There are two measures of a school’s worth:

• The objectivity criterion

An objective classification is one that represents a real, unambiguous property of nature; as opposed to a subjective classification, in which the classification represents some property arbitrarily chosen by the taxonomist. The objectivity test measures whether a classificatory system has some compelling justification, external to the method it uses, for classifying in the way it does.

• The naturalness of a classification

A natural classification is one in which the members of a group resemble one another not only in the characters that define the group (as they must, by definition) but also for many other non-defining characters too. This is opposed to an artificial classification in which the members of a group only resemble each other in the defining characters; they show no similarities for non-defining characters The advantage of natural classification is that it is possible to predict the distribution of other characters from the classificatory groupings alone.

Objective, and natural, classifications are preferable to subjective and artificial ones. If classification is objective and natural, then people working independently should be able to agree that it is the way to classify and predict the distribution of other characters from the classificatory groupings alone. The results should then be relatively stable and repeatable.