A certain monk went to another monk and, on arrival, said to him, "To what extent, my friend, is a monk's vision said to be well-purified?"

"When a monk discerns, as it actually is, the origination & passing away of the six media of sensory contact, my friend, it is to that extent that his vision is said to be well-purified."

The first monk, dissatisfied with the other monk's answer to his question, went to still another monk and, on arrival, said to him, "To what extent, my friend, is a monk's vision said to be well-purified?"

"When a monk discerns, as it actually is, the origination & passing away of the five clinging-aggregates, my friend, it is to that extent that his vision is said to be well-purified."

The first monk, dissatisfied with this monk's answer to his question, went to still another monk and, on arrival, said to him, "To what extent, my friend, is a monk's vision said to be well-purified?"

"When a monk discerns, as it actually is, the origination & passing away of the four great elements [earth, water, wind, & fire], my friend, it is to that extent that his vision is said to be well-purified."

The first monk, dissatisfied with this monk's answer to his question, went to still another monk and, on arrival, said to him, "To what extent, my friend, is a monk's vision said to be well-purified?"

"When a monk discerns, as it actually is, that whatever is subject to origination is all subject to cessation, my friend, it is to that extent that his vision is said to be well-purified."

The first monk, dissatisfied with this monk's answer to his question, then went to the Blessed One and, on arrival, having bowed down to him, sat to one side. As he was sitting there he [reported to the Blessed One his conversations with the other monks. The Blessed One then said:]

"Monk, it's as if there were a man who had never seen a riddle tree.[1] He would go to another man who had seen one and, on arrival, would say to him, 'What, my good man, is a riddle tree like?"

"The other would say, 'A riddle tree is black, my good man, like a burnt stump.' For at the time he saw it, that's what the riddle tree was like.

"Then the first man, dissatisfied with the other man's answer, went to still another man who had seen a riddle tree and, on arrival, said to him, 'What, my good man, is a riddle tree like?'

"The other would say, 'A riddle tree is red, my good man, like a lump of meat.' For at the time he saw it, that's what the riddle tree was like.

"Then the first man, dissatisfied with this man's answer, went to still another man who had seen a riddle tree and, on arrival, said to him, 'What, my good man, is a riddle tree like?'

"The other would say, 'A riddle tree is stripped of its bark, my good man, and has burst pods, like an acacia tree.' For at the time he saw it, that's what the riddle tree was like.

"Then the first man, dissatisfied with this man's answer, went to still another man who had seen a riddle tree and, on arrival, said to him, 'What, my good man, is a riddle tree like?'

"The other would say, 'A riddle tree has thick foliage, my good man, and gives a dense shade, like a banyan.' For at the time he saw it, that's what the riddle tree was like.

"In the same way, monk, however those intelligent men of integrity were focused when their vision became well purified is the way in which they answered.

"Suppose, monk, that there were a royal frontier fortress with strong walls & ramparts and six gates. In it would be a wise, experienced, intelligent gatekeeper to keep out those he didn't know and to let in those he did. A swift pair of messengers, coming from the east, would say to the gatekeeper, 'Where, my good man, is the commander of this fortress?' He would say, 'There he is, sirs, sitting in the central square.' The swift pair of messengers, delivering their accurate report to the commander of the fortress, would then go back by the route by which they had come. Then a swift pair of messengers, coming from the west... the north... the south, would say to the gatekeeper, 'Where, my good man, is the commander of this fortress?' He would say, 'There he is, sirs, sitting in the central square.' The swift pair of messengers, delivering their accurate report to the commander of the fortress, would then go back by the route by which they had come.

"I have given you this simile, monk, to convey a message. The message is this: The fortress stands for this body — composed of four elements, born of mother & father, nourished with rice & barley gruel, subject to constant rubbing & abrasion, to breaking & falling apart. The six gates stand for the six internal sense media. The gatekeeper stands for mindfulness. The swift pair of messengers stands for tranquillity (samatha) and insight (vipassana). The commander of the fortress stands for consciousness. The central square stands for the four great elements: the earth-property, the liquid-property, the fire-property, & the wind-property. The accurate report stands for Unbinding (nibbana). The route by which they had come stands for the noble eightfold path: right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration."