/summa/FP/FPVIII/FP_Q67:
We must consider next the work of distinction in itself. First, the work of the first day; secondly, the work of the second day; thirdly the work of the third day.
Under the first head there are four points of inquiry:
(1) Whether the word light is used in its proper sense in speaking of spiritual things?
(2) Whether light, in corporeal things, is itself corporeal?
(3) Whether light is a quality?
(4) Whether light was fittingly made on the first day?