Looks like it has more to do with day-to-day science, than to spirituality. Each of our eyes has a single lens reflecting the image on the retina in inverted form. The retinas send signals to be brain on ' as is where is' format. It is left to the brain to interpret the images sent by the retinas. In the first few frames, the brain thinks' that the images sent are actually Upside Up. After gaining some experience, the brain learns to perceive things before our eyes as they actually are, taking the signals sent by the retinas with a pinch of salt'. The man that Jesus healed would have been born-blind and cent per cent blind. The first images he sees after the healing, are of people moving. Naturally, the images of the first few shots of his eyes, are inverted. With the given method of dressing in Jesus'time people naturally looked like tree trunks in inverted position, to a man with newly opened eyes !It would take his brain many more views to adjust to the up-side-upness of people and things. Now, from where did he get the idea of trees ? Science, again says that in blind people, absence of sense of sight is compensated by enhanced senses of touch, smell, hearing and taste. So, the blind man had plenty of idea on how trees looked like , deep inside his brain! Finally, by touching the eyes of the man Jesus sped up his reasoning power to enable him see thing upside up, which otherwise would have taken him days together.
