When Jesus and his disciples arrived at Bethsaida, people brought to him a blind man and begged Jesus to touch him.
He took the blind man by the hand and led him outside the village. Putting spittle on his eyes he laid his hands on him and asked, "Do you see anything?"
Looking up he replied, "I see people looking like trees and walking."
Then he laid hands on his eyes a second time and he saw clearly; his sight was restored and he could see everything distinctly.
Then he sent him home and said, "Do not even go into the village."

Gospel commentary

Saint Jerome (347-420), priest, translator of the Bible, Doctor of the Church
Homilies on Saint Mark's Gospel, no. 8, 235 (trad. SC 494, p. 143)

Jesus put spittle on his eyes, placed his hands on him and asked him whether he could see anything. Knowledge always comes by degrees (...). It is only after a great deal of time and a long apprenticeship that we are able to attain perfect knowledge. First the impurities are removed, blindness goes, and thus light enters. The Lord's spittle is perfect teaching: to teach perfectly it comes from the Lord's mouth. The Lord's spittle, which comes forth, so to speak, from his substance, is understanding, just as the word coming forth from his mouth is a cure. (...)

“I see people looking like trees and walking”: I still see the shadow but not yet the truth. The meaning of these words is: I can see something in the Law but as yet I don't perceive the blazing light of the Gospel. (...) “Then he laid hands on his eyes a second time and he saw clearly.” He could see, I say, everything that we can see: he saw the mystery of the Trinity and he saw all the holy mysteries contained in the Gospel. (...) And we, too, see them since we believe in Christ, the true light.