Thomas Aquinas, Summa Theologiae

Objection 1: It would seem that light is a body. For Augustine says (De Lib. Arb. iii, 5) that "light takes the first place among bodies."Therefore light is a body.

Objection 2: Further, the Philosopher says (Topic. v, 2) that "light is a species of fire." But fire is a body, and therefore so is light.

Objection 3: Further, the powers of movement, intersection, reflection, belong properly to bodies; and all these are attributes of light and its rays. Moreover, different rays of light, as Dionysius says (Div. Nom. ii) are united and separated, which seems impossible unless they are bodies. Therefore light is a body.

On the contrary, Two bodies cannot occupy the same place simultaneously. But this is the case with light and air. Therefore light is not a body.

I answer that, Light cannot be a body, for three evident reasons. First, on the part of place. For the place of any one body is different from that of any other, nor is it possible, naturally speaking, for any two bodies of whatever nature, to exist simultaneously in the same place; since contiguity requires distinction of place.

The second reason is from movement. For if light were a body, its diffusion would be the local movement of a body. Now no local movement of a body can be instantaneous, as everything that moves from one place to another must pass through the intervening space before reaching the end: whereas the diffusion of light is instantaneous. Nor can it be argued that the time required is too short to be perceived; for though this may be the case in short distances, it cannot be so in distances so great as that which separates the East from the West. Yet as soon as the sun is at the horizon, the whole hemisphere is illuminated from end to end. It must also be borne in mind on the part of movement that whereas all bodies have their natural determinate movement, that of light is indifferent as regards direction, working equally in a circle as in a straight line. Hence it appears that the diffusion of light is not the local movement of a body.

The third reason is from generation and corruption. For if light were a body, it would follow that whenever the air is darkened by the absence of the luminary, the body of light would be corrupted, and its matter would receive a new form. But unless we are to say that darkness is a body, this does not appear to be the case. Neither does it appear from what matter a body can be daily generated large enough to fill the intervening hemisphere. Also it would be absurd to say that a body of so great a bulk is corrupted by the mere absence of the luminary. And should anyone reply that it is not corrupted, but approaches and moves around with the sun, we may ask why it is that when a lighted candle is obscured by the intervening object the whole room is darkened? It is not that the light is condensed round the candle when this is done, since it burns no more brightly then than it burned before.

Since, therefore, these things are repugnant, not only to reason, but to common sense, we must conclude that light cannot be a body.

Reply to Objection 1: Augustine takes light to be a luminous body in act---in other words, to be fire, the noblest of the four elements.

Reply to Objection 2: Aristotle pronounces light to be fire existing in its own proper matter: just as fire in aerial matter is "flame," or in earthly matter is "burning coal." Nor must too much attention be paid to the instances adduced by Aristotle in his works on logic, as he merely mentions them as the more or less probable opinions of various writers.

Reply to Objection 3: All these properties are assigned to light metaphorically, and might in the same way be attributed to heat. For because movement from place to place is naturally first in the order of movement as is proved Phys. viii, text. 55, we use terms belonging to local movement in speaking of alteration and movement of all kinds. For even the word distance is derived from the idea of remoteness of place, to that of all contraries, as is said Metaph. x, text. 13.

Articulus 2

Argumentum 1

Ad secundum sic proceditur. Videtur quod lux sit corpus.

Dicit enim Augustinus, in libro de Lib. Arbit., quod lux in corporibus primum tenet locum.

Ergo lux est corpus.

Argumentum 2

Praeterea, philosophus dicit quod lumen est species ignis.

Sed ignis est corpus. Ergo lumen est corpus.

Argumentum 3

Praeterea, ferri, intersecari, et reflecti est proprie corporum, haec autem omnia attribuuntur lumini vel radio. Coniunguntur etiam diversi radii et separantur, ut dionysius dicit, II cap. De div. Nom., quod etiam videtur non nisi corporibus convenire posse. Ergo lumen est corpus.

Sed Contra

Sed contra, duo corpora non possunt simul esse in eodem loco.

Sed lumen est simul cum aere. Ergo lumen non est corpus.

Corpus

Respondeo dicendum quod impossibile est lumen esse corpus. Quod quidem apparet tripliciter. Primo quidem, ex parte loci. Nam locus cuiuslibet corporis est alius a loco alterius corporis, nec est possibile, secundum naturam, duo corpora esse simul in eodem loco, qualiacumque corpora sint; quia contiguum requirit distinctionem in situ. Secundo, apparet idem ex ratione motus. Si enim lumen esset corpus, illuminatio esset motus localis corporis. Nullus autem motus localis corporis potest esse in instanti, quia omne quod movetur localiter, necesse est quod prius perveniat ad medium magnitudinis quam ad extremum. Illuminatio autem fit in instanti.

Nec potest dici quod fiat in tempore imperceptibili. Quia in parvo spatio posset tempus latere, in magno autem spatio, puta ab oriente in occidentem, tempus latere non posset, statim enim cum sol est in puncto orientis, illuminatur totum hemisphaerium usque ad punctum oppositum. Est etiam aliud considerandum ex parte motus. Quia omne corpus habet motum naturalem determinatum, motus autem illuminationis est ad omnem partem, nec magis secundum circulum quam secundum rectitudinem. Unde manifestum est quod illuminatio non est motus localis alicuius corporis.

Tertio, apparet idem ex parte generationis et corruptionis.

Si enim lumen esset corpus, quando aer obtenebrescit per absentiam luminaris, sequeretur quod corpus luminis corrumperetur, et quod materia eius acciperet aliam formam. Quod non apparet, nisi aliquis dicat etiam tenebras esse corpus. Nec etiam apparet ex qua materia tantum corpus, quod replet medium hemisphaerium, quotidie generetur. Ridiculum est etiam dicere quod ad solam absentiam luminaris, tantum corpus corrumpatur. Si quis etiam dicat quod non corrumpitur, sed simul cum sole accedit et circumfertur, quid dici poterit de hoc, quod ad interpositionem alicuius corporis circa candelam, tota domus obscuratur? nec videtur quod lumen congregetur circa candelam, quia non apparet ibi maior claritas post quam ante. Quia ergo omnia haec non solum rationi, sed sensui etiam repugnant, dicendum est quod impossibile est lumen esse corpus.

Ad 1

Ad primum ergo dicendum quod Augustinus accipit lucem pro corpore lucido in actu, scilicet pro igne, quod inter quatuor elementa nobilissimum est.

Ad 2

Ad secundum dicendum quod Aristoteles lumen nominat ignem in propria materia, sicut ignis in materia aerea dicitur flamma, et in materia terrea dicitur carbo.

Non tamen est multum curandum de eis exemplis quae Aristoteles inducit in libris logicalibus, quia inducit ea ut probabilia secundum opinionem aliorum.

Ad 3

Ad tertium dicendum quod omnia illa attribuuntur lumini metaphorice, sicut etiam possent attribui calori. Quia enim motus localis est naturaliter primus motuum, ut probatur in VIII physic., utimur nominibus pertinentibus ad motum localem, in alteratione et in omnibus motibus, sicut etiam nomen distantiae derivatum est a loco ad omnia contraria, ut dicitur in X metaphys..