Rules of Classification
Consistent Principle

In classifying, we should try to follow a consistent principle.


If we classify college courses by subject matter (such as ART, BIOLOGY, HISTORY, etc.), we should stick to that principle throughout, and not include species like INTRODUCTORY that involve a different principle.

There are various ways to classify furniture: by function (tables, chairs, etc.), by style of design (Danish, colonial, Louis XIV, etc.), by material (wood, plastic, chrome, etc.). However, whichever principle we choose, we should follow it consistently.

Every Classification Should Have Species That Are Mutually Exclusive

In a classification, the species must be mutually exclusive: each species must exclude all the members of every other species. The species must not overlap.


Suppose you tried to classify your courses into the following categories: art, biology, history, economics, and introductory. Where would you put Introduction to Art? Because your categories overlap, we don't know whether to classify this as an introductory course or an art course.

Every Classification Should Have Species That Are Jointly Exhaustive

A classification must be jointly exhaustive: the species taken together (jointly) must cover (exhaust) all the objects in the genus.

A good classification divides up the genus completely, allowing us to assign every member of the genus to one or another of the species.


If you also are taking a philosophy course, for example, then the classification of courses prevously given (ART, BIOLOGY, HISTORY) is not jointly exhaustive. It leaves out something that should be included. Another way to state this is that a classification should be complete.

Consistent Principle | Essential Attributes

Rules of Classification

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