Thales thought that water was the primordial substance of all things. Heraclitus of Ephesus... thought that it was fire. Democritus and his follower Epicurus thought that it was the atoms, termed by our writers "bodies that cannot be cut up" or, by some "indivisibles." The school of the Pythagoreans added air and the earthy to the water and fire. Hence, although Democritus did not in a strict sense name them, but spoke only of indivisible bodies, yet he seems to have meant these same elements, because when taken by themselves they cannot be harmed, nor are they susceptible of dissolution, nor can they be cut up into parts, but throughout time eternal they forever retain an infinite solidity.
Chapter II "On the Primordial Substance according to the Physicists" Sec. 1 - De architectura (The Ten Books On Architecture) (~ 15BC) - Book II