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Thinking critically requires the ability to categorize. This ability is basic (foundational) because proper language use depends on it. Your ability to communicate, both to understand what others are saying and to express what you are thinking, depends on categorization. Categorization is needed for clear and useful definitions, which are also basic to critical thinking. Finally, categorization is the basis for an entire branch of logic which we will study here as the Square of Opposition, Immediate Inference and the Categorical Syllogism.

Some Definitions:

Ideas that represent categories or types of things
The individual things referred to by concepts.
A broad concept which includes narrower concepts that pick out subgroups from all the referents in the genus.
A narrower concept included under some genus. A species picks out one type of the larger class. Genus and Species are relative terms. For example DOG is a species of MAMMAL, but DOG is a genus for POODLE.

 A Basic Principle:

In a good categorization every item to be categorized will fit under some category and no item will fit under more than one category. (Nothing will be left out and there will be no overlap.)