Classical elements typically refer to the pre-scientific concepts in Ancient Greece, of earth, water, air, fire, and aether, which were proposed to explain the nature and complexity of all matter in terms of simpler substances. Ancient cultures in Egypt, Babylonia, Japan, Tibet, and India had similar lists, sometimes referring in local languages to “air” as “wind” and the fifth element as “void”. The Chinese Wu Xing system lists Wood (木 mù), Fire (火 huǒ), Earth (土 tǔ), Metal (金 jīn), and Water (水 shuǐ), though these are described more as energies or transitions than as types of material.
These different cultures and even individual philosophers had widely varying explanations concerning their attributes and how they related to observable phenomena as well ascosmology. Sometimes these theories overlapped with mythology and were personified in deities. Some of these interpretations included atomism (the idea of very small, indivisible portions of matter) but other interpretations considered the elements to be divisible into infinitely small pieces without changing their nature.
While the classification of the material world by the ancient Indians and Greeks into Air, Earth, Fire and Water was more philosophical, during the Islamic Golden Age medieval middle eastern scientists used practical, experimental observation to classify materials. In Europe, the Ancient Greek system of Aristotle evolved slightly into the medieval system, which for the first time in Europe became subject to experimental verification in the 1600s, during the Scientific Revolution.
Centuries of empirical investigation have proven that all the ancient systems were incorrect explanations of the physical world. It is now known that atomic theory is a correct explanation, and that atoms can be classified into more than a hundred chemical elements such as oxygen, iron, and mercury. These elements form chemical compounds and mixtures, and under different temperatures and pressures, these substances can adopt different states of matter. The most commonly observed states of solid, liquid, gas, and plasma share many attributes with the classical elements of earth, water, air, and fire, respectively, but it is now known that these states are due to similar behavior of different types of atoms at similar energy levels, and not due to containing a certain type of atom or a certain type of infinitely divisible substance or energy.