Gettan Oshõ said, "Keichû, the first wheelmaker, made a cart whose wheels had a hundred spokes. Now, suppose you took a cart and removed both the wheels and the axle. What would you have?"
If anyone can directly master this topic, his eye will be like a shooting star, his spirit like a flash of lightning
When the spiritual wheels turn,
Even the master fails to follow them.They travel in all directions, above and below,
North, south, east, and west.
This is a logical problem similar to the Ship of Theseus, in which the question is asked, if, gradually over time, you were to replace every plank forming a ship, when no pieces of the original construction remain, is it still the same ship? Or a more simplified version I've come across: "This hatchet has been in my family for so long that the head has been replaced four times, and the handle three." If both the head and handle have been replaced and nothing of the original hatchet remains, is it the same hatchet?
In this case, we have the story of Keichu the Wheelmaker, and his miraculous cart. If you dismantle it such that the wheels and the axle are gone, what do you have? Is it still a cart? More directly, is it only a cart if it has the function of a cart? Is that function what determines that it is the form of "cart"? Or is it a cart whether it functions as a cart or not? The similarity to the Ship of Theseus is in this issue of how we recognize something as what it is.
This is not just limited to things. Scientifically we know that your body's cells are largely replaced such that you are not made of the same pieces today that you were ten years ago. Even setting that modern understanding aside though, it is clear that everything is in a state of constant change. There is no permanence. So the question begins to strike close to home very quickly. What are you if we were to take away the parts that make you, you.
Of course, it's rarely a simple matter of choosing a correct answer with Zen. This is a practice geared toward casting aside dualistic, conceptual thought, so "is it a cart or not" must be immediately thrown out. "Cart" is simply a conceptual label, and distinguishing "cart" from "non-cart" is engaging in dualism. By entertaining the debate of form and function, we've stepped into the trap. When conceptual and dualistic thought are cast aside, there is no separation between cart and non-cart.
No thing is distinct unto itself, all are simply components and expressions of one absolute, or what Huang Po called the One Mind. Mumon, in his verse, calls it the "Spiritual Wheel." From the perspective of this absolute, the forms and functions of "cart" take shape and dissipate freely. Nothing is created or destroyed, only expressed in one form or another, one function or another, with those forms and functions always in a state of flux. This is what is meant when Mumon says that the Spiritual Wheel turns in all directions. The Absolute, the totality of everything, the one whole that is the Universe, which delusively divide into chunks we identify as "carts" and "ships" and "hatchets" and "I," that Absolute is in a state of constant motion, constant change, in all directions.