Part I (Prima Pars) Part 103 (2/2)

SECOND ARTICLE [I, Q. 67, Art. 2]

Whether Light Is a Body?

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.

Obj. 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.

Obj. 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 pa.s.s through the intervening s.p.a.ce 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 Obj. 1: Augustine takes light to be a luminous body in act--in other words, to be fire, the n.o.blest of the four elements.

Reply Obj. 2: Aristotle p.r.o.nounces 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 Obj. 3: All these properties are a.s.signed 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.

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THIRD ARTICLE [I, Q. 67, Art. 3]

Whether Light Is a Quality?

Objection 1: It would seem that light is not a quality. For every quality remains in its subject, though the active cause of the quality be removed, as heat remains in water removed from the fire.

But light does not remain in the air when the source of light is withdrawn. Therefore light is not a quality.

Obj. 2: Further, every sensible quality has its opposite, as cold is opposed to heat, blackness to whiteness. But this is not the case with light since darkness is merely a privation of light. Light therefore is not a sensible quality.

Obj. 3: Further, a cause is more potent than its effect. But the light of the heavenly bodies is a cause of substantial forms of earthly bodies, and also gives to colors their immaterial being, by making them actually visible. Light, then, is not a sensible quality, but rather a substantial or spiritual form.

_On the contrary,_ Damascene (De Fide Orth. i) says that light is a species of quality.

_I answer that,_ Some writers have said that the light in the air has not a natural being such as the color on a wall has, but only an intentional being, as a similitude of color in the air. But this cannot be the case for two reasons. First, because light gives a name to the air, since by it the air becomes actually luminous. But color does not do this, for we do not speak of the air as colored. Secondly, because light produces natural effects, for by the rays of the sun bodies are warmed, and natural changes cannot be brought about by mere intentions. Others have said that light is the sun's substantial form, but this also seems impossible for two reasons. First, because substantial forms are not of themselves objects of the senses; for the object of the intellect is what a thing is, as is said _De Anima_ iii, text. 26: whereas light is visible of itself. In the second place, because it is impossible that what is the substantial form of one thing should be the accidental form of another; since substantial forms of their very nature const.i.tute species: wherefore the substantial form always and everywhere accompanies the species. But light is not the substantial form of air, for if it were, the air would be destroyed when light is withdrawn. Hence it cannot be the substantial form of the sun.

We must say, then, that as heat is an active quality consequent on the substantial form of fire, so light is an active quality consequent on the substantial form of the sun, or of another body that is of itself luminous, if there is any such body. A proof of this is that the rays of different stars produce different effects according to the diverse natures of bodies.

Reply Obj. 1: Since quality is consequent upon substantial form, the mode in which the subject receives a quality differs as the mode differs in which a subject receives a substantial form. For when matter receives its form perfectly, the qualities consequent upon the form are firm and enduring; as when, for instance, water is converted into fire. When, however, substantial form is received imperfectly, so as to be, as it were, in process of being received, rather than fully impressed, the consequent quality lasts for a time but is not permanent; as may be seen when water which has been heated returns in time to its natural state. But light is not produced by the trans.m.u.tation of matter, as though matter were in receipt of a substantial form, and light were a certain inception of substantial form. For this reason light disappears on the disappearance of its active cause.

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