Chapter forty is the shortest in the book and the most quoted for its size. Returning is how the way moves. Weakness is how the way works. The ten thousand things under heaven are born from what is there, and what is there is born from what is not.
The first two clauses are a pair of definitions in the same frame: one names the characteristic motion of the way, the other its characteristic manner of operating. Both are stated without argument, and both are the summary of arguments made at length elsewhere. Returning is what chapter sixteen describes as things going back to their root, what chapter twenty-five reaches when it unfolds greatness into going, far and returning, and what chapter sixty-five calls turning back with all things. Weakness is chapter seventy-six, chapter seventy-eight and the entire water sequence.
The word rendered returning is worth pausing on. It does not mean retreat and it does not mean decline. It names the movement of something that goes out and comes back, the pattern of a season or a breath, and the claim is that this is not one behaviour among others but the way the way moves at all.
The third clause is a different kind of statement and is the reason the chapter is disputed. Being born from non-being can be read as a cosmological sequence in which nothing precedes something, or as a structural claim that whatever exists depends on what is absent, in the manner of chapter eleven with its hub, its vessel and its room. The two readings produced quite different later traditions, and the Chinese does not settle the question.