Tao can be talked about, but not the eternal Tao. Names can be named, but not the eternal name. As the origin of heaven-and-earth, it is nameless: As 'the mother' of all things, it is nameable. So, as ever hidden, we should look at its inner essence: As always manifest, we should look at its outer aspects. These two flow from the same source, though differently named; And both are called mysteries. The mystery of all mysteries is the door of all essence.