A. What Light is

10. About the first I say that light cannot be set down as a substance, because it is a per se sensible; a substance is not such;a     therefore etc     .

a. a[Interpolation] save per accidens; the thing is plain in the sacrament of the altar after consecration, where there is no substance of bread but it is transubstantiated into the body of Christ, and yet whatever was discernible before by any sense remains there.

11. Light is also an accident in something, therefore in nothing is it a substance (for what is truly a substance is not an accident of anything, from Physics 1.3.186b4-5). The antecedent is plain, because if light were the substantial form in fire, it would be either the ultimate specific difference or some more imperfect difference that was in potency to the specific form; not in the first way, because then everything that had light would be of the same species; not in the second way, because then the form in question would be more imperfect than the specific form of fire and so would not be the substantial form of any celestial body, in which there is no substantial form more imperfect than the form of the element.

12. Third, it is probable that some active form follows the substantial form of a celestial body, just as the active qualities follow the substantial forms of generable and corruptible things; but there is no other quality than light that follows the substantial form of a celestial body (this is confirmed from Damascene Orthodox Faith ch.8 [“But the illuming generated by fire, remaining inseparable and always in it, does not have a proper hypostasis (that is, subsistence) besides the fire; for it is a natural quality of fire”]).