|Trail to Taisan summit (photo by Scott on summitpost.org)|
It's helpful - but not essential - to know that Taisan is a sacred mountain full of temples to which many people make pilgrimages. So the old woman's comment appears to be a criticism of pilgrims for their practice of going to a 'sacred place' of any kind. This is solid Zen philosophy: Place or location have little or nothing to do with the Path or gaining enlightenment. If you cannot find it where you are right now, moving 5 feet away to the left or making a pilgrimage to a sacred mountain will not make any difference. I believe the disciples tell Joshu about the old woman because they assume her criticism reveals an understanding of Zen.
Joshu investigates the old woman, perhaps to determine whether she truly does have some understanding. If that is the purpose of his investigation then, as soon as she answers the question in the same way and gives him the same criticism, he realizes she possesses no true wisdom.
First, to provide the same guidance or criticism to everyone you meet is wrong-minded. It is generally true that a pilgrimage is not essential to enlightenment, yet even so and even if we assume that most or all of the people who pass her have wrong-minded reasons for heading to Taisan, one still cannot mouth the same platitude to all of them and pretend that it is wisdom (or even relevant to the person being addressed). This is why Zen does not proselytize nor transmit wisdom through literal explanation. At best, even the greatest teacher can only point the way. A seeker must find the Way for themselves.
Second, since Joshu is a master who is clearly already living on Taisan at this particular time, the old woman's answer proves that she is merely mouthing a platitude - because in this case the platitude is completely meaningless. Joshu was not asking her the way Taisan because he wanted to go there (he already was there!) and not because he was seeking enlightenment (he already had attained it!). Thus her criticism is off the mark. She is not evaluating the situation or person she speaks to, her pronouncement is like the repeated words of a parrot.
Joshu's remark at the temple lacks an explanation of what he found for two reasons: 1) He found nothing, and 2) It would be wrong for him to feed his conclusion to the disciples. They must think over what occurred and what Joshu said and perhaps find their own understanding of this event. By not explaining his conclusion, Joshu avoids making the same error as the old woman.