ABSTRACT
CLASSIFICATION is one of the classic methods of science and is fundamental in all fields of study. This is equally true in biology as in physics. Systems of classification are always at first simple, governed by common-sense appearances, and far removed from the complexities of later developments. Thus Thales, the first of the Greek philosophers to think about the constitution of the world and its elements, held the theory that everything was originally water, from which earth, air and living things were later separated out. Later on Anaximander and Anaximenes modified this hypothesis to include earth, air and fire as well as water as the main elements. These of course were mere pre-scientific guesses of little value in the actual development of chemistry and physics, but at least they served to pose a problem.
