In the Shobogenzo, one of the many stories related by Zen Master Dogen is the following, which I find to be particularly interesting:
The venerable Nāgārjuna, while speaking from the Dharma Seat, revealed himself to be so free of any worldly ways that he looked like the orb of the Moon at its full. But all those assembled there merely heard the sounds of the teaching and did not observe the Master’s appearance.
One amongst them, however, Kānadaiba by name, the son of a town elder, said to those assembled, “Don’t you see his appearance?”
Those in the assembly said, “What we do not see with our eyes or hear with our ears right now does not exist, for it is not something that we can know with our minds or experience with our bodies.”
Kānadaiba said, “This is the Venerable One’s manifesting his Buddha Nature, by means of which he shows us how we can know it. By being cloaked in it, his meditative state, which is free of attachments, takes on a form resembling the Moon at its full, for the meaning of ‘Buddha Nature’ is that which is utterly unbounded and radiant.”
Once Kānadaiba had finished speaking, the orb-like look seemed to disappear.
If I can re-tell this story for a moment, Nāgārjuna is speaking and then suddenly, one of them, the son of a town elder, gets up and asks "Don't you see his appearance?" To everyone else, Nāgārjuna still looked perfectly normal, but to Kānadaiba he looked like the orb of the full moon.
Now that's just nutty stuff. If you or I were at an assembly, and someone suddenly declared that the speaker had changed form and now looked like the orb of the full moon and asked why we couldn't see it, we wouldn't conclude that the speaker had indeed transformed in some way that we couldn't perceive, but that the person who saw "the moon" was just plain nuts. The headline would read something like, "Local Lunatic Claims Ordinary Speaker Transforms Into Full Moon," if the press even bothered covering his outburst at all.
But Dogen's story doesn't read that way. In his story, Nāgārjuna does indeed transform into the orb of the full moon, but no one other than Kānadaiba can see it. The only record we have of this startling transformation is Kānadaiba's claim. To everyone else, Nāgārjuna looked perfectly normal. This really says something remarkable about Kānadaiba's powers of persuasion. The unusual thing in the story as Dogen tells it is not that Nāgārjuna transformed in appearance to the orb of the full moon, but that no one other than Kānadaiba was able to see it.
This story has historical precedents. Faiths around the world are based on a sole individual claiming to see remarkable things that no one else could perceive, and yet for some reason, everyone eventually accepts the otherwise irrational or supernatural version of the story rather than their own, subjective experience. There's a story of a man who heard God speak to him from a burning bush. Another claimed to literally be the son of God. A man upstate New York claimed to have been visited by angels who presented him with a radical new gospel. If anyone made similar claims today, they'd be locked up "for observations," yet whole religious traditions are based around just such claims.
For what it's worth, Buddhism doesn't use this account to support any claims of divinity for Nāgārjuna, or for Kānadaiba for that matter. The point of the story is that Buddha Nature is utterly unbounded and radiant, and that we are more than the bags of skin into which we were born.
Synesthesia is a neurological condition in which the sensations of one sense organ are perceived by the mind as a different sense, for example, "hearing" colors or "seeing" sounds. I wonder if Kānadaiba didn't have a form of synesthesia in which concepts, such as the radiance and unbound nature of Buddha Nature, was perceived in his mind as a "sight," in this case, the orb of the full Moon. To support this conjecture, once Kānadaiba finished speaking and other thoughts presumably entered his mind, the orb-like appearance disappeared.
Synesthesia, hallucinations, and remarkable powers of persuasion may be the one common element of all the world's religions.