Brahma Knowledge, by L. D. Barnett, , at sacred-texts.com
§ 7. Ātmā, Self or Spirit.—The word ātmā is several times used in the Ṛig-veda with the meaning of "breath," "spirit," in the literal sense; and so far there was little to distinguish it from the word prāṇa (§ 5). But from this
sense was further evolved the meaning "self." Then we can imagine that men began to reflect upon their own words. What, they doubtless asked, is the "self" of which we speak when we use sentences like "he finds it out by [him]self," "he goes by [him]self," "he sees [him]self"? It must be the inmost essence, the indwelling reality, the αὐτό of each agent, the informing εἶδος of subjecthood. Therefore it must be thought itself. For subjecthood is a mode of thought; and to thought or will all action is finally traceable. And the Ātmā, the Self, the consciousness of self-identity on which is based all further ideation of the thinking subject, is one with Brahma, the universal Power. My Idea is the World-Idea; "I am Brahma."
There is another link in this chain. The Vedic poets speak now and then of a god Daksha, who, as his name implies, is simply the abstract idea of "skill" or "intelligence" rather vaguely personified; and twice (x. v. 7, lxxii. 4, 5) Daksha is regarded as primal Being and universal father, from whom sprang the great gods and the universe. Thus Vedic mythology furnished two fruitful ideas—that the objective universe sprang from Intelligence, Daksha, and from an ideal Man, Purusha; and hence grew up gradually the idealistic conception of the universe as arising from, and existing in, the Thought of man.