Thirteenth Distinction

Single Question. Whether Light Generates Illuming as its Proper Sensible Speciesa

1. About the thirteenth distinction I ask at the same time about illuming [lumen] and about light [lux], and my question is single: whether light generates illuming as its proper sensible species.The distinction Scotus means here is between the light at the light source (lux) and the light illuming the air (lumen). The first is the light we see, as the burning candle, the lit light bulb, the shining sun, etc.; the second is the light we see things through, as the air illumined by the light source. The two are the same, for both are light, and also not the same, for one is seen and not seen through, while the other is seen through and not seen. ²⁶

a. a[Interpolation] In this thirteenth distinction, where the Master deals with the distinction of created things, and first of light, which was produced on the first day, the question is asked:

2. That it does not:

Augustine Literal Commentary on Genesis 7.15.n.21 says, “The soul administers bodies through light and air     etc .”

3. Further, illuming generates a substance, as is plain of fire [sc. which can be generated by the illuming of the sun]; therefore      it is a substance.

4. Further, illuming is refracted and reflected; therefore it is a body, because these are properties of body.

5. Further, that illuming is not a species:

Because it denominates the medium; not so the species of color [sc. air is said to be illumined but not to be colored].

6. It also excludes the opposite, namely darkness; not so species, because the species of opposites are present together (as the species of white and black are together in the medium).

8. To the contrary:

Light is of itself sensible (On the Soul 2.7.418b4-17, 419a1-6);     therefore it has some sensible species; not a species different from the illuming; therefore etc     .

I. To the Question

9. Here one must look at three things: first what light is; third what illuming is; third, how illuming is generated by light.

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”]).

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).

C. How Illuming is Generated by Light

18. And as to the third article [n.9], which makes the second article clear, one needs to note that illuming is generated by light according to a threefold root: direct ray, refracted ray, and reflected ray (according to Alhazen in his Optics 7.1 n.1).“Sight comprehends visibles in three ways, namely directly, by reflection from clean bodies, and by refraction on the other side of transparent bodies that differ in transparency from the transparency of the air.” 4.1 n.1 “The acquisition of seen objects is diversified in three ways, either directly, or by reflection on polished bodies, or by penetration, as in rare bodies whose rarity is not like the rarity of air...” ²⁷

19. A direct ray is one that flows from a luminous body in a medium of the same transparency in a direct line, and stops at the limit of the terminating or terminated body as long as the power of the luminous body lasts. A reflex ray is one that, when an opaque body intervenes (before the active power of the luminous body reaches its limit), flows in an opposed direction, not by choice, but naturally, because the natural agent whose active power is not completely used up in direct line acts as much as it can, and when it cannot act on a straight line it does so on an oblique one. A refracted ray is one that, when a medium of different transparency (but not completely opaque) intervenes, is continued in that medium though not in a straight line but there is an angle incident there; now when a more dense medium intervenes the ray is refracted toward the perpendicular, because from the fact that a perpendicular force is strongest in acting (hence it is not fractured) a position closer to a perpendicular ray is required for acting on a denser medium (such is the position of a ray refracted to the perpendicular); but when a less dense medium intervenes, the ray is refracted away from the perpendicular, for the opposite reason.The description here (whatever is said about the proposed reason) is correct. A ray of light coming from the air and falling on water at an angle is bent inwards, as it were, toward the perpendicular, and vice versa when coming from water into air. ²⁸

20. I say then that the illuming continued according to these three rays is generated by the light itself, and is also immediately a sensible species of the light.

21. Direct ray is plain. The proof about reflected ray is, first, that by this ray the thing is seen in itself and not in any species impressed by the reflecting mirror; second (see the conclusion below, nn.24-27). In the same way too is the thing itself seen by the refracted ray continuing itself, and not any species of the thing seen.

22. And just as these three primary illumings, diffused by multiplication, are immediately species of the light generating them, perhaps they are likewise as immediately generated by it, speaking of their immediacy to the cause, not excluding the order of the effects ordered in respect of the same cause.

