11 R. But all this forest of facts, unless I am mistaken, may be divided into two kinds. For it lies partly in equal, partly in inferior things. They are equal, when we say that this is as like to that as that to this, as is said of twins, or impressions of a ring. Inferior, when we say that the worse is like the better. For who, looking in a mirror, would dream of saying that he is like that image, and not rather that like him? And this class consists partly in what the soul undergoes, and partly in those things which are seen.
And that again which the soul undergoes, it either undergoes in the sense, as the unreal motion of a building; or in itself from that which it has received from the senses, such as are the dreams of dreamers, and perhaps also of madmen. Furthermore, those things which appear in the things themselves which we see, are some of them from nature, and some expressed and framed by living creatures. Nature either by procreation or reflection effects inferior similitudes. By procreation, when to parents children like them are born; by reflection, as from mirrors of various kinds.
For although it is men that make the most of the mirrors, yet it is not they that frame the images given back. On the other hand, the works of living creatures are seen in pictures, and creations of the like kind: in which may also be included (conceding their occurrence) those things which demons produce. But the shadows of bodies, because with but a slight stretch of language they may be described as like their bodies and a sort of false bodies, nor can be disputed to be submitted to the judgment of the eyes, may reasonably be placed in that class, which are brought about by nature through reflection.
For every body exposed to the light reflects, and casts a shadow in the opposite direction. Or do you see any objection to be made? A. None. I am only awaiting anxiously the issue of these illustrations.
Source: Soliloquies (New Advent)