The door is a symbol of secrecy and avoidance, a symbol of the quiet or sad secret struggle of the mind hiding in bliss. A house without a door is not a house, but a hallway. Wherever a man is, he can make himself at ease as long as he is behind a closed door. The mind works most effectively in a closed door. People are not horses that graze together.
People need to have a way to eliminate external interference in order to improve the thinking power of the brain is the correct interpretation of this paragraph.