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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does not remain in the air when the source of light is withdrawn.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.
Objection 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.
Objection 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 constitute 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would be destroyed when light is withdrawn.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 to Objection 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
transmutation 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.
Reply to Objection 2: It is accidental to light not to have a
contrary, forasmuch as it is the natural quality of the first corporeal
cause of change, which is itself removed from contrariety.
Reply to Objection 3: As heat acts towards perfecting the form of
fire, as an instrumental cause, by virtue of the substantial form, so
does light act instrumentally, by virtue of the heavenly bodies,
towards producing substantial forms; and towards rendering colors
actually visible, inasmuch as it is a quality of the first sensible
body.
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