A few reckless interpretations of mine: Joshu was able to determine what the monk ate, what type of dish he ate it from and whether he had washed it. This made the monk realize that his life was highly predictable, and he needed to vary it more. Or it made the monk realize that Joshu had Sherlock Holmes-style deductive capabilities. Or it made the monk realize that Joshu was telepathic. Alternatively, Joshu guessed incorrectly about the monk's diet and dishes, which made the monk realize that Joshu was no less susceptible to mistakes than anyone else.
Or it made the monk realize that Joshu was telepathic.
This seems refutable on the basis that monks probably enjoy a very restricted diet, and inferring telepathy from a correct guess in such a limited domain is both pretty unlikely and pretty silly.
Alternatively, Joshu guessed incorrectly about the monk's diet and dishes, which made the monk realize that Joshu was no less susceptible to mistakes than anyone else.
I really like this one. It contains a legitimate and worthwhile insight (that teachers are fallible), and I don't know how I'd go about refuting it.
Joshu Washes the Bowl
A monk told Joshu: `I have just entered the monastery. Please teach me.'
Joshu asked: `Have you eaten your rice porridge?'
The monk replied: `I have eaten.'
Joshu said: `Then you had better wash your bowl.'
At that moment the monk was enlightened.
Mumon's Comment: Joshu is the man who opens his mouth and shows his heart. I doubt if this monk really saw Joshu's heart. I hope he did not mistake the bell for a pitcher.
It is too clear and so it is hard to see.
A dunce once searched for fire with a lighted lantern.
Had he known what fire was,
He could have cooked his rice much sooner.