CLASSIFICATION is the process of recognizing classes or kinds, each class or kind consisting of members having certain characteristics in common. The members may themselves be classes or they may be individuals. In a complete system of classi fication the lowest classes (in which only individual members can be distinguished) are subordinated to higher ones, and these again to others still higher until the most inclusive category with which the science in question is concerned is reached. In Logic, such an inclusive category or highest class is called a suinmum genus, and the lowest, in Pima species; each intermediate class is called a genus of the one below it, and a species of the one above. In the biological sciences the terms genus and species are not used in this relative sense, but denote certain definite classes in the biological scheme.
Classification is one method, probably the simplest method, of discovering order in the world. By noting similarities between numerous distinct individuals, and thinking of these individuals as forming one class or kind, the many are in a sense reduced to one, and to that extent simplicity and order are introduced into the bewildering multiplicity of Nature. In the history of every science classification is the very first method to be employed; but it is much older than science. Every name, indeed almost every word, of a language is the expression of some implicit classifi cation; and language is older than science. The classifications expressed in ordinary language are, however, the result of prac tical needs rather than of scientific interests, so that science has to correct them even when it starts from them. The scientific classification of such things as coal, or whales, or sea-anemones, or of processes like rusting and breathing, is very different from the popular way of classing them.
Scientific classification seeks to formulate a scheme of mutually exclusive and collectively exhaustive categories based on the most important characteristics of the things concerned and on the actual relations between them. This is frequently a very difficult matter, as the history of biology shows. The more nearly a classification approaches the aforementioned ideal the better is its claim to be called a natural one; a classification that deviates from this ideal, as usually happens when it is made for some practical human purpose, is called artificial (e.g., a herbalist's classification of herbs with reference to their medicinal uses). Border cases are especially apt to cause trouble. To this day there is uncertainty whether certain micro-organisms should be classed as plants or as animals. Volvocinae and the Euglena viridis, for example, have some of the characteristics of vege tables and also some of those of animals, so they are simply listed as "organisms" for the time being.
In one sense classification is the basic method of science. For science is mainly interested in general truths, that is, in truths relating to classes of things or events rather than to individual instances. Whatever other methods it may employ in discovering a certain truth, that truth is implicitly applied at once to the whole class of phenomena to which the instance in connection with which the discovery has been made belongs. This obviously as sumes a correct classification of the phenomena in question.
Some classifications are based on evidence of common descent or biological kinship. Such are known as phylogenetic classifica tions.
BIBLIOGRAPHY.--J. S. Mill, System of Logic (1875 etc.) ; J. Venn, Bibliography.--J. S. Mill, System of Logic (1875 etc.) ; J. Venn, Empirical Logic (1907) ; A. Wolf, Essentials of Scientific Method (1928) . (A. Wo.)