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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.
Another possible explanation is to understand by the firmament that was made on the second day, not that in which the stars are set, but the part of the atmosphere where the clouds are collected, and which has received