Cognitive Psychology: Connecting Mind, Research and Everyday Experience, 3rd Edition

How Are Objects Placed Into Categories? • 241

How Are Objects Placed Into Categories?

A time-honored approach to determining the characteristics of an object is to look up

its defi nition. We begin by describing how cognitive psychologists have shown that

this “defi nitional approach” to sorting objects into categories doesn’t work. We then

consider another approach, which is based on determining how similar an object is to

other objects in a category.

WHY DEFINITIONS DON’T WORK FOR CATEGORIES

According to the defi nitional approach to categorization, we can decide whether some-

thing is a member of a category by determining whether a particular object meets the

defi nition of the category. Defi nitions work well for some things, such as geometric

objects. Thus, defi ning a square as “a plane fi gure having four equal sides” works.

However, for most natural objects (such as birds, trees, and plants) and many human-

made objects (like chairs), defi nitions do not work well at all.

The problem is that not all of the members of everyday categories have the same

features. So, although the dictionary defi nition of a chair as “a piece of furniture con-

sisting of a seat, legs, back, and often arms, designed to accommodate one person” may

sound reasonable, there are objects we call “chairs” that don’t meet that defi nition. For

example, although the objects in ● Figure 9.2a and b would be classifi ed as chairs by

this defi nition, the ones in Figures 9.2c and d would not. Most chairs may have legs and

a back, as specifi ed in the defi nition, but most people would still call the disc-shaped

furniture in Figure 9.2c a chair, and might go so far as saying that the rock formation

in Figure 9.2d is being used as a chair.

● FIGURE 9.1 Knowing that something is in a category provides a great deal of

information about it.

Sleeps a lot,

but more active

at night

Has whiskers

Difficult to train

Catches mice

Likes milk,

fish

Likes to

rub up against

people and other

objects

A feline: related

to lions and tigers

Has nine lives

Bruce Goldstein

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