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
Every Classification Should Have Species That Are Mutually
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
A classification must be jointly exhaustive: the species taken
together (jointly) must cover (exhaust) all the objects in the
A good classification divides up the genus completely, allowing
us to assign every member of the genus to one or another of the
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