Concepts and Categories

Psy315h - Class 21

(This page last updated 1 November, 2005.)

 We will explore four ways of representing concepts. This page will describe one method, and the remaining three will be described on the next two pages.

1. Defining Features

• Example: Triangle
  • Closed figure
  • Made of straight line segments
  • 3 sides and 3 vertices
• Each feature is necessary to be defined as a triangle.
• Together these features are sufficient to define the category.

Conditions for Category Membership

• A Necessary Condition: Must have this property to be a member, but this property by itself may not be enough to make it a category member.
• A Sufficient Condition: Anything that has this property will be a member, but there may be other members that do not have this property.

Difficulties in Representing Categories by Defining Features

• Only works for certain well defined concepts, such as "triangle".
• For most common categories, it is difficult to find defining features that work for all members.
• Some members seem to fit better than others (graded membership).
• It may be difficult to draw a boundary between members and nonmembers (fuzzy boundary)
• Categories often exhibit a "family resemblance" structure.
  • Many features shared across members.
  • No feature shared by all members.

next class: Prototype and Exemplar Theories of Concepts
  
Psych 315H: Cognitive Psychology Kyle Cave Psychology Dept. U. Mass.