Dear Stephen,

SH> I normally translate "nimitta" as "perceptual form" -- I would
SH> prefer "perceptual image" but I use that for "aakaara".

I wholeheartedly agree with you. Hopefully the translators with some
experience in meditation will make the translations deeper.

The practitioner develops mindfulness (sati) of what happens, naming
(apilapana) it in the certain frame of reference (satipatthana).

Next, he develops sampajana, fishing out the causal relations

Satisutta.m (уо 5.180)

“Sato, bhikkhave, bhikkhu vihareyya sampajaano. Aya.m vo amhaaka.m
anusaasanii”.

“Katha~nca, bhikkhave, bhikkhu sato hoti? Idha, bhikkhave, bhikkhu
kaaye kaayaanupassii viharati aataapii sampajaano satimaa, vineyya
loke abhijjhaadomanassa.m; vedanaasu …pe… citte …pe… dhammesu
dhammaanupassii viharati aataapii sampajaano satimaa, vineyya loke
abhijjhaadomanassa.m. Eva.m kho, bhikkhave, bhikkhu sato hoti.

“Katha~nca, bhikkhave, bhikkhu sampajaano hoti? Idha, bhikkhave,
bhikkhuno viditaa vedanaa uppajjanti, viditaa upa.t.thahanti, viditaa
abbhattha.m gacchanti. Viditaa vitakkaa uppajjanti, viditaa
upa.t.thahanti, viditaa abbhattha.m gacchanti. Viditaa sa~n~naa
uppajjanti, viditaa upa.t.thahanti, viditaa abbhattha.m gacchanti.
Eva.m kho, bhikkhave, bhikkhu sampajaano hoti. Sato, bhikkhave,
bhikkhu vihareyya sampajaano. Aya.m vo amhaaka.m anusaasanii”ti.

That is, practitioner tracks not only the phenomena per se, but
cause-and-effect relations of their origination and disappearance.

For example, feelings (vedana) are analysed in regard of specific
impressions (phassa) due to which they arise.

Samudayasutta.m (SN 5.184)

"Catunna.m, bhikkhave, satipa.t.thaanaana.m samudaya~nca
attha'ngama~nca desessaami. Ta.m su.naatha. Ko ca, bhikkhave, kaayassa
samudayo?
Aahaarasamudayaa kaayassa samudayo; aahaaranirodhaa kaayassa attha'ngamo.
Phassasamudayaa vedanaana.m samudayo; phassanirodhaa vedanaana.m
attha'ngamo.
Naamaruupasamudayaa cittassa samudayo; naamaruupanirodhaa cittassa
attha'ngamo.
Manasikaarasamudayaa dhammaana.m samudayo; manasikaaranirodhaa dhammaana.m
attha'ngamo"ti

As can be seen from Pathamarahogata sutta, the practice of
satipatthana is closely linked to the control of apperception
(sa~n~naa):

§ 45. Now when Ven. Anuruddha was meditating in solitude, this train
of thought appeared in his awareness: 'Whoever neglects the four
frames of reference neglects the noble path going to the right ending
of stress. Whoever undertakes the four frames of reference undertakes
the noble path going to the right ending of stress.'

Then Ven. Maha Moggallana, as soon as he perceived with his awareness
the train of thought in Ven. Anuruddha's awareness -- as a strong man
might extend his flexed arm or flex his extended arm -- appeared in
front of Ven. Anuruddha and said to him, 'To what extent are the four
frames of reference undertaken?'

Anuruddha: 'There is the case, my friend, of a monk who internally
remains focused on the phenomenon of origination with regard to the
body, remains focused on the phenomenon of passing away with regard to
the body, remains focused on the phenomenon of origination & passing
away with regard to the body -- ardent, alert, & mindful -- putting
aside greed & distress with reference to the world.

'Externally he remains focused on the phenomenon of origination with
regard to the body...

'Internally & externally he remains focused on the phenomenon of
origination with regard to the body, remains focused on the phenomenon
of passing away with regard to the body, remains focused on the
phenomenon of origination & passing away with regard to the body --
ardent, alert, & mindful -- putting aside greed & distress with
reference to the world.

'If he wants, he remains percipient of loathsomeness in the presence
of what is not loathsome. If he wants, he remains percipient of
unloathsomeness in the presence of what is loathsome. If he wants, he
remains percipient of loathsomeness in the presence of what is not
loathsome & what is. If he wants, he remains percipient of
unloathsomeness in the presence of what is loathsome & what is not. If
he wants -- in the presence of what is loathsome & what is not --
cutting himself off from both, he remains equanimous, alert, &
mindful.

(Similarly with regard to feelings, mind & mental qualities.)

'It is to this extent, my friend, that the four frames of reference
are undertaken...'

