When the First Chinese Patriarch Bodhidharma was asked, "Who are you?" he famously replied, "I don't know," or more accurately "Mu knowing," or "not knowing," or "not known," or "unknown," or even, "the unknown." Personally, I like "that which is unknown."
Five generations of students and teachers later, the Sixth Chinese Patriarch Hui-Neng asks a monk, "What is this that comes thus?" Both Hui-Neng's question and Bodhidharma's earlier answer were directly pointing at buddha-nature, that which cannot be grasped with the mind, but is the substrate from which the mind divides all things into separate entities. It is both empty of any individual identity and it is our potential to become a Buddha. The monk answered Hui-Neng by saying "To describe a thing misses the mark."
The Fourth Chinese Patriarch, meeting the Fifth Patriarch when the latter was just a young boy, asked "What is your name?" "What is your name?," "Who are you?," and "What is this that comes thus?" are all tests to see if the one being questioned grasps the essence of buddha-nature, whether the one asking the question realizes it or not.
The boy answers "I have a name, but it is an unusual one."
Intrigued, the Fourth Patriarch asks what this unusual name is.
"It is Buddha-nature," the boy replies.
"You do not have buddha-nature," the Fourth Patriarch tells the boy.
"Buddha-nature is emptiness," the boy replies, "so we call it being without."
Years later, the Fifth Patriarch asks Hui-Neng, the Sixth Patriarch, "Where are you from?" Do not think that just because the question this time is about geographical origination rather than individual identification that it is any different from the other three questions.
Hui-Neng answers straightforwardly, "I am from south of the Great Mountains."
The Fifth Patriarch asks him, "Why do you come here?"
Still answering straightforwardly, Hui-Neng says "I come to become a Buddha."
The Fifth Patriarch tells him, "People from south of the Great Mountains do not have buddha-nature. How can you expect to become a Buddha?"
This sounds rude, certainly provincial and possibly even racist, but the Fifth Patriarch was correct - people from south of the Great Mountains do not, in fact, have buddha-nature. But then, neither do the people from north of the Great Mountains. No one "has" buddha-nature in the same sense that no one "has," say, atoms. To say that I "have" atoms implies that there is a "me" not composed of atoms that can possess these particles, yet there is no "me" that isn't itself completely composed of atoms. I do not "have" atoms, I AM atoms. Similarly, we do not "have" buddha-nature, we completely and totally ARE buddha-nature through and through.
Hui-Neng understood. He calmly told the Fifth Patriarch, "North and south exist in the minds of people, but in buddha-nature there is no north and south."
So this test, "Who are you?" was thrown at Bodhidharma, the First Chinese Patriarch, and has been passed down from generation of teacher to generation of student over the years, being posed by the Fourth Patriarch to the Fifth, and by the Fifth Patriarch to Hui-Neng, the Sixth Patriarch, and by Hui-Neng to his students, the many teachers of the seventh generation.
This question still is being asked this day. Quick, without referring to any other person or relying on a name (a mere label applied on you by some other), who are you?