2 Yet it is not enough to observe these things in the miracles of Christ. Let us interrogate the miracles themselves, what they tell us about Christ: for they have a tongue of their own, if they can be understood. For since Christ is Himself the Word of God, even the act of the Word is a word to us. Therefore as to this miracle, since we have heard how great it is, let us also search how profound it is; let us not only be delighted with its surface, but let us also seek to know its depth.
This miracle, which we admire on the outside, has something within. We have seen, we have looked at something great, something glorious, and altogether divine, which could be performed only by God: we have praised the doer for the deed. But just as, if we were to inspect a beautiful writing somewhere, it would not suffice for us to praise the hand of the writer, because he formed the letters even, equal and elegant, if we did not also read the information he conveyed to us by those letters; so, he who merely inspects this deed may be delighted with its beauty to admire the doer: but he who understands does, as it were, read it.
For a picture is looked at in a different way from that in which a writing is looked at. When you have seen a picture, to have seen and praised it is the whole thing; when you see a writing, this is not the whole, since you are reminded also to read it. Moreover, when you see a writing, if it chance that you can not read, you say, “What do we think that to be which is here written?” Thou ask what it is, when already you see it to be something. He of whom you seek to be informed what it is that you have seen, will show you another thing.
He has other eyes than you have. Do you not alike see the form of the letters? But yet you do not alike understand the signs. Well, you see and praisest; but he sees, praises, reads and understands. Therefore, since we have seen and praised, let us also read and understand.
Source: Tractates on the Gospel of John (New Advent)