The Enchiridion by St. Augustine
Chapter 59.--The Bodies Assumed by Angels Raise a Very Difficult, and Not Very Useful, Subject of Discussion.
Further, who will tell with what sort of bodies it was that the angels appeared to men, making themselves not only visible, but tangible; and again, how it is that, not through material bodies, but by spiritual power, they present visions not to the bodily eyes, but to the spiritual eyes of the mind, or speak something not into the ear from without, but from within the soul of the man, they themselves being stationed there too, as it is written in the prophet, |And the angel that spake in me said unto me| (he does not say, |that spake to me,| but |that spake in me|); or appear to men in sleep, and make communications through dreams, as we read in the Gospel, |Behold, the angel of the Lord appeared unto him in a dream, saying|? For these methods of communication seem to imply that the angels have not tangible bodies, and make it a very difficult question to solve how the patriarchs washed their feet, and how it was that Jacob wrestled with the angel in a way so unmistakeably material. To ask questions like these, and to make such guesses as we can at the answers, is a useful exercise for the intellect, if the discussion be kept within proper bounds, and if we avoid the error of supposing ourselves to know what we do not know. For what is the necessity for affirming, or denying, or defining with accuracy on these subjects, and others like them, when we may without blame be entirely ignorant of them?