The Aitareya Upanishad is one of the shortest of the thirteen principle Upanishads. It is a concise mythological account of the creation of the world. "In the beginning, Atman (Self, Soul), verily, one only, was here -- no other winking thing whatever. He bethouhght himself: 'Let me now create worlds,'" and out of Atman, the elements of the world separate. Thus it begins as a theistic treatise.
From the creation of the worlds, Atman creates world-guardians and to nourish them, food; however, the food he offers them is not sufficient until he offers them a person. At least this is how section 1.2 describes it. Section 1.3 describes the food of the guardians as fleeing from him, until "this one living food," the wind, is consumed. The centrality of food is a bit puzzling; however, understood as that which is consumed and which is necessary for a thing's existence, makes it critical to understanding creation and being while indicating the ephemeral, constantly changing nature of creation.
Having created the worlds and all that is in them, Atman enters the body of the world as the self; hence, the distinction between the creator and the creation is blurred. The final section expands the return of the Atman into its creation to all things "born from an egg,..."born from a womb,...born from sweat, and...born from a sprout."
While the Aitareya Upanishad is almost exclusively cosmological in subject, it does makes brief mention of righteous actions as a final "birth" of the self, suggesting a naturalistic basis for morality and indicating the expansiveness of the what is born of and returns to the Atman.