In the previous chapter, we saw how knowledge can be represented and organised. In this chapter, we examine the more circumscribed area of object concepts and categories. Object concepts have been, by far, the most researched topic in the study of concepts. Beyond this chapter, much of the remainder of the book is about how these object concepts and other schematic knowledge is used in the important activities of reading, problem solving, reasoning, and decision making. Before we plunge into this chapter, we should pause to consider why we need knowledge and why that knowledge needs to be organised.

Constraints on concepts: Economy, informativeness, and naturalness

Why do we need knowledge? We need to know about things to behave and act in the world. In its most general sense our knowledge is all the information that we have inherited genetically or learned through experience. Without this knowledge we simply cannot do certain things. If you have not acquired the knowledge for bicycle riding, by spending hours falling off bicycles and grazing your knees, then you cannot carry out this behaviour. If you have not studied the recipe for hollandaise sauce, then the likelihood is you will not produce a decent meal using this sauce. In short, knowledge informs and underlies all of our daily activities and behaviour.

Why do we need to organise knowledge? It is not enough just to acquire experience and store it; we need to organise this knowledge in an economic and informative fashion. The South American writer Jorge-Luis Borges (1964, pp. 93-94) describes a fictional character who had a perfect memory of every second of his life, a man called Funes, who had no need to organise or categorise his experience:

.. .Funes remembered not only every leaf of every tree of every wood, but also every one of the times he had perceived or imagined it.He was, let us not forget almost incapable of ideas of a general, Platonic sort. Not only was it difficult for him to comprehend that the generic symbol dog embraces so many unlike individuals of diverse size and form; it bothered him that the dog at three fourteen (seen from the side) should have the same name as the dog at three fifteen (seen from the front). His own face in the mirror, his own hands, surprised him every time he saw them.

No human being is like Funes, because we have to organise our knowledge. We identify categories of things, like dogs, in part to avoid having to remember every individual dog we have seen (or indeed every different angle from which we have seen a specific dog). Our memory systems clearly require a certain economy in the organisation of our experience. If we were like Funes, our minds would be cluttered with many irrelevant details. So, we seem to abstract away from our experience to develop general concepts (indeed, Borges suggests that Funes could not think and reason because he lacked abstract categories). Cognitive economy is achieved by dividing the world into classes of things to decrease the amount of information we must learn, perceive, remember, and recognise (Collins & Quillian, 1969). Once concepts have been formed they can, in turn, be organised into hierarchies; where animal is a superordinate concept (i.e., more general or encompassing) of dog and where living thing is a superordinate of animal and plant. However, this sort of cognitive economy has to be balanced by informativeness.

If our minds went too far in applying the economy constraint then we would end up with too many general concepts and lose many important details. If we generalised all of our object concepts to be just three (animals, plants, and everything else) then we would have a very economic conceptual system, but we would not have a very informative system; for instance, we would not have abstractions to distinguish between, say, chairs and tables.

Finally, there is a sense in which some concepts are more "natural" than others. A category that included pints-of-Guinness and birds-that-flew-on-one-wing, does not seem likely or natural Human concepts cohere in certain ways making certain groupings of entities more likely to occur than other groupings. One problem is to specify the basis for this naturalness or cohesiveness.

In short, for reasons of storage and effective use it seems to be necessary to organise and categorise experience. In human memory, this organisation appears to be guided by the principles of cognitive economy, informativeness, and natural coherence. One of the marvels of human memory is that it balances these principles in the acquisition of conceptual knowledge that allows us to get around and understand our world. A marvel whose extent is most horribly revealed when it becomes damaged in brain injury or by a disease like Alzheimer's.

Outline of chapter

In the next section, we consider some of the main findings in the object concept literature before outlining how the various theoretical perspectives deal with these findings. In the course of reviewing each theory we outline some of the supportive evidence that has been garnered for that specific theoretical approach. Then, in the latter sections of the chapter we consider wider evidential shores against which the various theories should also be tested, by reviewing the literatures on conceptual combination, concept formation, and the cognitive neuropsychological evidence on categorisation.

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