Now Thales had made the ultimate principle of the universe, water. Anaximander agrees with Thales that the ultimate principle of things is material, but he does not name it water, does not in fact believe that it is any particular kind of matter. It is rather a formless, indefinite, and absolutely featureless matter in general. {25} Matter, as we know it, is always some particular kind of matter. It must be iron, brass, water, air, or other such. The difference between the different kinds of matter is qualitative, that is to say, we know that air is air because it has the qualities of air and differs from iron because iron has the qualities of iron, and so on. The primeval matter of Anaximander is just matter not yet sundered into the different kinds of matter. It is therefore formless and characterless. And as it is thus indeterminate in quality, so it is illimitable in quantity. Anaximander believed that this matter stretches out to infinity through space. The reason he gave for this opinion was, that if there were a limited amount of matter it would long ago have been used up in the creation and destruction of the "innumerable worlds." Hence he called it "the boundless." In regard to these "innumerable worlds," the traditional opinion about Anaximander was that he believed these worlds to succeed each other in time, and that first a world was created, developed, and was destroyed, then another world arose, was developed and destroyed, and that this periodic revolution of worlds went on for ever. Professor Burnet, however, is of opinion that the "innumerable worlds" of Anaximander were not necessarily successive but rather simultaneously existing worlds. According to this view there may be any number of worlds existing at the same time. But, even so, it is still true that these worlds were not everlasting, but began, developed and decayed, giving place in due time to other worlds.