The Linnaean Hierarchy and ‘Extensional Thinking’

Abstract

The Linnaean Hierarchy has been opposed from a number of different perspectives, but most of this opposition was based upon the confusion of two aspects of every taxon—extension and intension. These aspects account for its dual nature of simultaneous change and constancy. "Constancy" is apparent only in the differences that exist among groups of organisms; these differences appear in the very first origin of taxa and persist throughout their entire evolutionary history. The sequence of differences over time is the only basis upon which to judge the history of the organic world. The Linnaean system addresses this sequence by attempting to distribute these differences among hierarchical levels. Taxonomic hierarchical levels, or ranks, do not imply a rigid adherence to certain kinds of characters, since characters can change their rank during group evolution. As such, taxonomic levels can serve only as a tool to coordinate chaotically segregated groups of organisms.