"Then He came to Bethsaida; and they brought a blind man to Him, and begged Him to touch him. So He took the blind man by the hand and led him out of the town. And when He had spit on his eyes and put His hands on him, He asked him if he saw anything. And he looked up and said, 'I see men like trees, walking.' Then He put His hands on his eyes again and made him look up. And he was restored and saw everyone clearly. Then He sent him away to his house, saying, 'Neither go into the town, nor tell anyone in the town.'"
As this narrative unfolds, a blind man is brought to Jesus. Our Lord had already healed many blind men, but the miracle He would perform here would be distinct. Jesus preferred to heal the man privately (cf. Mark 7:33), so he took him aside and led him out of town.
Jesus then proceeded to spit in the man's eyes. Can you imagine a doctor doing such to a patient today? Why would Jesus do this? It seems unlikely that the spit would have anything to do with the miraculous cure, but since we have no understanding of how this (or any) miracle worked, we cannot discount the possibility. It is reasonable to suggest that the purpose of the saliva was to soften and soothe the man's eyes, which were probably sore. If Jesus used spit on the man's eyes simply because of compassion, then another lesson is suggested; that is, miracles were not used for a task that could be accomplished by natural means.
After laying His hands on him, Jesus asked the man if he saw anything. The man responded in Mark 8:24 - "I see men like trees, walking." This statement causes many to think that the man was not born blind since he was able to recognize (to some extent) men and trees. However, it also seems very possible that even if the man was born blind he would be able to distinguish between the two because of his tactile experiences with both.
"Then He put His hands on his eyes again and made him look up. And he was restored and saw everyone clearly" (Mark 8:25). This miracle is unique among all of the recorded miracles of Jesus in that it was done in two-parts. Many scholars suggest that Jesus healed this man in two stages to show that He had complete control over His miracles. If He chose to heal instantly, He could; but, if He chose to heal in stages, He could do that too. Jesus didn't fail in performing the miracle the first time; He merely chose to heal the man progressively.
Although the above interpretation is certainly correct, I personally believe there is more to this issue. There is a phenomenon known by neurologists as agnosia which may offer some insight into understanding the two-step process of this miracle. Agnosia is when an individual, though his sensory powers are not impaired in any way, becomes conceptually (not perceptually) blind (e.g., his eyes work perfectly, but when he looks at a staircase he sees nothing but a flat surface of parallel and criss-crossing lines, not solid objects going up and coming down in three-dimensional space). If it is the case that this blind man had never seen, is it not possible that Jesus completely restored his perceptual sight the first time, yet since the man had never interacted with the world by sight, it was very confusing to him? Humans who are born with sight gradually learn from infancy how to conceptually understand the stimuli that are taken in through the eyes. But, if an adult who has never seen is suddenly able to see, he would be overwhelmed by the "visual chaos" of his surroundings. It would take a great deal of time and effort for him to learn how to comprehend the information his eyes are relaying to his brain (e.g., color, distance, shape, shadow, texture, etc.). Thus, I believe that in the second stage Jesus miraculously "recalibrated" the man's brain, so to speak, so he would understand what he was seeing (i.e., He instantaneously gave him the conceptual understanding necessary to interpret what he was seeing).
Whichever view one prefers, the fact is unchanged that Jesus performed another amazing miracle. Praise be to the Lord Jesus Christ for His compassion and power!