Chapter thirty-one carries a textual problem that has been discussed for centuries, and it is the clearest example in the book of a difficulty that affects the whole text.
Parts of the chapter read less like the compressed, rhythmic and frequently rhymed material around them than like somebody explaining that material. Several passages are longer, flatter and more expository than the book's usual manner, and they gloss terms that other chapters simply use. The traditional observation is that the Wang Bi recension, which carries a running commentary, has little or no comment attached to those particular stretches, and the natural explanation offered is that they were commentary in the first place and were absorbed into the main text by a copyist who could not tell the two apart.
That explanation is an inference, not a demonstration, and it is not settled. What makes it worth knowing is the general lesson rather than the verdict on this chapter. An early Chinese text transmitted by hand copying, without punctuation and often without any visual distinction between a passage and a note on it, will accumulate material of this kind, and the eighty-one chapters as they stand are the product of that process rather than of a single act of composition.
The manuscripts recovered in the twentieth century are useful here precisely because they are older than the recensions and were copied by people with different habits. They are not a pristine original, since they too are copies with their own errors, but comparing them with the received text shows where a passage is stable and where it is not.