[1] A certain monk visited another monk and said: "Tell me, friend, how does a monk's vision become fully purified?"

"Friend, when a monk fully comprehends as they really are the arising and passing away of the six spheres of contact,[2] to that extent his vision becomes fully purified."

But that monk was dissatisfied with the other's answer, so he went to another monk...

"Friend, when a monk fully comprehends as they really are the arising and passing away of the five groups of clinging,[3] to that extent his vision becomes fully purified."

But that monk was dissatisfied with the other's answer, and he went to another monk...

"Friend, when a monk fully comprehends as they really are the arising and passing away of the four great elements,[4] to that extent his vision becomes fully purified."

But that monk was dissatisfied with the other's answer, so he went to another monk...

"Friend, when a monk fully comprehends as it really is that whatever is of a nature to arise is bound to pass away,[5] to that extent his vision becomes fully purified."

But that monk was dissatisfied with the other's answer, and he went to the Blessed One...

"Suppose, monk, a man had never seen a 'what's it' tree. So he went to someone who had seen one,[6] and said: 'Tell me, good sir, what does a "what's it" tree look like?' The other man replied: 'Well now, a "what's it" tree is sort of blackish, like a burnt stump.' But that man was not long satisfied with this answer, so he went to another man... 'Well now, a "what's it" tree is sort of reddish, like a lump of meat.' But that man was not long satisfied with this answer, so he went to another man... 'Well now, a "what's it" tree is stripped of its bark, with burst pods, rather like an acacia.' But that man was not long satisfied with this answer, so he went to another man... 'Well now, a "what's it" tree has very thick leaves. It gives close shade like a banyan.' So for a while he sees the 'what's it' tree as that man sees it.

"In the same way, monk, according to the way the vision of these wise monks[7] has become purified, so they explain it, each according to his disposition.

"It is, monk, just like some king's border-city, with strong walls and arches, and with six gates. And there is a wise, skilled and experienced gatekeeper who keeps out the unknown and admits the known. From the east there come a swift pair of messengers, and they say to the gatekeeper: 'Where is the lord of this city?' — 'He is over there, sirs, at the cross-roads in the center.' Then the two messengers take a truthful report to the lord of the city, and return the way they came. [Likewise from the west, south, north.]

"I have given you a parable, monk, and so that you can understand it, this is the explanation: 'The city' is a name for the body, composed of the four great elements, born of mother and father, an accumulation of rice and gruel, impermanent, subject to abrasion, dissolution and disintegration. 'The six gates' denote the six internal sense-spheres.[8] 'The gatekeeper' denotes mindfulness. 'The two swift messengers' denote calm and insight.[9] 'The lord of the city' denotes consciousness. 'The cross-roads at the center' denotes the four great elements: the earth-element, the water-element, the fire-element, the air-element. 'The truthful report' denotes Nibbaana. 'The way they came,' monk, denotes the Noble Eightfold Path, that is to say Right View, Right Thought, Right Speech, Right Action, Right Livelihood, Right Effort, Right Mindfulness, Right Concentration."