Objection 1. It would seem the Scripture does not use suitable words to express the works of the six days. For as light, the firmament, and other similar works were made by the Word of God, so were the heaven and the earth. For "all things were made by Him" (John 1:3). Therefore in the creation of heaven and earth, as in the other works, mention should have been made of the Word of God.
Objection 2. Further, the water was created by God, yet its creation is not mentioned. Therefore the creation of the world is not sufficiently described.
Objection 3. Further, it is said (Genesis 1:31): "God saw all the things that He had made, and they were very good." It ought, then, to have been said of each work, "God saw that it was good." The omission, therefore, of these words in the work of creation and in that of the second day, is not fitting.
Objection 4. Further, the Spirit of God is God Himself. But it does not befit God to move and to occupy place. Therefore the words, "The Spirit of God moved over the waters," are unbecoming.
Objection 5. Further, what is already made is not made over again. Therefore to the words, "God said: Let the firmament be made... and it was so," it is superfluous to add, "God made the firmament." And the like is to be said of other works.
Objection 6. Further, evening and morning do not sufficiently divide the day, since the day has many parts. Therefore the words, "The evening and morning were the second day" or, "the third day," are not suitable.
Objection 7. Further, "first," not "one," corresponds to "second" and "third." It should therefore have been said that, "The evening and the morning were the first day," rather than "one day."
Reply to Objection 1. According to Augustine (Gen. ad lit. i, 4), the person of the Son is mentioned both in the first creation of the world, and in its distinction and adornment, but differently in either place. For distinction and adornment belong to the work by which the world receives its form. But as the giving form to a work of art is by means of the form of the art in the mind of the artist, which may be called his intelligible word, so the giving form to every creature is by the word of God; and for this reason in the works of distinction and adornment the Word is mentioned. But in creation the Son is mentioned as the beginning, by the words, "In the beginning God created," since by creation is understood the production of formless matter. But according to those who hold that the elements were created from the first under their proper forms, another explanation must be given; and therefore Basil says (Hom. ii, iii in Hexaem.) that the words, "God said," signify a Divine command. Such a command, however, could not have been given before creatures had been produced that could obey it.
Reply to Objection 2. According to Augustine (De Civ. Dei ix, 33), by the heaven is understood the formless spiritual nature, and by the earth, the formless matter of all corporeal things, and thus no creature is omitted. But, according to Basil (Hom. i in Hexaem.), the heaven and the earth, as the two extremes, are alone mentioned, the intervening things being left to be understood, since all these move heavenwards, if light, or earthwards, if heavy. And others say that under the word, "earth," Scripture is accustomed to include all the four elements as (Psalm 148:7-8) after the words, "Praise the Lord from the earth," is added, "fire, hail, snow, and ice."
Reply to Objection 3. In the account of the creation there is found something to correspond to the words, "God saw that it was good," used in the work of distinction and adornment, and this appears from the consideration that the Holy Spirit is Love. Now, "there are two things," says Augustine (Gen. ad lit. i, 8) which came from God's love of His creatures, their existence and their permanence. That they might then exist, and exist permanently, "the Spirit of God," it is said, "moved over the waters"--that is to say, over that formless matter, signified by water, even as the love of the artist moves over the materials of his art, that out of them he may form his work. And the words, "God saw that it was good," signify that the things that He had made were to endure, since they express a certain satisfaction taken by God in His works, as of an artist in his art: not as though He knew the creature otherwise, or that the creature was pleasing to Him otherwise, than before He made it. Thus in either work, of creation and of formation, the Trinity of Persons is implied. In creation the Person of the Father is indicated by God the Creator, the Person of the Son by the beginning, in which He created, and the Person of the Holy Ghost by the Spirit that moved over the waters. But in the formation, the Person of the Father is indicated by God that speaks, and the Person of the Son by the Word in which He speaks, and the Person of the Holy Spirit by the satisfaction with which God saw that what was made was good. And if the words, "God saw that it was good," are not said of the work of the second day, this is because the work of distinguishing the waters was only begun on that day, but perfected on the third. Hence these words, that are said of the third day, refer also to the second. Or it may be that Scripture does not use these words of approval of the second days' work, because this is concerned with the distinction of things not evident to the senses of mankind. Or, again, because by the firmament is simply understood the cloudy region of the air, which is not one of the permanent parts of the universe, nor of the principal divisions of the world. The above three reasons are given by Rabbi Moses [Perplex. ii.], and to these may be added a mystical one derived from numbers and assigned by some writers, according to whom the work of the second day is not marked with approval because the second number is an imperfect number, as receding from the perfection of unity.
