Classifications and other ordering systems

The enormous variety of things in nature must be ordered before it can be studied and understood. Unfortunately in spite of their great importance, the methods of ordering have been greatly neglected by the philosophers. In this article, we distinguish six systems of ordering. Classification, in which similar entities are grouped in classes (taxa), is one such ordering system, but not all ordering systems are classifications. The Hennigian system of cladification consists of the ordering of branches of the phylogenetic tree, strictly on the basis of a single criterion, the branching points of the phylogeny (holophyly) (Hennigian phylogeny). It is not a system of classification, as it does not lead to classes of entities possessing similar phenotypic attributes. A Darwinian classification, by using two criteria, similarity and common descent, leads to the recognition of classes (taxa) of similar entities consistent with common descent (monophyly) (Haeckelian phylogeny).