Have you seen a tree? I bet that strictly speaking you haven't，for you have seen a willow，a peach tree，or any particular tree，but you haven't seen an abstract tree. The same is true with the word dog. Have you seen a dog? You haven't，not in the abstract sense of the word. You have seen a white dog，a yellow dog，your own dog or your neighbor's dog，but you haven't seen an abstract dog. A dog (or a tree for that matter) stands for ail the characteristics of the species it refers to. The special term for this phenomenon is called category. The mental process of classification is called categorization，which is one of the important capabilities of the human mind.
Categorization occurs everywhere around us，without which a lot of information would be in disorder. In fact，people are able to divide the world into categories，and they can learn to distinguish between different categories such as CAR and BUS. For example，as a student，you may be classified as a FRESHMAN，SOPHOMORE，JUNIOR or SENIOR；you may also be classified by your major. As to the color system，we have color categories，such as RED，WHITE，BLACK，YELLOW and BLUE. Movies in America have already been categorized as G (General Audiences：used to mean that a film is suitable for anyone to watch)，PG (Parental Guidance：used to describe a film containing scenes，subjects，or language considered unsuitable for children unless they have a parent with them)，R (Restricted：used to mean that the people under the age of 17 cannot go to a particular film unless they go with an adult)，or X (used to mean that a film is not suitable for people under the age of 18). The list of categorization examples is practically endless.
But what principles do people use when they do categorizing. Generally speaking，there are two major theories which deal with this problem. One is the classical theory，and the other is the prototype theory.
2.1 The classical theory
The classical theory of categorization can be traced back to Aristotle，and it is carried forward by structuralist and transformationalist linguists.
According to this theory，in the BIRD category，for example，if a creature has two wings，two legs，a beak，feathers and lays eggs (these are the necessary conditions)，then it is a bird；on the other hand，if a creature has all these features，this is also sufficient for classifying it as a bird. This seems to tell us that categories are defined by a limited set of necessary and sufficient conditions (these conditions are regarded as features). In other words，a thing cannot both be and not be，it cannot both have a feature and not have it， it cannot both belong to a category and not belong to it. This is the first assumption of the classical theory.
In the definition of a category，one of the two values，either [+] or [-]，can be used. For example，the BIRD category has the feature [+two legs]，but [-four legs]. This means that a feature is either in the definition of a category，or it is not；an entity either has this feature，or it does not. That is，features are binary. This is the second assumption.
Once a category has been established，it divides the universe into two sets of entities. In this case，some entities are the members of the category，while the others are not. There are no borderline cases. For instance，after the BIRD category is formed，some creatures such as the sparrow，the robin，and the swallow are the members of BIRD，but others such as lions，tigers，and bears are not，because the latter are the members of the BEAST category. The view can be demonstrated in the following figure：
This shows that the two categories BIRD and BEAST have clear boundaries.In general，categories have clear boundaries. This is the third assumption.The fourth assumption is that all members of a category have equal status.
This means that a member that has all the features of a category is a full member of the category；a member that does not have all the features is not a member. There is no such case in which one member is better than others in the same category. Therefore，according to this view，we cannot say that the sparrow is a better member than the ostrich in the BIRD category.
The classical theory has dominated for a long time，but it has also suffered from criticisms. Such a view of categorization is bound to run into difficulties when it is used to describe categories which have good and bad members and fuzzy boundaries. Things in the world are much too complex for a theory as neat as the classical theory. Certain things do not fall into clear-cut categories. For instance，do ostriches and penguins belong to the BIRD category? Even if they do，aren't they less of birds than robins are? Then what about ducks and peacocks? Are they more of birds than penguins? (See Figure 1 in the next section.)
