From the very beginning of the kind of disciplined thinking about the physical world that we now call science, one of the major objectives has been to identify its basic constituent, or constituents; to answer the question, What is the world made of? The earliest theories of which we have detailed knowledge, those developed by the Ionians in the years from about 600 to about 400 B.C., and by the Chinese around the same time, were of two general types. One group of philosophers, reasoning from an assumption as to the unity of nature, argued for a single constituent. Water was the usual choice, although there was some support for air. Another group contended that the great multiplicity of physical forms required the existence of a number of basic constituents. The most popular choice among the early investigators in the West was a four-element universe, constructed of earth, water, air, and fire, an identification that achieved a kind of an official status when it was accepted by Aristotle. The Chinese recognized five basic elements, omitting air and adding metal and wood.
