Jigme Lingpa recounts a visionary encounter with Yeshe Tsogyal in which she presents him with King Trisong Detsen's skull. (Jigme Lingpa was the King in a previous lifetime.)
Surprisingly, Tsogyal quickly takes back the skull, disappears and returns with two skulls—the first one and an identical one that is an optical illusion.
She demands that Jigme Lingpa distinguish between the two. When he is unable to do this,Tsogyal vanishes back into space and never returns.
When she disappeared, Jigme Lingpa realized that the correct answer was not about which was which. He understood that the answer was to recognize the unformulated, ineffable nature (spros bral) of everything.
But, that answer created another problem. To cite the the unformulatedness of everything would be formulated or conceptual. Then he realized that only a wordless way of indicating nonconceptuality would be correct.
The Flower Sermon, an 11th century story of the origin of Zen Buddhism recounts a similar situation where the Buddha held up a flower indicating:
Not everything, if anything, is knowable through the intellect!