Taiji is 3
Suppose you have only a 4 liter container and a 9 liter container. The containers have no measurement lines on either of them. How could you measure exactly 6 liters of water using only those containers assuming you can have as much extra water as you need?
After I solved the above puzzle, I had a realization on the statement "taiji is 3".
Taiji is separation of yin and yang. Yin and Yang are entities that are clearly separated from each other. In order to have yin and yang, there is something that separates/differentiates/demarks yin and yang, so it is neither yin or yang, and it is called Ling (灵）。 This is what makes taiji being 3, and not 2.
In the puzzle above, there are clearly 2 containers of size 9 liters and 4 liters respectively. The question is asking us to get 6 liters of water using only those 2 containers. 6 is neither the size of either container, nor the delta size between them. However, this task is made possible because there is a difference in the container sizes. If the two containers are of the same size, we can only get the amount that is single or double the container size.
9, 4 and the delta (5) make taiji. With the above setup, 0, 1, 3, 4, 5, 6, 8, 9, 10, 13 can be realized. Master Chen Zhonghua has said that we need 3 to make other things possible as illustrated in this example.
For the full solution to the puzzle, please see https://www.geeksforgeeks.org/measuring-6l-water-4l-9l-buckets/
Source: https://practicalmethod.com/?p=69875