Objection 1. It would seem that the production of light is not fittingly assigned to the first day. For light, as stated above (Article 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.
Objection 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.
Objection 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.
Objection 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 (63, 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.
Other writers think that the production of spiritual creatures was purposely omitted by Moses, and give various reasons. Basil [Hom. i in Hexaem.] says that Moses begins his narrative from the beginning of time which belongs to sensible things; but that the spiritual or angelic creation is passed over, as created beforehand.
Chrysostom [Hom. ii in Genes.] gives as a reason for the omission that Moses was addressing an ignorant people, to whom material things alone appealed, and whom he was endeavoring to withdraw from the service of idols. It would have been to them a pretext for idolatry if he had spoken to them of natures spiritual in substance and nobler than all corporeal creatures; for they would have paid them Divine worship, since they were prone to worship as gods even the sun, moon, and stars, which was forbidden them (Deuteronomy 4).
But mention is made of several kinds of formlessness, in regard to the corporeal creature. One is where we read that "the earth was void and empty," and another where it is said that "darkness was upon the face of the deep." Now it seems to be required, for two reasons, that the formlessness of darkness should be removed first of all by the production of light. In the first place because light is a quality of the first body, as was stated (3), and thus by means of light it was fitting that the world should first receive its form. The second reason is because light is a common quality. For light is common to terrestrial and celestial bodies. But as in knowledge we proceed from general principles, so do we in work of every kind. For the living thing is generated before the animal, and the animal before the man, as is shown in De Gener. Anim. ii, 3. It was fitting, then, as an evidence of the Divine wisdom, that among the works of distinction the production of light should take first place, since light is a form of the primary body, and because it is more common quality.
Basil [Hom. ii in Hexaem.], indeed, adds a third reason: that all other things are made manifest by light. And there is yet a fourth, already touched upon in the objections; that day cannot be unless light exists, which was made therefore on the first day.
Reply to Objection 1. According to the opinion of those who hold that the formlessness of matter preceded its form in duration, matter must be held to have been created at the beginning with substantial forms, afterwards receiving those that are accidental, among which light holds the first place.
Reply to Objection 2. In the opinion of some the light here spoken of was a kind of luminous nebula, and that on the making of the sun this returned to the matter of which it had been formed. But this cannot well be maintained, as in the beginning of Genesis Holy Scripture records the institution of that order of nature which henceforth is to endure. We cannot, then, say that what was made at that time afterwards ceased to exist.
Others, therefore, held that this luminous nebula continues in existence, but so closely attached to the sun as to be indistinguishable. But this is as much as to say that it is superfluous, whereas none of God's works have been made in vain. On this account it is held by some that the sun's body was made out of this nebula. This, too, is impossible to those at least who believe that the sun is different in its nature from the four elements, and naturally incorruptible. For in that case its matter cannot take on another form.
I answer, then, with Dionysius (Div. Nom. iv), that the light was the sun's light, formless as yet, being already the solar substance, and possessing illuminative power in a general way, to which was afterwards added the special and determinative power required to produce determinate effects. Thus, then, in the production of this light a triple distinction was made between light and darkness.
First, as to the cause, forasmuch as in the substance of the sun we have the cause of light, and in the opaque nature of the earth the cause of darkness.
Secondly, as to place, for in one hemisphere there was light, in the other darkness.
Thirdly, as to time; because there was light for one and darkness for another in the same hemisphere; and this is signified by the words, "He called the light day, and the darkness night."
Reply to Objection 3. Basil says (Hom. ii in Hexaem.) that day and night were then caused by expansion and contraction of light, rather than by movement. But Augustine objects to this (Gen. ad lit. i), that there was no reason for this vicissitude of expansion and contraction since there were neither men nor animals on the earth at that time, for whose service this was required. Nor does the nature of a luminous body seem to admit of the withdrawal of light, so long as the body is actually present; though this might be effected by a miracle. As to this, however, Augustine remarks (Gen. ad lit. i) that in the first founding of the order of nature we must not look for miracles, but for what is in accordance with nature. We hold, then, that the movement of the heavens is twofold. Of these movements, one is common to the entire heaven, and is the cause of day and night. This, as it seems, had its beginning on the first day. The other varies in proportion as it affects various bodies, and by its variations is the cause of the succession of days, months, and years. Thus it is, that in the account of the first day the distinction between day and night alone is mentioned; this distinction being brought about by the common movement of the heavens. The further distinction into successive days, seasons, and years recorded as begun on the fourth day, in the words, "let them be for seasons, and for days, and years" is due to proper movements.
Reply to Objection 4. As Augustine teaches (Confess. xii; Gen. ad lit. 1,15), formlessness did not precede forms in duration; and so we must understand the production of light to signify the formation of spiritual creatures, not, indeed, with the perfection of glory, in which they were not created, but with the perfection of grace, which they possessed from their creation as said above (Question 62, Article 3). Thus the division of light from darkness will denote the distinction of the spiritual creature from other created things as yet without form. But if all created things received their form at the same time, the darkness must be held to mean the spiritual darkness of the wicked, not as existing from the beginning but such as God foresaw would exist.
Source: Summa Theologica (New Advent)