Wumen says, “In studying Zen, one must pass the barriers set up by the ancient Zen Masters. What is the barrier of the Zen Masters? Just this Mu.” Feeling the definitive tone of this commentary, we are invited to turn to it with a serious and genuine attitude, with an openness of mind and heart. It’s a first meeting with yourself. You don’t want to begin falsely.

In our practice we sit with Mu. We use it to deepen our concentration and our appreciation of the nature of mind, the nature of our attachments and of those things that obscure the inherent clarity of who we are. We sit into bottomless zazen, but we sit with a specific question: what is Mu? We may want to let go into some effusive vastness of the universe, and yet we are held back by this demand. What is it? To sit with Mu is to sit with a gatekeeper to the realm of reality. It is to make contact with a teacher in the Zen tradition, with those ancient masters who set up this path of mystic realism, and with your teacher who pares away the layers of falseness and frivolity. Because just as there is a gate, a gateless gate, there is a gatekeeper that will either let you in or not. And that entry point is based on something real, something concrete—your relationship to your mind, as opened and entered through, “What is Mu?”

Mu is set up for you and me to enter and recognize the nature of the mind-to-mind transmission, to recognize what it means to live in a tradition that for eighty-two generations has been able to perpetuate a spark of clarity and offer it to others. Wumen is sitting with Zhaozhou. Maezumi Roshi, our dharma grandfather, is sitting with Wumen, Daido Roshi is sitting with Maezumi Roshi. You are sitting with all of them. And all of it begins as a spontaneous encounter between Zhaozhou and a monk who one day asks a simple question: “Has a dog the buddha nature, or not?” The response is Mu. No. What happened in that encounter—two people, two sentences, five seconds? It’s a dialogue that has reverberated, been repeated and implanted in the minds of thousands of Zen practitioners. In fact, I suspect that more people might have taken up these two simple sentences with full weight of seriousness, maybe desperation, than any other words in the human discourse. Imagine how much energy has been offered to this simple exchange that on the surface seems almost trivial. Someone is asking Zhaozhou about the presence of buddha nature, the absolute principle of reality, in a dog. Shakyamuni Buddha and all the teachings up till that point unequivocally declared that every thing—every atom, every thought, every aspect of the universe, every creature regardless of its state of clarity or depravity—is buddha nature itself. No exceptions. Yet here comes Zhaozhou, not only denying the fact that buddha nature is all-pervasive, but apparently placing himself on a cusp of dualistic thinking. Does a dog have buddha nature? No, despite what has been said previously, it doesn’t. What’s Zhaozhou doing?

There’s an illuminating exchange between another monk and Zhaozhou in which the practitioner asks, “What is the meaning of Bodhidharma’s coming from the West?” Zhaozhou responds, “The oak tree in the courtyard.” The monk persists, “Please don’t teach me with a reference to outside things.” Abandon the dualistic framework. Zhaozhou corrects, “I don’t teach with a reference to outside things.” There’s nothing dualistic here. The monk continues, “Then what is the meaning of Bodhidharma’s coming from the West?” Zhaozhou rests, “The oak tree in the courtyard.”

What realm is Zhaozhou inhabiting? Is it the realm of freedom? Is it the realm where no is something other than no, yes surpasses yes? Mu doesn’t weigh you down or tangle you up like a ghost clinging to a branch. Instead, it opens the doorway to freedom. Mu drives you into doubt. Mu breaks that doubt open. Dogen’s teacher, Jujing, has the following commentary on Mu:

Thoughts in the mind are confused and scattered; they’re clinging. How can they be controlled? In the story about Zhaozhou and whether or not a dog has buddha nature, there is an iron broom named Wu. If you use it to sweep thoughts, they just become more numerous. Then you frantically sweep harder, trying to get rid of even more thoughts. Day and night, you sweep with all your might, furiously working away. All of a sudden, the broom breaks into vast emptiness, and you instantly penetrate the myriad differences and thousand variations of this universe.