Aquinas' Commentary on the Gospel of John

Aquinas uses the analogy of light to speak about our capacity to understand. The dog has sense impressions and a kind of sense imagination that is able to reproduce an image, you might say, a representative likeness of the tree. But the light of understanding proceeds from that representative image, which is also in our mind. And so he begins asking about the nature of light and its relation to our minds. He says the physical sun is a light that sense can perceive.