The question was not casual. In Buddhist philosophy, all sentient beings possess Buddha-nature — the seed of awakening, the inherent capacity for liberation. A dog is a sentient being. Therefore, by the logic of the doctrine, a dog has Buddha-nature.
The monk asking the question knew this. He had studied it. He was asking something else — perhaps whether the doctrine was literally true, perhaps whether he himself had Buddha-nature, perhaps what Buddha-nature actually was and where it was located and whether the doctrine was pointing at something real or merely being logically consistent.
Zhaozhou said: Mu.
In Chinese, wu means no, not, without, negation. It is a denial. But it is also emptiness. And it is also the specific sound that the tradition holds as the most direct available pointer at what cannot be said.
If the monk says: you said Mu, which means no, so dogs do not have Buddha-nature — he is wrong. The doctrine says they do. If the monk says: you said Mu which is a trick, you really mean yes — he is also wrong. Zhaozhou said what he said. If the monk says: Mu is emptiness, so you are pointing at the emptiness that is Buddha-nature — he is getting closer but he is still constructing an interpretation rather than encountering the thing.
Every answer is wrong. The wrong answers are the teaching.
The Zen master Wumen, who compiled the Mumonkan — the collection in which this koan appears — wrote: do not try to explain Mu through reasoning. Do not understand Mu as nothingness. Do not understand Mu as yes or no. Arouse the entire three hundred and sixty bones and joints, and the eighty-four thousand pores, to become one mass of doubt about Mu. Work at this day and night. When you have done this for a long time and the questioning becomes completely unbroken, you will find that you are like a stupid person who cannot sleep at night or find peace by day. Then one morning there will be an explosion — greater than a thunderclap — and you will have opened your own eye. The koan is not a puzzle with a hidden answer. It is a device for exhausting the mind that wants to find a hidden answer, until the mind that wants to find gives up — and in the giving up, the question that was generating the wanting dissolves.
Mu has been worked for twelve centuries. Millions of students in the Rinzai Zen tradition have sat with Mu as their first koan — the entrance to the practice. Most of them sat with it for years before something shifted. Some sat with it for decades.
The dog is still there. The question is still being asked. Zhaozhou is still saying Mu.
The mind that wants the answer is the obstacle. When the obstacle exhausts itself looking for what is not an answer — the specific quality of attention that remains when the looking has stopped is the only thing the koan was ever trying to produce.
Mu. Work on this.