are relevant for the comparison? The answer is far from trivial.

Consider a second case. Mary has two cousins, Marc and Tom. They are brothers and physically very similar. Let us assume that we know that Marc is a fairly prototypical member of the category MAN. The physical resem- blance of Tom would then entitle us to categorize him too as a man. Yet if Marc is a butcher, Tom’s physical similarity to Marc does not entitle us to conclude that he has the same profession. Rather, the relevant aspect of Marc and Tom would be the kind of job they have or they are qualified for. Categorization in the category MANrequires similarity to the prototype in quite different respects from categorization in the category BUTCHER, or in the category BACHELORor NEIGHBOURor HIP-HOP FREAK, for that matter.

These considerations show again that in order for prototypes to serve as reference points of categorization they must be defined by a set of crucial features. And it is these features that must be checked in order to judge the similarity to the prototype. The features may be of different weight or interdependent. For example, for birds the feature of being able to fly may depend on the body weight and the form and relative size of the wings. In this sense, the resulting model of categorization is not the same as the simple checklist model of NSC, where each feature is supposed to be necessary, independent of the other features, but the difference is not as radical as PT made it appear.

Another difficult question concerns the relevant scale of similarity on which the degree of membership depends. Let us assume similarity is measured in values between 0 and 1. Given a prototype and the set of criteria relevant for the comparison, it is clear which cases have the value 1 on the scale of similarity: all those that completely agree with the prototype in all relevant aspects. However, in the other direction the scale is unde- fined and, in fact, indefinable. Consider again the case of the category DOG. Since wolves are definitely non-members of the category DOG, they must be 09-UnderSemantics-Ch9-cp 04/04/2002 7:51 am Page 182

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assigned the value 0.0 on the scale of similarity. But some of the crucial defining properties of the prototypical dog must also be properties of wolves. Certainly wolves are more similar to dogs than to cows. For that reason, the categories WOLFand DOGbelong to the same superordinate cate- gory CANINEwhileCOWdoes not. It then appears we should assign cows the value 0.0 and wolves some value reasonably greater than 0.0. Of course, this kind of adjustment of the scale can be repeated arbitrarily. Cows are more similar to dogs than crabs are, because both cows and dogs are mammals; crabs are more similar to dogs than potatoes, potatoes more similar than stones, and stones more similar than, say, French personal pronouns. The more kinds of cases we include in the assignment of similarity values, the greater the value of wolves becomes and the more similar (in terms of the scale) they become to dogs. It is just impossible to fix, for a given category, a general zero point of the scale and degrees of membership for potential and actual members – except for uncontroversial members. Apparently the degree of similarity and membership depends on the given context, namely on the range of rival categories.

The considerations about the nature of the prototype, its defining properties and the notion of similarity show that PT is not as unproblematic as it appears at first sight. The claim that categorization is a matter of similarity to the prototype raises a lot of non-trivial questions.