If the Sun comes from the Good Nature, why does it hurt the eye?

For they say that everything which injures is from the Evil (Nature), just as everything which helps is from the Good (Nature). And they say concerning the Sun that it purifies from Evil, because it goes and comes every day to the Domain of the Good one, which is a purification. And yet [Cf. p. xli.] the eye which fixes its gaze much upon it is injured by its strength, but if it fixes its gaze to look on the shadow or thick darkness it is not injured, and so it is found that the Sun of the Good (Being) is harmful.