The first I can remember anyone talking about this wall is when I read some Plato. He wrote about a cave in which there was a light that threw shadows on the walls. These were the shadows of the ideals of the things we see here in this realm. It seemed to me to be his description of the fact that we cannot see or know directly the source of the things that exist and live around us.
I used to live in a farming community and would ask the people working there if anyone understood what made a seed grow. Some of them had, after all, been to school to study agriculture and would presumably understand the chemical processes that resulted in the transformation from a kernel to a plant. I would point out, however, that in order to answer the question properly you would have to have a comprehensive understanding of the universe – what ultimately controlled the climate, what brought the soil to the point at which plants could be nurtured in it, etc.
This shadow of life that we see extends also to ourselves. We seem only to understand ourselves indirectly, and most of the time, not at all. In the midst of this, we experience the pushes and pulls of life around us, making it all the more inexplicable and uncontrollable as we go along. Being a craftsperson in this is an exhilarating and frightening ride, sitting as you do between the wall of the wood and the wall of yourself.