Reply Obj. 2: Fire and air, as not distinctly known by the unlettered, are not expressly named by Moses among the parts of the world, but reckoned with the intermediate part, or water, especially as regards the lowest part of the air; or with the heaven, to which the higher region of air approaches, as Augustine says (Gen. ad lit. ii, 13).
Reply Obj. 3: The production of animals is recorded with reference to their adorning the various parts of the world, and therefore the days of their production are separated or united according as the animals adorn the same parts of the world, or different parts.
Reply Obj. 4: The nature of light, as existing in a subject, was made on the first day; and the making of the luminaries on the fourth day does not mean that their substance was produced anew, but that they then received a form that they had not before, as said above (Q. 70, [A. 1] ad 2).
Reply Obj. 5: According to Augustine (Gen. ad lit. iv, 15), after all that has been recorded that is assigned to the six days, something distinct is attributed to the seventh—namely, that on it God rested in Himself from His works: and for this reason it was right that the seventh day should be mentioned after the six. It may also be said, with the other writers, that the world entered on the seventh day upon a new state, in that nothing new was to be added to it, and that therefore the seventh day is mentioned after the six, from its being devoted to cessation from work. _______________________
SECOND ARTICLE [I, Q. 74, Art. 2]
Whether All These Days Are One Day?
Objection 1: It would seem that all these days are one day. For it is written (Gen. 2:4, 5): "These are the generations of the heaven and the earth, when they were created, in the day that the Lord . . . made the heaven and the earth, and every plant of the field, before it sprung up in the earth." Therefore the day in which God made "the heaven and the earth, and every plant of the field," is one and the same day. But He made the heaven and the earth on the first day, or rather before there was any day, but the plant of the field He made on the third day. Therefore the first and third days are but one day, and for a like reason all the rest.
Obj. 2: Further, it is said (Ecclus. 18:1): "He that liveth for ever, created all things together." But this would not be the case if the days of these works were more than one. Therefore they are not many but one only.
Obj. 3: Further, on the seventh day God ceased from all new works. If, then, the seventh day is distinct from the other days, it follows that He did not make that day; which is not admissible.
Obj. 4: Further, the entire work ascribed to one day God perfected in an instant, for with each work are the words (God) "said . . . . and it was . . . done." If, then, He had kept back His next work to another day, it would follow that for the remainder of a day He would have ceased from working and left it vacant, which would be superfluous. The day, therefore, of the preceding work is one with the day of the work that follows.