coffins of black – The claustrophobic confines of grimy chimneys may have seemed like living coffins to their young occupants, many of whom lost their lives through their job. It also refers to the idea that bodies are dead things. A platonic belief was that human bodies were more or less prisons for the soul.
However, Blake believed that it was mistaken to look for ‘release' in the future. He felt that humans do not need freeing from their bodies, but from the perception that reality can only be experienced through the senses. (Compare The Garden of Love and To Tirzah.) Therefore, focussing the child only on the hope of release in the future gives him a false idea about his body, and so about his freedom to change his life.