ABSTRACT
All of us make sense of experience by placing the objects and actions we observe into categories according to shared properties. Science does this systematically and explicitly. The members of a category have one or more characteristics in common. For example, consider the letter
x
in ten different font styles:
x
,
x
,
x
,
x
,
x
,
x
,
x
,
x
,
x
, x.
