A door is to be opened, and a different world may be experienced when entered. As another world begins when we open a book, a door is a boundary at the beginning of another world. However, we cannot enter a new world as we would when opening a book with a curious mind if we open a door without giving any thought to the significance of the act.
Sometimes, I have a fancy that the door through which I entered may be a completely different door if I looked back at it. A door the front and the back of which are completely different. The size, the color, and material of either side are so different that it makes me think I entered into a totally different world. Looking into a mirror, I realized that my face and the back of my head were so different that I became curious how I myself had known nothing of it at all. Out of curiosity, I looked at things around me and found out there were so many things with completely different fronts and backs. Then, the thought did cross my mind that my appearance in everyday life and my appearance as I perceived it in my mind were totally different. Dr. Jekyll and Mr. Hyde are the same person but look completely unlike each other. Likewise, a door to another world is a plane of other world that is different.