Mk.8:17,18 – But Jesus, being aware of it, said to them, “Why do you reason because you have no bread? Do you not yet perceive nor understand? Is your heart still hardened? Having eyes, do you not see? And having ears, do you not hear? And do you not remember?
This conversation with His disciples took place between two miracles where we see Jesus using very personal and unusual measures. In Mk. 7:31-37 Jesus healed a deaf man. The method Jesus used was very personal, so personal as to be quite embarrassing. First, He physically led the deaf man away from the crowd, implying personal touch. Next, Jesus put His finger into the man’s ears. Then, He spat and touched the man’s tongue with His spittle. In Mk. 8:22-26 we see Jesus ministering to a blind man. His tactics were strangely similar. He grabbed the man by the hand and led him outside of the village. Then Jesus actually spat on his eyes and laid His hands on him. Partial healing occurred and then Jesus laid His hands on him again.
The message of these miracles is one and the same for His disciples. Here is an explanation from the ESV Study Bible.
“The disciples’ hearts are still partially closed to the depth of Jesus’ teaching and person, for he asks if they do not yet perceive or understand. Although Jesus does not rebuke them as hard-hearted “hypocrites” like the Pharisees and scribes (7:6), they still lack full understanding of who Jesus is. While the Pharisees reject Jesus’ teaching outright, the disciples are slow to appreciate it. The figurative reference to eyes and ears echoes the healings of the deaf man (7:31–35) and the blind man (8:22–26). Jesus wants to open the “ears” and “eyes” of the disciples’ hearts.”
Specifically what was the message of the healings? First, get out of the influence of your old “village”, come out of the world. Next, allow Jesus to move in close, maybe closer than you are comfortable with. Also, don’t shrink back from His unusual tactics. Obviously the spit and the touching is about as close as someone can get to us. This closeness is where we receive His impartation. The disciples had seen many miracles and heard great teachings from being with Jesus. Even with all of that, their vision and ability to hear Christ was clouded. They too saw the spiritual world as trees walking. Jesus was drawing them in closer that they could understand who He was and the powerful message of the cross.