To early Greek philosophers, all matter was made of a single substance, even if they couldn't agree what it was. For astronomer and geometrician Thales, it was water; for Anaximenes, it was air (both lived in sixth century B.C.E.). Far from arbitrary, these choices stemmed from observations of changing states of matter. Anaximenes, for example, saw air grow visible and dense as it cooled into mist and then rain, and assumed it would condense further into earth and rock [source: Encyclopaedia Britannica; Encyclopaedia Britannica; Cohen].
Later, Plato, always the overachiever, tapped four elements for his world: earth, air, fire and water. Aristotle added a fifth, ether, to describe heavenly bodies. By mixing and matching these elements, they could explain, for example, why wood was solid (part earth), but also floated (part air) and burned (part fire) [sources: Armstrong; Plato].
The underlying idea -- that, as Democritus said around 440 B.C.E., all matter consists of imperceptibly tiny things -- approached the truth, but useful evidence of real atomic theory lay far in the future, in Robert Boyle's 1662 experiments with air pressure and vacuums. It would take another century-and-a-half before English chemist John Dalton would advance an accepted atomic theory in 1803 [source: Berryman].