NvG> Just a remark about ruupasa~n~nii : does this not refer to ruupajhaana,
NvG> whereas aruupasa~n~nii refers to aruupajhaana?
The kasinas allow to attain only form jhanas, and in the cited sutta
DN 33, MN 77 there is a clear reference to aruupasa~n~nii with colour
kasinas. Moreover, after this stage and the stage of 'subha' proper
aruupa jhanas are described. Thus 'aruupasa~n~nii' here should mean
something else.
NvG> When the
NvG> yogavacara sees colour or form, colour of kashmire cloth, etc. he is
NvG> reminded of the kasina which is his meditation subject and can attain fine
NvG> material jhaana.
This is the function of apperception: selective discrimination of
sense impressions. When the apperception is properly trained, any
similar object can serve as a kasina.
We often see the reverse: when the apperception is untrained, any
similar object can serve as a reminder for passion.
Here's a definition from Vimuttimagga (page 78):
"And again, it is said that the meaning of wisdom is the meaning of
sign [nimitta]. The Buddha has declared: "With trained perception
[sa~n~naa] one should forsake". This is called wisdom."
I am intrigued reading that nimitta can be a basis of concentration.
Apparently nimitta is something quite tangible, not just a designation
(pa~n~natti).
'Lakkha.na' is an essential characteristic, whereas 'nimitta' is more
on the subjective, outward side, appearance, - which again reminds of
apperception.