"A monk once asked Master Joshu, 'Has a dog the Buddha Nature, or not? Joshu said, 'Mu!"
In his preface to the Mumonkan Mumon tells us that when he compiled these 48 koan, he did so randomly; they are in no special order. One must wonder, however, if this holds true of the first which has laid such a claim of importance among the practitioners of Rinzai Zen. And it may be only coincidental, but Mumon himself, in discussing this koan, makes such an impassioned and inspired appeal to the pursuit of the ultimate Zen experience through it that one cannot help but see it as special.
"Mu" simply means "no". But it also, less simply, means "nothing" or "nothingness".
Watts tells us that the orthodox view answers "yes", and thus Mumon's "no" is intended to break through the normal operation of the discriminating mind, which is essentially the aim of every koan. The question we might first ask then is whether Mu is the equivalent of, in answer to what is the Buddha, "the cypress in the courtyard" or "three pounds of flax". In other words, does it have any particular meaning beyond these nonsensical curveballs? Clearly, it does. Mumon writes, "Now, tell me, what is the barrier of the Zen Masters? Just this 'Mu' — it is the barrier of Zen." And again: "Concentrate yourself into this 'Mu'....Now, how should one strive? With might and main work at this 'Mu', and be 'Mu'." We are never exhorted to become the cypress in the courtyard.
So, Mu seems to have some definite content. Yet, of course, it cannot. It is the gateless barrier, Mumon tells us, and it opaquely stands like every other 'answer' as an occasion for the transcendence of the discriminating mind. But this, of course, would be to "be Mu". It both is and is not the essential Zen experience.
"Do not attempt nihilistic or dualistic interpretations. It is like having bolted a red hot iron ball. You try to vomit it but cannot." Mu offers nothing if we simply try to understand what it means. If it means 'nothing', then the only way to 'understand' it is to be nothing. Yet, within nothing, there is not even any conception of nothing. Every attempt to understand it on this side of the barrier is its negation. The “red hot iron ball” in our guts is this dilemma and the agony of knowing this while still in the thrall of the discriminating mind.
To my thinking, Mu should not be thought of as a negation — nothingness is beyond negation and indeed, equally affirms all things. If there is nothing anywhere, there is nothing everywhere. It is all things. I make this point to suggest that, in going beyond the discriminating mind to where yes and no do not apply, “Yes” is an exclamation ever much as helpful a point of departure as “Mu”. This is where Daoism and Zen seemingly diverge.
You can check out Scott's other miscellaneous writings here.