15 Now listen, so far as the present time requires, while from that similitude of sensible things I now teach also something concerning God. Namely, God is intelligible, not sensible, intelligible also are those demonstrations of the schools; nevertheless they differ very widely. For as the earth is visible, so is light; but the earth, unless illumined by light, cannot be seen. Therefore those things also which are taught in the schools, which no one who understands them doubts in the least to be absolutely true, we must believe to be incapable of being understood, unless they are illuminated by somewhat else, as it were a sun of their own.
Therefore as in this visible sun we may observe three things: that he is, that he shines, that he illuminates: so in that God most far withdrawn whom you would fain apprehend, there are these three things: that He is, that He is apprehended, and that He makes other things to be apprehended. These two, God and yourself, I dare promise that I can teach you to understand. But give answer how you receive these things, as probable, or as true? A. As probable certainly; and, as I must own, I have been hoping more: for excepting those two illustrations of the line and the globe, nothing has been said by you which I should dare to say that I know. R. It is not to be wondered at: for nothing has been yet so set forth, as that it exacts of you perception.
Source: Soliloquies (New Advent)