Atomism is a doctrine that has a long history in both philosophy and science. For this reason it is not easy to define its content in such a way as to comprehend all the historical variations and especially the historical development of the doctrine. In a very general sense, however, atomism may be defined as the doctrine that material reality is composed of simple and unchangeable minute particles, called atoms. It holds that all observable changes must be reduced to changes in the configuration of these particles. The multiplicity of visible forms in nature must likewise be based upon differences of configuration. The best way to discuss the variations of this general idea of atomism is to follow the historical development, which shows a gradual shift of emphasis from philosophical to scientific considerations. Consequently, the first part of this article, covering the period from the sixth century BCE...