Thales (c. 624–c. 545 B.C.E.), Anaximander (c. 610–545 B.C.E.), and Anaximenes (c. 580–500 B.C.E.), who were all from the city of Miletus, thought that the natural world was made up of one kind of material, such as water, the “unbounded,” or air. (The “unbounded” probably meant something like what we mean by something that is infinite.) Pythagoras thought that everything was made up of number. This did not mean that everything was based on mathematics, as we might think, but rather that numbers themselves were real things that existed in everything else that existed. Heraclitus (c. 540–480 B.C.E.) noted that the world and things in it are constantly in flux, and he claimed that change was more important than what the world was made up of. Parmenides (c. 515–450 B.C.E.), on the other hand, thought that change requires that things come into existence from non-being, and for that reason he believed that change was not possible or real. Heraclitus and other Milesians held that the real stuff or substance that makes up the world cannot change, so that to account for change there has to be a number of substances making up the world. Empedocles (c. 495–435 B.C.E.) built on this idea to posit the four elements: earth, wind, water, and fire. Anaxagoras (c. 500–428 B.C.E.) thought there were more than four basic elements—perhaps as many as an infinite number. Democratus (c. 460–371 B.C.E.) posited that everything is made up of atoms.