Conceptual Combination

The bulk of this chapter has been given over to examining the acquisition and organisation of single concepts. However, new concepts can also be created by combining existing concepts in novel ways. We develop concepts like pet fish, fake gun, and blue-striped shirt. These conceptual combinations or complex concepts should also be explained by concept theories (Osherson & Smith, 1981). Concept combinations come in a number of different forms: including adjective-noun combinations (e.g., red fruit, large bird), adverb-adjective-noun combinations (e.g., very red fruit, slightly large bird) and noun-verb combinations (e.g., birds eat insects). We will concentrate on the most commonly examined of these; namely, noun-noun and adjective-noun combinations (see Costello & Keane, 1992, 1997, 2000, in press; Hampton, 1983, 1987, 1988; Jones, 1982; Osherson & Smith, 1981, 1982; Smith, 1988; Smith & Osherson, 1984; Smith, Osherson, Rips, & Keane, 1988; Wisniewski, 1996, 1997; Zadeh, 1982).

Defining-attribute theories predict that a combined concept should contain a set of entities that are a conjunction of the members that belong to the two constitutent concepts. So, "red apple" should refer to objects that are in both the categories red-things and apples. However, this is nowhere near a complete account. As Lakoff (1982) indicates, a fake gun is not a member of the category gun. Osherson and Smith (1981, 1982) pointed out several serious problems for any prototype explanation of conceptual combination. They proved formally that the typicality of the member of the conjunction of two concepts could not be a simple function of the two constitutent typicalities. Intuitively, a guppy fish is a good example of a pet fish, but a guppy is not typical of the category of pets (who are generally warm and furry) nor is it typical of the category of fish (who are generally larger; see Hampton, 1988).

Several models have been proposed to account for conceptual combination and to predict the typicality of members of the combined concept (Cohen & Murphy, 1984; Hampton, 1983; Murphy & Medin, 1985; Smith & Osherson, 1984; Thagard, 1984). Hampton's (1983) model talks of the formation of a composite prototype, by combining various attributes of the constitutent concepts in an interactive fashion. Hampton (1987, 1988) produced evidence in favour of this model which shows that the similarity of an object to the composite prototype of the combined concepts determines the typicality and class membership of that object. Murphy and Medin (1985) maintain that conceptual combination is another case where background conceptual knowledge or theories about the concepts in question play a role. They point out that ocean drives are not both oceans and drives, and horse races are not both horses and races. It is clear from these examples that there are some combinations—intersective combinations—that do conform to prototype accounts (e.g., orphan girl), but that many combinations are not intersective in any sense (e.g., ocean drive).

More recently, Costello and Keane (1992, 1997, in press) have proposed a theory of conceptual combination for noun-noun compounds that combines aspects of instance-based and explanation-based approaches to categorisation. Costello and Keane maintain that combinations are interpreted by forming subsets of the attributes and relations in both concepts according to the constraints of informativeness, diagnosticity, and plausibility. When people interpret novel compounds, like "cactus fish", to produce the meaning "a fish with spikes on its skin" they use diagnostic attributes of the cactus (e.g., spiky) rather than non-diagnostic ones (e.g., green), they apply these attributes in a plausible way (e.g., they do not say that it is a fish with spikes on its eyes), and the meaning produced is always informative (e.g., people never say that a cactus fish is a fish that is alive; although alive is an attribute of cacti it conveys no new information about fish). Costello and Keane have produced a computational model of the combination process that uses parallel constraint satisfaction (see also Estes & Glucksberg, in press; Markman & Wisniewski, 1997; Wisniewski, 1996, 1997; Wisniewski & Love, 1998).

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