Prototype theory

In cognitive psychology, prototype theory describes how people categorize things. A "prototype" is considered a central member (or the central member) of a category.

Contrast with the classical theory
The classical theory of concepts views concept formation as definitional. That is, concept x is composed of simpler concepts that describe necessary and sufficient conditions for being a member of x. For example, a triangle may be defined as a "three-sided polygon" where a shape is a triangle if and only if it satisfies the condition of being both a polygon and being three-sided. However, the classical theory came to be seen as increasingly insufficient during the 20th century. Protracted disagreements over definitions led to dissatisfaction with classical theory in philosophical circles, and the theory also failed to explain new empirical results in psychology.

Prototype theory was developed as an alternative to the classical theory. Categories and concepts, according to this theory, are probabilistic rather than definitional in nature. They are radially structured, with some members more central than others. The items in a category generally have graded degrees of membership. For example, sparrow is more central to the category bird than are kiwi or emu, even though the last two examples are just as much members of the Linnaean class Aves as the first one. When asked to name an example member of a given category, respondents most often mention prototypical members of that category. The boundaries and central members of a category may change over time as a person takes in more exemplars.

Early research
Taking inspiration from Ludwig Wittgenstein's concept of family resemblance, Eleanor Rosch developed prototype theory during the 1970s. Rosch first tested her ideas among the Dani people of New Guinea. The Dani have only two concepts of color in their language. Rosch selected focal colors as "natural prototypes" to teach to the Dani participants in her experiments. She found that the participants learned new color concepts much faster when these natural prototypes were used.

Basic level categories
Categories exist at various levels, described as basic, subordinate, or superordinate. The basic level is not the "lowest" level in a hierarchy, but may be considered the most convenient mid-level entry point to a set of related categories. The basic level is neither too specific nor too general. Visualizing any given basic level category is easily possible, using a generalized prototypical member. Subordinate categories are more specific than their basic category, and superordinate categories more general. A superordinate category may be difficult to visualize in terms of a single example. For example, chair is a basic category, with subordinate categories including rocking chair, recliner, Windsor chair, etc. Chair belongs to the superordinate category furniture. When asked what they are sitting on, most people will respond using the basic category, "a chair," rather than a specific type of chair. Very seldom will anyone respond, "I am sitting on furniture."

Criticism
Hofstadter & Sander (2013) don't dispute graded categorization, but point out that on any level the categorization is always fuzzy and not at all as nicely structured as proposed by the Prototype Theory. No level of scrutiny seems to bring someone closer to understanding, since all the parts are themselves concepts with fuzzy outlines:

A source of problems, however, is the fact that the words used to express the membership criteria are not any more precise than the concept that one is trying to pin down — in this case, bird. What, for instance, is a foot? And what does “to possess” mean? What does “covered with” mean? And of course, everyone knows that there are all sorts of birds that don’t have two feet (perhaps because of an injury or a genetic defect) or that are not covered with feathers (ducklings and chicks, for example). And turning things around, we human beings have two feet, but if we hold a spray of feathers in our hand, this "possession" does not suffice to turn us into birds. And the famous plume de ma tante — my ancient aunt’s quill pen, which she loved to use to make beautiful calligraphy — would that count as a feather? And if so, would possession thereof make my bipedal old aunt a bird? — Douglas Hofstadter, 2013