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 (Q[62], A[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.