Chapter Summary

The organisation of knowledge is one of the oldest and most researched areas in cognitive psychology. As such, it should act as a barometer of the state of the discipline. Has progress been made? Well, it is clear that research is not standing still. Researchers have used everything in the cognitive science cupboard (from empirical tests, to formal tests and computational models) to challenge each others' and often their own theories (e.g., Medin appears as the proposer of several different theories). It is clear that certain theoretical views are modified by the evidence found or in some cases wholly defeated. For instance, straight defining-attribute views have had their day, even though they have been the dominant view of conceptualisation for most of the intellectual history of western Europe.

In summary, we have examined the following main points throughout this chapter:

• People need to organise their knowledge into categories and concepts to deal efficiently with the world; conceptual systems need to have cognitive economy, need to be informative, and need to cohere naturally.

• The critical evidence on categorisation comes from category membership judgements showing typicality effects, the different levels of conceptual hierarchies, the fuzziness of category boundaries; prediction tasks showing how people make inductive inferences from categories; and concept instability effects.

• The defining-attribute view characterises concepts in terms of necessary and jointly sufficient attributes. This view asserts that (i) such attributes can be found, (ii) membership of a category is not a matter of degree but is an all-or-nothing affair, (iii) there are clear boundaries between conceptual categories, (iv) a subordinate concept should contain all the attributes of its superordinate concept. However, all of these assumptions are either highly questionable or have not been supported by the evidence found.

• The prototype view characterises concepts as being organised around prototypes, expressed as a central tendency in the attributes of members of the category. This view can account for gradients of typicality, for fuzzy boundaries, and for levels of abstraction in both natural and artificial categories. There are some queries about the generality of the prototype view, as some abstract concepts do not exhibit prototype structure and it does not account for the use of variability and correlational aspects of concept definitions.

• The exemplar-based view characterises categories as collections of instances and explains the main finding in the literature by the way different example instances come to mind in different task contexts. It has been quite successful in its predictions about human conceptual behaviour.

• Theories of concepts have also been tested by being applied to other phenomena such as conceptual combination; that is, to explain what happens to the typicalities of combined concepts (e.g., red apple, ocean drive).

• Similarity is central to all theories of categorisation. Tversky's contrast model, and variants of it, have been extensively used to account for the judgement of conceptual similarity. Recently, this theory as been modified by research showing that when conceptual representations are characterised as having relations, then an account based on structural alignment or analogy is more appropriate.

• Cognitive neuropsychological accounts of category-specific impairments and other deficits in semantic memory have continued to help constrain the various theories of categorisation.

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