One is laughing

The Reading:

Shoyoroku - Case 46: Tokusan's "Study Accomplished"

Great Master Tokusan Emmyo instructed his assembly and said, "If you have exhausted to the end, you will realize right away that all buddhas in the three worlds have stuck their mouths to the wall [1]. Yet there is still one person he is giving a great laugh. If you can recognize that person, you have accomplished your study."


[1]: I.e., they are unable to open their mouths.



Discussion: Why could Tokusan at least given us a hint, so that there was room for doubt?