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. _______________________
FOURTH ARTICLE [I, Q. 67, Art. 4]
Whether the Production of Light Is Fittingly Assigned to the First
Day?
Objection 1: It would seem that the production of light is not fittingly assigned to the first day. For light, as stated above (A. 3), is a quality. But qualities are accidents, and as such should have, not the first, but a subordinate place. The production of light, then, ought not to be assigned to the first day.
Obj. 2: Further, it is light that distinguishes night from day, and this is effected by the sun, which is recorded as having been made on the fourth day. Therefore the production of light could not have been on the first day.
Obj. 3: Further, night and day are brought about by the circular movement of a luminous body. But movement of this kind is an attribute of the firmament, and we read that the firmament was made on the second day. Therefore the production of light, dividing night from day, ought not to be assigned to the first day.
Obj. 4: Further, if it be said that spiritual light is here spoken of, it may be replied that the light made on the first day dispels the darkness. But in the beginning spiritual darkness was not, for even the demons were in the beginning good, as has been shown (Q. 63, A. 5). Therefore the production of light ought not to be assigned to the first day.
On the contrary, That without which there could not be day, must have been made on the first day. But there can be no day without light. Therefore light must have been made on the first day.
I answer that, There are two opinions as to the production of light. Augustine seems to say (De Civ. Dei xi, 9,33) that Moses could not have fittingly passed over the production of the spiritual creature, and therefore when we read, "In the beginning God created heaven and earth," a spiritual nature as yet formless is to be understood by the word "heaven," and formless matter of the corporeal creature by the word "earth." And spiritual nature was formed first, as being of higher dignity than corporeal. The forming, therefore, of this spiritual nature is signified by the production of light, that is to say, of spiritual light. For a spiritual nature receives its form by the enlightenment whereby it is led to adhere to the Word of God.