Concepts and Categories

Psy315 - Class 31

(This page last updated 30 March 2009.)

 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.