19 Categories Defined by Similarity

So far we’ve studied three types of category structures:

  • A LIST category is just a list of its members.   There is no property test that decides if something goes in the category.
  • SINGLE-PROPERTY categories define their members using a property test.  We have to be careful in the choice of the property;  if the members differ a lot on other properties, the category won’t make much sense.
  • MULTIPLE-PROPERTY categories define their members using two or more property tests.  The order of the tests is important because that  determines the hierarchy or taxonomy in which broader categories contain the members of  their sub-categories.

In this lesson we’ll study another type of category structure — defining categories based on SIMILARITY.  These categories define membership by considering multiple properties, but the properties are used in a looser way.  Sometimes things go in the same category because they have many but not all of the same properties — they are similar but not exactly the same.

This idea is easier to explain with an example.


We all know the “bird” category, and we can reliably distinguish birds from dogs using property tests.   But being precise about whether an animal is a bird is a little tricky.   The “bird” category cannot be defined using “yes or no” property tests the way that the shapes and shirts were in the previous lessons.


  • All have feathers, wings, beaks, and two feet
  • But some birds fly or swim and some don’t, so flying and swimming can’t be used as property tests
  • The shape of bird beaks, wings, and feet varies a great deal, so shape can’t be used as a property test


So birds resemble each other, but there is a lot variation within the category. This means that membership in the bird category is based on SIMILARITY or TYPICALITY.
      • The properties that most birds share make them resemble each other, but some properties are shared more than others
      • Birds that share the most characteristics properties are “better” category members
      •  This makes some birds more typical of the bird category than others


TOP ROW: Penguin, Pigeon, Swan
BOTTOM ROW: Frigate Bird, Flamingo, Pelican

Many of the categories we use a lot are based on similarity rather than on property tests; try to define “friend” or “game” using property tests and you will be stumped!   So when you have to organize a collection of things, use property tests to create a hierarchy if you can.


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"The Discipline of Organizing" for Kids Copyright © 2022 by Robert J Glushko is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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