23. But besides these three there is another, secondary illuming (which is called ‘accidental illuming), of the sort that is where a shadow is; for actual shadow is distinguished from darkness in this, that darkness is a privation of illuming both primary and secondary, but shadow is not a privation of secondary illuming. Now although this secondary illuming is sometimes diffused by reflection, though not from a polished body, for reflection from a polished body is the generation of a primary illuming because it is immediately a species of the light generating it; but not so this secondary illuming, which rather is generated by the primary illuming, so that if, per impossibile, reflection were to remain within the illuming of the impressing ray, it would generate this secondary illuming in a spherical, or semi-spherical, manner; but reflected illuming is not generated in this way by a primary ray of the sort that reflection from a polished body is.

So therefore all illuming is a species; but primary illuming is a species of the light generating it, and secondary illuming a species of primary illuming.

24. Hereby is the response clear to the arguments that could be made against this section [nn.20-23], that, since illuming is seen, it will not be a visible species.

Response:

Notwithstanding that an intended species is the intention and species of another visible thing, yet it can be visible in itself, as the Commentator says on Sense and Thing Sensed 2.437a23-24 [Averroes Paraphrase thereon ch.3], “when illuming is reflected by green plants, the walls appear colored.”

25. Likewise species are not visible save when they are stopped at opaque bodies, so that, when they are not in contact with the opaque, they cannot generate a species of themselves whereby they may be seen (as the species of color is not seen while it is in the medium); but when a species is in contact with the opaque, then it can be seen, as is plain about a ray passing through colored glass; the color appears on the wall when the ray is stopped at it and the wall is seen colored like the glass, and yet the colored ray was not visible in the medium; nor for this reason is the red that appears on the wall denied to be a species, for the eye focused on the wall would, if the red were put in its way, see the red glass or the redness of the glass.

26. Hence this proposition is false: ‘that which is the intention of the visible object is not visible’; but this one is true: ‘every intention is not only visible but is a reason for seeing’. Hence the form that illumines the moon is illuming (and not light, because it is not a body terminated in the moon), and yet it is sensible and is a species of light.

27. Likewise, from Aristotle at the beginning of Sense and Thing Sensed 2.437a23-24, “the gleam emitted from an eye in darkness is visible,” and yet it is a species of light only.

28. In the same way, illuming not terminated at an opaque body is not seen. The point is clear, first, because the solar rays do not appear at night, and yet they intercept between us and the sky at the tip of the cone of shadow cast by the earth; second, because someone in an opaque and dark prison, where a solar ray passed through opposite holes, would not see - so that, when the solar ray is not reflected anywhere on something opaque, he would not see the ray, unless the corpuscles are refracted.

29. Likewise, if an illumined medium were infinite and did not end at an opaque body, nothing would be seen when the sun was above the horizon, because there would be no end to the illuming.

30. Illuming then has the same condition as to being seen and not being seen as other principal species of colors have; and just as the principal species of colors, namely those that are continuied in rays, can generate other accidental secondary species not continued in rays, and can be seen through them (and yet there is no denying that the first species are intentions, because they are simply likenesses of the first generating colors); so it is in the issue at hand.

31. But that the whole ray is generated immediately by the light and not one part of the ray by another [n.20] is proved thus:

Because if it is posited that illuming in one part of the medium generates illuming from itself, then any point of illuming multiplies itself spherically (because every natural active principle acts spherically on the surrounding medium if it is equally disposed on all sides); and even if there were an opaque body in the way on one side of it, at any rate it multiplies itself hemi-spherically; therefore in the same way, if illuming were to be generative with respect to another illuming, then, from its not having an opaque body in the way on any side of it, it multiplies itself spherically; and thus, just as a principal illuming generates further another secondary, indirect illuming, so a secondary illuming would be generated from every part of a distant ray, which is contrary to the senses.

