The basic problem is to determine what the basic stuff is, of which the universe is made.
As Aristotle puts it, what is the arche, the first "principle" or, we would say, the first (material) cause?
The very diversity of things gives no suggestion of what this basic stuff or material might be, - but the idea of change does lead to the idea of permanence.
At least this is the fundamental presupposition -- one to be challenged by one or two later thinkers: a substance metaphysics -- underlying the development of any thing (e.g., child / adult / old person) is assumed to be a oneness, a unity -- something which it always is, despite the obvious changes or motion. In spite of change -- something is assumed to remain constant.
(Belief in change is possible only if a permanence is granted: otherwise, change would consist in merely a series of substitutions.)
The argument for one stuff emerges with Diogenes of Apollonia, a disciple of Anaximenes. Simply, unless all things derive from one stuff -- then things could have no subsequent connection or interaction with one another:
Neither could a plant grow out of the earth, nor any animal nor any thing else come into being unless things were composed in such a way as to be the same. But all these things arise from the same thing; they are differentiated and take different forms at different times, and return again to the same thing.
(Burnet, Early Greek Philosophy, pp. 353f.)
--> the character of the origin explains a subsequent character: in this case an original connection explains subsequent connection.
[When Thales spoke of water, he likely was not thinking of H20, but of liquids generally, and anything which could become liquid. Similarly, when Anaximenes speaks of air, he probably means not only a gas, but all things "gaseous" -- breath, wind, mind, and soul.]