This corresponds to the philosophers' distinction between the intension of a concept (the meaning or idea or definition) and the extension (the set of things to which the concept refers). For example, the intension of bird is 'animal with wings and feathers' and the extension is the set of all birds. It also corresponds to two ways of defining a set in mathematics - by giving a predicate ( like X = { x | x is an even number less than 12 } ) or by enumeration ( X = { 2,4,6,8,10} )
2.6.2.1 The feature list approach
Going beyond a simple definition approach, one possibility is the feature list idea. For example, the 'robin' concept might have features such as bipedal, has wings, has feathers,
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flies, and has a red breast. Smith Shoben and Rips (1974) review how the feature list idea has been used to account for many aspects of language. In particular, Lakoff (1973) suggests that some features are characteristic, whilst some are defining. His evidence is based on hedges, which are qualifying phrases such as 'technically speaking' and 'loosely speaking'. For example, 'Technically speaking, a chicken is a bird', because it has a defining feature (feathers) but lacks a characteristic feature (flies). Again, 'Loosely speaking, a bat is a bird', because bats have the characteristic feature of wings, but lack the defining feature of feathers. Lakoff argues that 'Technically speaking, a robin is a bird' sounds odd, since robins have both defining and characteristic features, and so the hedge is not required.
2.6.2.2 Typicality
Wittgenstein (1958 page 66) showed that for some concepts (his famous example was 'game'), there were no defining features which applied to every instance. Instead there was the idea of 'family resemblance' - for each instance there was at least one other instance which had a lot in common with it (soccer and rugby), but there were many pairs of instances which had very little in common (water polo and chess).
In the absence of defining features, only characteristic features are left. Rosch and Mervis (1975) developed this idea to consider if it might be possible to rank features in terms of
how characteristic they were. They demonstrated this experimentally by showing close
agreement between subjects in judging how characteristic the features of instances of furniture were. Ripps Shoben and Smith (1973) also supported this notion. A development of this was the idea of prototype (Smith and Medin 1981). The idea of this was that if people were making judgements of typicality, there had to be a perfect 'ideal' with which they were making the comparison, and this was termed the prototype. Another variation is the notion of exemplar (Medin and Schaffer 1978). They proposed that concepts consisted of recalled instances of observed examples of the concept. Their evidence was derived from experiments with artificial categories of geometric forms of different size and colour.
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Does the exemplar approach relate to an educational context? It is a common practice when introducing a new idea to provide examples, but there is a distinction between the
idea of the concept (see section 2.6.2.4) and the examples provided. But this does raise
the point that if students are constructing their own idea of the concept on the basis of examples (along the lines of Fodor's hypothesis testing and Skemp's 'build and test') then the examples must be very carefully chosen, for otherwise students might use inappropriate aspects of the examples.
2.6.2.3 Category construction
Many experiments in this area are designed on the basis of subjects trying to choose between (usually artificial) categories constructed by the experimenter. However an interesting area of work involves looking at how subjects will construct categories of their own. For example, Medin, Wattenmaker and Hampson (1987) carried out a sequence of experiments where subjects were given a number of items and asked to partition them into 2 sets. The idea was to see if these would be chosen according to Rosch's family resemblance notion - but in fact the sorting was what they called uni-dimensional. Figure 2-1 illustrates this:
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D1 to D4 are 4 attributes, and a 1 means the example had that attribute, and 0 means it did
not. A possible family resemblance sort is shown, into one set containing the examples which had all or all but one attribute, and the other having none or just one attribute. A uni-dimensional sort is also shown, with the partition depending solely on whether the example possessed attribute D1 or not.
This was carried out in several ways, using cartoon-like drawings of animals and insects, and descriptions of people with differing personality traits. Almost all the subjects used a uni-dimensional sort, not family resemblance. This was still the case when the examples were set up to discourage this. For example with insects with short medium and long tails, they were split into long and short, and then the medium were split into longer than average and shorter than average.
However when there was some correlation between the attributes, subjects were more likely to use a sort based on that. For example, with the examples in figure 2.2,
attributes 2 and 4, and 3 and 5, are correlated. Subjects would sort on the basis of these, and especially if there was some 'theory' to explain the correlation. For example in one experiment the examples were patients and the attributes were disease symptoms.
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Attributes like 'dizzy' and 'ear ache' were correlated, and might both be explained by an ear infection - in which case subjects used these as the basis of sorting. Similar family resemblance sorts were obtained if subjects were given a hint as to the basis of the categorization, such as 'things that can fly' or 'introverted people'.
The uni-dimensional sort is like Aristotle's definitional idea of category. It has the advantages of simplicity and precision, and it is the approach people will take, unless they are given correlated attributes or suggested concepts to sort on.
But consider this in relation to the touch-stone question. If students observed instances of static and non-static in this way, they would arrive at a normative meaning of the term. But learning Java takes place in the real world, not in the psychology lab, and students bring with them the „met-before‟ meaning of static, that is, constant.
However the idea of a concept as something which is coherent and 'makes sense' leads on to the next section.
2.6.2.4 The theory theory
The approaches of simple definition, family resemblance, typicality, prototype and exemplar all assume that category members have, in some sense, something in common. Murphy and Medin (1985) challenged this view. They gave several arguments against the similarity view, and instead :