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What Is the Psychology of Concepts a Psychology of ?
There is a methodological pitfall masquerading as an advantage that accounts for some part of the problem I have described. In order to study concepts, all I have to do is to make up two sets of entities (which can be anything, although I will usually call them objects because that is usually what they are) and persuade subjects to give one response to one set and a diVerent response to the other set. The problem with this is that I can make up any arbitrary sets, use any procedure to try to get subjects to learn, and use anyresponse that is at all distinctive. Hull (1920),whowasfaced with the task of developing a methodology for studying concepts experimentally, listed a number of desiderata for studying concepts. The Desiderata included the use of distinct classes, each receiving diVerent responses. However, they also included constraints on the concepts themselves, namely that each concept should contain an element that is unique to it. This desideratum reflected Hull’s assumption about the structure of categories in the world, what has come to be known as the classical view of categories (Smith & Medin, 1981).

Let us imagine for a moment that Hull had been right about categories, that each category has a unique element or some set of defining features that determine category membership. What would we then think about the vast majority of modern experiments on concepts, which lack such defining features? These experiments might be interesting as studies of abstract learning, but they would simply not be about how people learn concepts. A study of how people learn nonlinearly separable categories might have some interest regarding the nature of memory and learning in general, but it would tell us little about how people learn real categories because
nonlinearly separable categories by definition do not have definitions (sic). Studies of family resemblance concepts (Rosch & Mervis, 1975), in which category members tend to share features but have no feature common to the whole category, would also not be telling us how people learn real categories.These studies would be uninformative because the requirements involved in learning a well-defined category are diVerent from those involved in learning family resemblance or NLS categories. Indeed, the study of logically defined concepts that was ushered in by Bruner, Goodnow, and Austin (1956) was essentially dropped when Rosch published her studies of the structure of natural categories (e.g., Rosch, 1973, 1975). The studies of concept attainment that Bruner et al. and many others carried out are now viewed as studies of a particular kind of reasoning or problem solving rather than studies of concept learning, precisely because we believe that real concepts are not like Bruner et al.’s concept.
In order to answer how people learn categories and form concepts, we cannot operate in a vacuum of knowledge about the real structure of categories. For Hull, it would have been pointless to study how people learn family resemblance categories because this could not tell you how people learn ‘‘real’’ categories. We are less certain now, perhaps, what the real categories are and therefore are less willing to reject any particular experiment as being irrelevant. But perhaps we have erred on the side of liberality and acceptingness. Perhaps some of the categories we have studied do not tell us about how people learn real categories, just as the studies of Bruner et al. do not tell us how people learn family resemblance categories.