In chemistry, the term element applies to any substance that can not be reduced by ordinary physical or chemical means to a simpler form. Thus, lead is an element because no method has ever been found by which the substance can be reduced into two or more parts.

The notion of elements first became popular among the natural philosophers of ancient Greece. These scholars found it difficult to imagine that nature was fundamentally as complex as it appears to be. They found it more satisfying intellectually to suppose that the wide variety of substances we encounter in nature are really composed of a small number of basic materials, that is, elements.

The Greeks devoted a great deal of thought to the question of what it is that is truly elemental in nature. Various philosophers proposed a variety of theories. In the fifth century, b.c., Empedocles...