32. In addition, any luminous body can act at a distance in place (because if it could only act where it was in place it could only illumine something indivisible [sc. place is the surface of the containing body, and surface is an indivisible quantity]). But if it is present, in idea of agent, in some distant place, then, and by parity of reasoning, it is present in the whole medium, to as great a distance as its power is sufficient to reach; and if it is present to the whole and can cause illuming in the whole, then it does so cause it -because, even given that the illuming generated by it could cause another illuming, yet the present luminous body is of greater power than the illuming generated, and so it will precede the illuming in acting on a remote part.The Vatican editors remark that the proof in these two paragraphs, nn.31-32, was omitted or forgotten here and placed by Scotus after n.39 (the same sort of thing has apparently happened elsewhere). The Vatican editors have restored it to its place here. ²⁹

II. To the Principal Arguments

33. To the arguments.

To the first [n.2] I say that Augustine takes light there for a subtle body (to wit, for fire, which is the more subtle body in a mixed body, or for some subtle body very much agreeing with fire in nature), just as Scripture too says that light was made on the first day; not because then an accident came to be without a subject, but a luminous body then came to be whose more known form was light; and so the subject is described through a more known term.

34. To the second [n.3] I say that the altering agent, and not the quality by which it generates, generates at the end of the alteration.

35. To the third [n.4] I say that those things are said metaphorically of light, as is plain from Avicenna On the Soul p.3 ch.2.“For as to our saying that a ray descends or goes out or enters, these words are taken transumptively, because a ray has nothing of this... But that a ray shines back is also transumptively taken; for when a body is illumined and is polished, another body opposite to it is said to be illumined by it, in such a way that the first body is not moved toward the second by local motion.” ³⁰ For no same illuming proceeds in a direct line as by local motion, and yet it is sometimes turned obliquely (by bending or refracting), but when the active power is not totally used up (that is, has not caused as much as it can cause), then, if it cannot then further act in a straight line (in which direction nature acts most of all, because a straight line is shortest and most effective for acting in), it acts in another line nearer to it if it cannot do anything further (and this a refracted or reflex line [n.19]), by reacting on the same thing it acted on before, causing there however, along with what was first caused, something simply more imperfect.

36. And if you object that a later more imperfect caused thing cannot stand together with a prior more perfect caused thing, and so there cannot be light along with a whole medium illumined by the primary ray, nor even can a reflex ray exist along with a medium illumined by a direct ray - I say that the falsity of the conclusion is manifest to sense. For it is plain that if a ray of the sun falls on water and is reflected into some dark place where there is no direct ray, the sense says that a reflection comes to be in that dark place; for it would not come to be there if the reflex ray was not first in the medium illumined by the direct ray. In the same way the sense says that a secondary illuming (or the species of a primary illuming) is diffused by the primary illuming (in contact with some opaque body) up to the eye; otherwise the primary illuming would not be seen, and yet the primary illuming is there through the whole medium. So then this proposition must be denied, ‘where there is a more perfect form, a more imperfect form (or a species, which is a more imperfect form) cannot be multiplied there’.

37. To the other arguments that are against the species or against the intention [nn.5-7].

I say to the first [n.5] that every accident could, perhaps, denominate its subject in which it is, if it were an imposed denominating term which would signify a denomination agreeing with such a form in respect of such a subject; in this way a denominative term is not imposed by the species of colors, because the denominatives imposed by colors denominate the subject that is possessed of the colors in its real being. But if a denominative were imposed that would denominate that a subject has a form in intentional being (not in real being), the medium could well be denominated by such a name as ‘white’; such a name is imposed by the illuming, and perhaps more here than in the case of colors, because of the greater perfection and evidence of this sort of intention than of other visible intentions.

38. As to what is added secondly about the opposite [darkness, n.6], I say that one illuming does not exclude another illuming of a different idea, as the illuming of the sun does not exclude the illuming of the moon in the same part of the medium, or of another star - just as neither does the species of black exclude the species of white (or conversely) in the same part of the medium; but just as any illuming excludes darkness from the medium, which darkness is a privation of illuming (not the contrary or the disparate of it), so the species of any color excludes the privation of that color in the medium.

39. To the third [n.7] I say that illuming is a thing and can have a real effect; but it is not a thing in such a way that it cannot be an intention, because along with its idea stands that it is per se the reason for tending to the object, and this suffices for the idea of intention.