Part IaThe Days of CreationQuestion 67
Light: Work of the First Day of Creation
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Light means what the eye requires so that it may seeand also what the mind requires that it may understand. We constantly use the word light in both senses; we speak of the light of day, and we also say that an explanation of a problem or difficulty “throws light on the subject.”
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Light in its meaning as the illumination of the bodily universe is not a substance.
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Bodily light is an active quality which pertains to a luminous bodily substance. The effect of light is different according to the different substances from which it comes.
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It is suitable that the creation of light be the work of the first day, for in light other works may fittingly proceed.
Full Summa Text · I, Q. 67
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