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Summa Theologica
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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 But light does not remain in the air when the source of light is withdrawn.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.

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 But light is not the substantial form of air, for if it were, the air would be destroyed when light is withdrawn.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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