B. What Illumining is

13. About the second [n.9] I say that illuming is not a complete substance, that is, subsistent per se, for it is neither spiritual (since it is extendable) nor corporeal (for then two bodies would be together simultaneously, for illuming is in the whole air).

14. Nor does the air have to be moved locally when illuming comes to it, nor moved when it departs and stops coming, because then there would be no breathing of air in the illumined medium if the air were moved aside locally because of the illuming. Nor even is illuming a substantial form, because what it is accidental to [e.g. the air] remains perfect in its species when the illuming goes away, as is clear of air when illumined and when dark. Nor is it matter (as is plain), nor anything pertaining to the genus of substance; nor is it plausible that it belongs to any other genus besides the genus of quality.

15. Now since the genus of quality, as to its third species [Categories 8.9a28-10a10], is distinguished into sensible quality and into quality that is a species or intention of sensible quality, one should notea that by ‘intention’ here is not meant what the sense intends (for in this way the object itself would be the intention), but by ‘intention’ is meant here that by which - as by formal principle - the sense tends to the object; and just as whatever is a sign is a thing (according to Augustine On Christian Doctrine 2.1. n.1), but not conversely (and therefore in distinguishing between thing and sign the thing is taken for the thing which is not a sign, though that which is a sign is also a thing), so in the distinguishing of thing and intention, although intention may be a thing (and perhaps a sensible thing) that the sense can tend to, yet ‘intention’ is said to be that which is not only the thing to which the sense tends but is the reason for tending to some other thing of which the intention is the proper likeness. I say that in this way illuming is properly the intention or sensible species of light itself.

a. a[Interpolation] the noun ‘intention’ is equivocal: in one way intention is said to be an act of will; in another way the formal reason in a thing (as the intention of a thing from which is taken the genus differs from the intention of it from which is taken the difference); in a third way it is said to be a concept, and in a fourth way the reason for tending to an object (as a likeness is said to be the reason for tending to that of which it is the likeness). Hence...

16. This is proved by the fact that, if it were not an intention, then it would, when placed on the sense, impede the sense, because what is only a sensible and not a reason for sensing impedes sensation if it is placed on the sense (because ‘a sensible placed on a sense is not sensed’ On the Soul 2.7.419a11-13, 28-30, 9.421b17-18), and so illuming placed on the eye would prevent it from seeing; but this is false and against the Commentator [Averroes] on Sense and Sensibles chs.2, 3, where he maintains that a proper illuming in the eye is necessary for it to receive the species of color and to see.

17. The way it is posited is this, that just as visible light is naturally prior to color, so too is its species naturally prior to the species of color, and this both in perfecting the medium (so that a non-illumined medium is not fit for the perceiving of colors) and in perfecting the sight (so that a non-illumined organ is not in proximate power to see).