From Ambrose, (De Fide, ad Gratianum) as cited by John Owen in the Preface to Christologia
I inquire of you when and how the Son was begotten? Impossible it is to me to know the mystery of this generation. My mind faileth, my voice is silent — and not only mine, but of the angels; it is above principalities, above angels, above the cherubim, above the seraphim, above all understanding. Lay thy hand on thy mouth; it is not lawful to search into these heavenly mysteries. It is lawful to know that he was born — it is not lawful to discuss how he was born; that it is not lawful for me to deny — this I am afraid to inquire into. For if Paul, when he was taken into the third heaven, affirms that the things which he heard could not be uttered; how can we express the mystery of the divine generation, which we can neither apprehend nor hear? Why do such tormenting questions delight thee?