Engraving by Jacques de Gheyn from a series entitled: The Four Elements, Agua (plate 2), Flanders, 1588. Many philosophies and world views have a set of classical elements believed to reflect the simplest essential parts and principles of which anything consists or upon which the constitution and fundamental powers of anything are based. Most frequently, classical elements refer to ancient beliefs inspired by natural observation of the phases of matter; with the classical elements: earth is equivalent to solid, water is equivalent to liquid, air is equivalent to gas and fire is equivalent to plasma.