Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
The traditional view that categories are defined by common properties of their members was quickly replaced by Eleanor Rosch's theory of prototypes. After all, the best way to teach a concept is to show an example of it.
The US psychologist Eleanor Rosch noted that some members of a category seem to be better examples of the category than others. Not all members are alike, even if they all share the same features of the category. This means that the features by themselves are not enough to determine the category. It also implies that there must exist a “best” example of the category, what she called the “prototype” of the category.
In the 1970s she founded her early theory on two basic principles of categorization: 1. The task of category systems is to provide maximum information with the least cognitive effort; and 2. The perceived world comes as structured information. In other words, we do categorization because it helps save a lot of space in our memory and because the world lends itself to categorization. Concepts promote a cognitive economy by partitioning the world into classes, and therefore allowing the mind to substantially reduce the amount of information to be remembered and processed.
In Rosch's theory of prototypes, a concept is represented through a prototype that expresses its most significant properties. Membership of an individual in a category is then determined by the perceived distance of resemblance of the individual to the prototype of the category.
Next, Rosch proposed that thought in general must be organized around a privileged level of categorization. In the 1950s the US psychologist Roger Brown (“Words and Things”, 1958) had noted that children tend to learn concepts at a level which is not the most general and not the most specific: say, “chair”, rather than “furniture” or “armchair”. And in the 1960s the US anthropologist Brent Berlin through his studies on colors and on plant-naming and animal-naming (“Covert Categories and Folk Taxonomies”, 1968) had reached a similar conclusion that applies to categories used by adults. The point was that we can name objects in many different ways: a cat is also a feline, a mammal, an animal, and it is also a specific variety of cat. However, we normally call it “a cat”. The level at which we "naturally" name objects is the level of what Brown termed “distinctive action”. The actions we perform on flowers are pretty much all the same, and certainly different from the actions that we perform on a cat (e.g., one we smell and one we pat). But the actions we perform on two different varieties of cats or two different types of flowers are the same (we pat both the same way, we smell both the same way). Our basic actions tell us that a cat is a cat and a flower is a flower, but they cannot tell us that a rose is not a lily. “Cat” and “flower” represent a “natural” level of categorization.
Berlin had found that people categorize plants at the same "basic level" anywhere in the world (which roughly corresponds to the genus in biology). It is a level at which only shape, substance and pattern of change are involved, while no technical details are required.
Rosch extended his ideas to artifacts and she found that we also classify artifacts at a "basic level" where technical details are not essential. We first create categories of "chair" and "car", and only later we specialize and generalize those categories (to "armchair", "furniture", "sport car", etc). At the basic level we can form a mental image of the category. We can form a mental image of "chair", but not of "furniture". We can form a mental image of "car", but not of "vehicle". We have a motor program for interacting with "chair", but not with "furniture". We have a motor program for interacting with "car", but not with "vehicle". Categorization initially occurs based on our interaction with the object. Meaning is in the interaction between the body and the world.
Rosch postulated a level of abstraction at which the most basic category cuts are made (i.e., where “cue validity” is maximized), which she called the “basic” level. Categories are not merely organized in a hierarchy, from more specific to more general. There is one level of the hierarchy that is somewhat privileged when it comes to perception of form, movement of body parts, organization of knowledge, etc. "Chair" and "car" are examples of basic categories. We can form a mental picture of them. We have a motor program for dealing with them. They are the first ones learned by children. The category of "furniture", for example, is different: I cannot visualize it, I do not have a motor program to deal with it, and it takes some time for a child to learn it.
Generalization tends to proceed upwards from this level, and specialization proceeds downward from this level. Superordinate categories are more abstract and more comprehensive. Subordinate categories are less abstract and less comprehensive. The most fundamental perception and description of the world occurs at the level of basic (or natural) categories.
Rosch also realized that categories occur in systems, not alone, and they depend on the existence of contrasting categories within the same system. Each contrasting category limits a category (e.g., if a category for birds did not exist, the category for mammals would probably be bigger). At the basic level, categories are maximally distinct, i.e. they maximize perceived similarity among category members and minimize perceived similarities across contrasting categories. Technically, one can use the notion of “cue validity”: the conditional probability that an object falls in a particular category given a specific feature. Category cue validity is the sum of all the individual cue-validities of the features associated with a category. The highest cue validity occurs at the basic level. The lowest cue-validities are those for super-ordinate categories.
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