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withdrawn.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 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would be destroyed when light is withdrawn.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 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
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 Obj. 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 Obj. 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.
_______________________