Obj. 2: Further, the work of the six days is ordered conformably to the order of Divine wisdom. Now it would ill become the Divine wisdom to make afterwards that which is naturally first. But though the firmament naturally precedes the earth and the waters, these are mentioned before the formation of light, which was on the first day. Therefore the firmament was not made on the second day.
Obj. 3: Further, all that was made in the six days was formed out of matter created before days began. But the firmament cannot have been formed out of pre-existing matter, for if so it would be liable to generation and corruption. Therefore the firmament was not made on the second day.
On the contrary, It is written (Gen. 1:6): "God said: let there be a firmament," and further on (verse 8); "And the evening and morning were the second day."
I answer that, In discussing questions of this kind two rules are to be observed, as Augustine teaches (Gen. ad lit. i, 18). The first is, to hold the truth of Scripture without wavering. The second is that since Holy Scripture can be explained in a multiplicity of senses, one should adhere to a particular explanation, only in such measure as to be ready to abandon it, if it be proved with certainty to be false; lest Holy Scripture be exposed to the ridicule of unbelievers, and obstacles be placed to their believing.
We say, therefore, that the words which speak of the firmament as made on the second day can be understood in two senses. They may be understood, first, of the starry firmament, on which point it is necessary to set forth the different opinions of philosophers. Some of these believed it to be composed of the elements; and this was the opinion of Empedocles, who, however, held further that the body of the firmament was not susceptible of dissolution, because its parts are, so to say, not in disunion, but in harmony. Others held the firmament to be of the nature of the four elements, not, indeed, compounded of them, but being as it were a simple element. Such was the opinion of Plato, who held that element to be fire. Others, again, have held that the heaven is not of the nature of the four elements, but is itself a fifth body, existing over and above these. This is the opinion of Aristotle (De Coel. i, text. 6,32).
According to the first opinion, it may, strictly speaking, be granted that the firmament was made, even as to substance, on the second day. For it is part of the work of creation to produce the substance of the elements, while it belongs to the work of distinction and adornment to give forms to the elements that pre-exist.
But the belief that the firmament was made, as to its substance, on the second day is incompatible with the opinion of Plato, according to whom the making of the firmament implies the production of the element of fire. This production, however, belongs to the work of creation, at least, according to those who hold that formlessness of matter preceded in time its formation, since the first form received by matter is the elemental.
Still less compatible with the belief that the substance of the firmament was produced on the second day is the opinion of Aristotle, seeing that the mention of days denotes succession of time, whereas the firmament, being naturally incorruptible, is of a matter not susceptible of change of form; wherefore it could not be made out of matter existing antecedently in time.
Hence to produce the substance of the firmament belongs to the work of creation. But its formation, in some degree, belongs to the second day, according to both opinions: for as Dionysius says (Div. Nom. iv), the light of the sun was without form during the first three days, and afterwards, on the fourth day, received its form.
If, however, we take these days to denote merely sequence in the natural order, as Augustine holds (Gen. ad lit. iv, 22,24), and not succession in time, there is then nothing to prevent our saying, whilst holding any one of the opinions given above, that the substantial formation of the firmament belongs to the second day.