10 I wonder, however, that, although you believe it possible for the sound of the voice which said, “You are my Son,” to have been produced through a divine act, without the intermediate agency of a soul, by something the nature of which was corporeal, you nevertheless do not believe that a bodily form and movements exactly resembling those of any real living creature whatsoever could be produced in the same way, namely, through a divine act, without the intermediate agency of a spirit imparting life. For if inanimate matter obeys God without the instrumentality of an animating spirit, so as to emit sounds such as are wont to be emited by animated bodies, in order to bring to the human ear words articulately spoken, why should it not obey Him, so as to present to the human eye the figure and motions of a bird, by the same power of the Creator without the instrumentalist of any animating spirit? The objects of both sight and hearing— the sound which strikes the ear and the appearance which meets the eye, the articulations of the voice and the outlines of the members, every audible and visible motion— are both alike produced from matter contiguous to us; is it, then, granted to the sense of hearing, and not to the sense of sight, to tell us regarding the body which is perceived by this bodily sense, both that it is a true body, and that it is nothing beyond what the bodily sense perceives it to be? For in every living creature the soul is, of course, not perceived by any bodily sense. We do not, therefore, need to inquire how the bodily form of the dove appeared to the eye, just as we do not need to inquire how the voice of a bodily form capable of speech was made to fall upon the ear. For if it was possible to dispense with the intermediate agency of a soul in the case in which a voice, not something like a voice, is said to have been produced, how much more easily was it possible in the case in which it is said that the Spirit descended “like a dove,” a phrase which signifies that a mere bodily form was exhibited to the eye, and does not affirm that a real living creature was seen! In like manner, it is said that on the day of Pentecost, “suddenly there came a sound from heaven as of a mighty rushing wind, and there appeared to them cloven tongues like as of fire,” in which something like wind and like fire, i.e. resembling these common and familiar natural phenomena, is said to have been perceived, but it does not seem to be indicated that these common and familiar natural phenomena were actually produced.
11. If, however, more subtle reasoning or more thorough investigation of the matter result in demonstrating that that which is naturally destitute of motion both in time and in space [i.e. matter] cannot be moved otherwise than through the intermediate agency of that which is capable of motion only in time, not in space [i.e. spirit], it will follow from this that all those things must have been done by the instrumentality of a living creature, as things are done by angels, on which subject a more elaborate discussion would be tedious, and is not necessary. To this it must be added, that there are visions which appear to the spirit as plainly as to the senses of the body, not only in sleep or delirium, but also to persons of sound mind in their waking hours—visions which are due not to the deceitfulness of devils mocking men, but to some spiritual revelation accomplished by means of immaterial forms resembling bodies, and which cannot by any means be distinguished from real objects, unless they are by divine assistance more fully revealed and discriminated by the mind's intelligence, which is done sometimes (but with difficulty) at the time, but for the most part after they have disappeared. This being the case in regard to these visions which, whether their nature be really material, or material only in appearance but really spiritual, seem to manifest themselves to our spirit as if they were perceived by the bodily senses, we ought not, when these things are recorded in sacred Scripture, to conclude hastily to which of these two classes they are to be referred, or whether, if they belong to the former, they are produced by the intermediate agency of a spirit; while, at the same time, as to the invisible and immutable nature of the Creator, that is, of the supreme and ineffable Trinity, we either simply, without any doubt, believe, or, in addition to this, with some degree of intellectual apprehension, understand that it is wholly removed and separated both from the senses of fleshly mortals, and from all susceptibility of being changed either for the worse or for the better, or to anything whatever of a variable nature.
Source: Letters (New Advent)