The motive follows which caused the lights to be created.  It was to illuminate the earth.  Already light was created; why therefore say that the sun was created to give light?  And, first, do not laugh at the strangeness of this expression.  We do not follow your nicety about words, and we trouble ourselves but little to give them a harmonious turn.  Our writers do not amuse themselves by polishing their periods, and everywhere we prefer clearness of words to sonorous expressions.  See then if by this expression “to light up,” the sacred writer sufficiently made his thought understood.  He has put “to give light” φαῦσις, the act of giving light, LXX. instead of “illumination.” φωτισμός, the condition produced by φαῦσις.  Now there is nothing here contradictory to what has been said of light.  Then the actual nature of light was produced:  now the sun’s body is constructed to be a vehicle for that original light.  A lamp is not fire.  Fire has the property of illuminating, and we have invented the lamp to light us in darkness.  In the same way, the luminous bodies have been fashioned as a vehicle for that pure, clear, and immaterial light.  The Apostle speaks to us of certain lights which shine in the world cf. Phil. ii. 15. without being confounded with the true light of the world, the possession of which made the saints luminaries of the souls which they instructed and drew from the darkness of ignorance.  This is why the Creator of all things, made the sun in addition to that glorious light, and placed it shining in the heavens.