-- SN LII.1

http://www.accesstoinsight.org/lib/modern/thanissaro/wings/2b.html#passages

Hence the practice of Satipatthana includes the development of
faculties, fully described in Indriya-bhavana sutta (MN 152)
http://www.accesstoinsight.org/canon/sutta/majjhima/mn152.html

The next factor to develop is dhamma-vicaya, i.e. discrimination of
dhammas (mental qualities). Practitioner generalizes cause-and-effect
relations into certain mental qualities, and discriminates skilful and
unskilful among them:

S v 102 (Ahara sutta):

"Ko ca, bhikkhave, aahaaro anuppannassa vaa
dhammavicayasambojjha'ngassa uppaadaaya, uppannassa vaa
dhammavicayasambojjha'ngassa bhaavanaaya paaripuuriyaa? Atthi,
bhikkhave, kusalaakusalaa dhammaa saavajjaanavajjaa dhammaa
hiinapa.niitaa dhammaa ka.nhasukkasappa.tibhaagaa dhammaa. Tattha
yonisomanasikaarabahuliikaaro- ayamaahaaro anuppannassa vaa
dhammavicayasambojjha'ngassa uppaadaaya, uppannassa vaa
dhammavicayasambojjha'ngassa bhaavanaaya paaripuuriyaa.

"And what is the food for the arising of unarisen analysis of
qualities as a factor for Awakening, or for the growth & increase of
analysis of qualities... once it has arisen? There are mental
qualities that are skillful & unskillful, blameworthy & blameless,
gross & refined, siding with darkness & with light. To foster
appropriate attention to them: This is the food for the arising of
unarisen analysis of qualities as a factor for Awakening, or for the
growth & increase of analysis of qualities... once it has arisen.

http://www.accesstoinsight.org/canon/sutta/samyutta/sn46-051.html

Next factor to develop is virya. Practitioner, with four right
endeavours, aroses and develops skilful qualities, abandons and
weakens unskilful ones.

How exactly does he do it? With the help of appropriate attention:

"[7] And what are the fermentations to be abandoned by developing?
There is the case where a monk, reflecting appropriately, develops
mindfulness as a factor for Awakening dependent on seclusion...
dispassion... cessation, resulting in letting go. He develops analysis
of qualities as a factor for Awakening...persistence as a factor for
Awakening...rapture as a factor for Awakening...serenity as a factor
for Awakening...concentration as a factor for Awakening...equanimity
as a factor for Awakening dependent on seclusion... dispassion...
cessation, resulting in letting go. The fermentations, vexation, or
fever that would arise if he were not to develop these qualities do
not arise for him when he develops them. These are called the
fermentations to be abandoned by developing.

http://www.accesstoinsight.org/canon/sutta/majjhima/mn002.html

Samudayasutta.m (SN 5.184)

Manasikaarasamudayaa dhammaana.m samudayo; manasikaaranirodhaa
dhammaana.m attha'ngamo"ti

As described in Ahara sutta, he directs his attention to those
nimittas (perceptual images) that encourage skilful qualities, and
does not pay attention to those nimittas that encourage unskilful
qualities. Samadhi-nimitta, conducive to jhana, is but one of those
perceptual images. The development of skilful qualities does not come
'naturally', but with fundamental attention.

The rendering of 'nimitta' as psychological term
'perceptual image', and 'sa~njaanana' as 'recognition' is evident from
the Visuddhimagga passage (XIV 130).

sabbaa va sa~njaanana-lakkha.naa, tad ev’etan ti puna
sa~njaanana-paccaya-nimitta-kara.na-rasaa daaru-aadiisu tacchakaadayo
viya, yathaa-gahita-nimitta-vasena abhinivesakara.na-paccupa.t.thaanaa
hatthi-dassaka-andhaa (udaa. 54) viya,
yathaa-upa.t.thita-visaya-pada-.t.thaanaa ti.na-purisakesu
miga-potakaana.m purisaa ti uppanna-sa~n~naa viyaati.

Nimitta is the image one forms in the process of apperception
(sa~n~naa), used to recognize the object in the future.

The same principle is applied in the practice of samadhi, when
practitioner reaches the jhana again with the help of grasped
perceptual image:

Samathova ta.m aakaara.m gahetvaa puna pavattetabbassa samathassa
nimittanti samathanimitta.m.

(Mohavicchedani Mya: .161)

The practitioner applies the four endeavours by directing his
attention to some perceptual images and away from others:

AN 2.16 = DN 3.225:

14. “Cattaarimaani, bhikkhave, padhaanaani. Katamaani cattaari?
Sa.mvarappadhaana.m, pahaanappadhaana.m, bhaavanaappadhaana.m,
anurakkha.naappadhaana.m. Katama~nca, bhikkhave, sa.mvarappadhaana.m?
Idha, bhikkhave, bhikkhu cakkhunaa ruupa.m disvaa na nimittaggaahii
hoti naanubya~njanaggaahii…
…
“Katama~nca, bhikkhave, anurakkha.naappadhaana.m? Idha, bhikkhave,
bhikkhu uppanna.m bhaddaka.m samaadhinimitta.m anurakkhati
a.t.thikasa~n~na.m pu.lavakasa~n~na.m viniilakasa~n~na.m
vicchiddakasa~n~na.m uddhumaatakasa~n~na.m. Ida.m vuccati, bhikkhave,
anurakkha.naappadhaana.m. Imaani kho, bhikkhave, cattaari
padhaanaanii”ti.

In this way, with fundamental attention, practitioner develops all
seven factors of Awakening, and thus reaches Awakening.

Best wishes,
Dimitry