This divine fire, or aether, is the basis for all activity in the universe, operating on otherwise passive matter, which neither increases nor diminishes itself. The primary substance in the universe comes from fire, passes through the stage of air, and then becomes water: the thicker portion becoming earth, and the thinner portion becoming air again, and then rarefying back into fire. Individual souls are part of the same fire as the world-soul of the universe. Following Heraclitus, Zeno adopted the view that the universe underwent regular cycles of formation and destruction.