Reply to Objection 4. Rabbi Moses (Perplex. ii) understands by the "Spirit of the Lord," the air or the wind, as Plato also did, and says that it is so called according to the custom of Scripture, in which these things are throughout attributed to God. But according to the holy writers, the Spirit of the Lord signifies the Holy Ghost, Who is said to "move over the water"--that is to say, over what Augustine holds to mean formless matter, lest it should be supposed that God loved of necessity the works He was to produce, as though He stood in need of them. For love of that kind is subject to, not superior to, the object of love. Moreover, it is fittingly implied that the Spirit moved over that which was incomplete and unfinished, since that movement is not one of place, but of pre-eminent power, as Augustine says (Gen. ad lit. i, 7). It is the opinion, however, of Basil (Hom. ii in Hexaem.) that the Spirit moved over the element of water, "fostering and quickening its nature and impressing vital power, as the hen broods over her chickens." For water has especially a life-giving power, since many animals are generated in water, and the seed of all animals is liquid. Also the life of the soul is given by the water of baptism, according to John 3:5: "Unless a man be born again of water and the Holy Ghost, he cannot enter into the kingdom of God."
Reply to Objection 5. According to Augustine (Gen. ad lit. i, 8), these three phrases denote the threefold being of creatures; first, their being in the Word, denoted by the command "Let... be made"; secondly, their being in the angelic mind, signified by the words, "It was... done"; thirdly, their being in their proper nature, by the words, "He made." And because the formation of the angels is recorded on the first day, it was not necessary there to add, "He made." It may also be said, following other writers, that the words, "He said," and "Let... be made," denote God's command, and the words, "It was done," the fulfilment of that command. But as it was necessary, for the sake of those especially who have asserted that all visible things were made by the angels, to mention how things were made, it is added, in order to remove that error, that God Himself made them. Hence, in each work, after the words, "It was done," some act of God is expressed by some such words as, "He made," or, "He divided," or, "He called."
Reply to Objection 6. According to Augustine (Gen. ad lit. iv, 22,30), by the "evening" and the "morning" are understood the evening and the morning knowledge of the angels, which has been explained (58, 6,7). But, according to Basil (Hom. ii in Hexaem.), the entire period takes its name, as is customary, from its more important part, the day. And instance of this is found in the words of Jacob, "The days of my pilgrimage," where night is not mentioned at all. But the evening and the morning are mentioned as being the ends of the day, since day begins with morning and ends with evening, or because evening denotes the beginning of night, and morning the beginning of day. It seems fitting, also, that where the first distinction of creatures is described, divisions of time should be denoted only by what marks their beginning. And the reason for mentioning the evening first is that as the evening ends the day, which begins with the light, the termination of the light at evening precedes the termination of the darkness, which ends with the morning. But Chrysostom's explanation is that thereby it is intended to show that the natural day does not end with the evening, but with the morning (Hom. v in Gen.).
Reply to Objection 7. The words "one day" are used when day is first instituted, to denote that one day is made up of twenty-four hours. Hence, by mentioning "one," the measure of a natural day is fixed. Another reason may be to signify that a day is completed by the return of the sun to the point from which it commenced its course. And yet another, because at the completion of a week of seven days, the first day returns which is one with the eighth day. The three reasons assigned above are those given by Basil (Hom. ii in Hexaem.).
Source: Summa Theologica (New Advent)