There are phenetic, cladistic, and evolutionary schools of classification. These schools will each be discussed in turn. Notice how each one utilizes different principles and offers different advantages. A method is needed to judge the merit of a school of classification. Natural is preferable to artificial .
Theoretical justification is also meritorious.
So is practicality. In other words, the groups exist in nature. Any classification is unlikely to be either objective or natural if it depends on one or a small set of arbitrarily chosen characteristics. Phenetic classification uses distance measures and cluster statistics. Phenetic classification does not depend on any knowledge or hypothesis of phylogeny.