In the Visuddhimagga, when discussing conformity knowledge and change-of-lineage knowledge Ven. Buddhaghosa makes use of the following metaphor:
- Herein, conformity is able to dispel the murk of defilements that conceals the truths, but is unable to make Nibbána its object. Change-of-lineage is only able to make Nibbána its object, but it is unable to dispel the murk that conceals the truths.
- Here is a simile:  A man with eyes went out at night, it seems, to find out the conjunction of the stars, and he looked up to see the moon. It was invisible because it was concealed by clouds. Then a wind sprang up and blew away the thick clouds; another blew away the medium clouds; and another blew away the fine clouds as well. Then the man saw the moon in the sky free from clouds, and he found out the conjunction of the stars.
- Herein, the thick, medium and fine kinds of darkness that conceal the truths are like the three kinds of cloud. The three kinds of conformity consciousness are like the three winds. Change-of-lineage knowledge is like the man with eyes. Nibbána is like the moon. The dispelling of the murk that conceals the truths by each kind of conformity consciousness is like the successive blowing away of the clouds by each wind. Change-of-lineage knowledge’s seeing the clear Nibbána when the murk that concealed the truths has disappeared is like the man’s seeing the clear moon in the sky free from cloud.
- Just as the three winds are able only to blow away the clouds that conceal the moon but cannot see the moon, so the three kinds of conformity are able only to dispel the murk that conceals the truths but cannot see Nibbána. Just as the man can only see the moon but cannot blow away the clouds, so change-oflineage knowledge can only see Nibbána but cannot dispel the defilements. Hence it is called “adverting to the path.”
- For although it is not adverting, it occupies the position of adverting; and then, after, as it were, giving a sign to the path to come into being, it ceases. And without pausing after the sign given by that the change-of-lineage knowledge, the path follows upon it in uninterrupted continuity, and as it comes into being it pierces and explodes the mass of greed, the mass of hate, and the mass of delusion never pierced and exploded before (cf. Paþis II 20).
CHAPTER XXII Purification by Knowledge and Vision
This ties in with the Theravādin view that nibbāna is an externally existing dhamma which is cognised at the mind base. One that exists right now, but is obscured by the defilements. Interestingly, in the Paṭisambhidāmagga section on mindfulness of breathing we find this stanza:
Whose mindfulness of breathing in
And out is perfect, well developed,
And gradually brought to growth
According as the Buddha taught,
'Tis he illuminates the world
Just like the full moon free from cloud.
Which is a direct quote from the Mahākappinattheragāthā (Thag 10.3):
One who has fulfilled, developed,
and gradually consolidated
mindfulness of breathing
as it was taught by the Buddha:
they light up the world,
like the moon freed from a cloud.
So here we have a metaphor found in the Teragāthā that relates to the meditator who perfects mindfulness of breathing, which is then lifted by the Paṭisambhidāmagga in its own discussion of ānāpānasati. By the time we get to the Visuddhimagga this metaphor has changed somewhat, as it is now used to refer directly to awakening itself. The “moon” is now being taken as a direct metaphor for nibbāna, rather than the original usage of referring to the actual meditator. A quick search on SuttaCentral also returned the Upakkilesa Sutta (AN 4.50), which also makes use of the cloud and moon imagery in relation to defilements and the mendicant rather than nibbāna itself.
It would be interesting to see if this metaphor is also found in the northern sutras or Abhidharma texts, and if it ever relates to nibbāna rather than simply referring to the defilements and the meditator. Does this imagery of moon and clouds show up in the northern texts?