Why the Holy Ghost Should Have Been Mentioned After the Mention of Heaven and Earth.

7. But what was the cause, O Thou true-speaking Light? Unto Thee do I lift up my heart, let it not teach me vain things; disperse its darkness, and tell me, I beseech Thee, by our mother charity, tell me, I beseech Thee, the reason why, after the mention of heaven, and of the earth invisible and formless, and darkness upon the deep, Thy Scripture should then at length mention Thy Spirit? Was it because it was meet that it should be spoken of Him that He was “borne over,” and this could not be said, unless that were first mentioned “over” which Thy Spirit may be understood to have been “borne?” For neither was He “borne over” the Father, nor the Son, nor could it rightly be said that He was “borne over” if He were “borne over” nothing. That, therefore, was first to be spoken of “over” which He might be “borne;” and then He, whom it was not meet to mention otherwise than as having been “borne.” Why, then, was it not meet that it should otherwise be mentioned of Him, than as having been “borne over?”

Conf 15.7.1