We can think of a Buddhist lineage or Buddha gotra as a tradition. The word 'tradition' comes from the Latin verb ‘tradere’ which means to transmit. Thus, in a tradition there is a transmission. But tradition also refers to what is transmitted, namely, a traditum. Putting this together, in a tradition something is transmitted. But what is transmitted, in the case of Buddhism, and in particular Zen? Is it a robe, or a bowl. Are documents, seals, or certificates transmitted? If we think about this and answer that the Dharma is transmitted then what is the substance of this transmitted Dharma?
With other traditions it is easy to guess what they transmit. It might be a skill or a secret recipe that has been in our family for many generations. In the example of this recipe we can actually taste the tradition. In other traditions, what is transmitted might be a certain blend of teas, or a particular trade skill that has been handed down as is often the case in Japan. It could also be the tradition of violin making or weaving Persian carpets.
But now we must ask ourselves, in the highest tradition of Buddhism, what is transmitted? What, for example, did the Buddha know and see that other Buddhas before him also realized? Of course, we can posit some answer like, true reality (yathabhutam). But what it is like to receive this traditum? How is it different than a strong belief by which I am persuaded that I have been transmitted?
For Zen master Huang-po (d. 850) the traditum was the One Mind (ekacittam) which, instead of being pluralized, is mind which is purely, itself, or one. In this transmission, even the plural form of mind is recognized to be mind-only whereas before we were unable to distinguish the One Mind from its pluralization into a riot of phenomena.
Who transmits the One Mind is simple: it is the Mind, itself, if we can get into the range for this event to happen by letting go of the baggage of the plural mind. What is important to grasp is that we don’t actually need a teacher to transmit us. The Buddha, the Pali Nikayas inform us, never transmitted anyone. He simply helped people to open their heart up to allow the Mind to Mind transmission as it is called in Zen literature.