This is the law of the middle line.
The donkey and the ox symbolize the right and left lines that can’t be joined together. The donkey is kindness, and the ox is stubbornness (the left line).
The left line is egoistic desires, and the right line is the altruistic qualities that we receive from above. However, they should be correctly balanced in the middle line.
The same is true regarding the ban on wearing clothes from mixed fabric. You can’t mix wool and linen that symbolize the two lines, because by this you cause a “short circuit,” and everything you do this way won’t give any good result.
From KabTV’s “Secrets of the Eternal Book,” 9/28/16