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