Thanks for Nina for supplying the classic Theravada explanation. As
often, the commentaries are concerned to show how integrated this
teaching is within the path as a whole. Note especially the Tika's
wise reminder that the phrase ekaayana was not meant to exclude the
other path factors, including right samadhi. The idea that ekaayana
was meant to imply a distinction between satipatthana and jhana is
just a product of modern polemics, i fear.
I have done extensive research on this point and have come to
exactly the opposite conclusion (so there you go!). The word 'eka'
in meditation contexts always means 'ekaggataa' or 'ekodibhuuta',
that is, jhana or samaadhi. Since satipatthana is
the 'samaadhinimitta', i.e., the cause or basis of samaadhi, i think
ekaayana means that the purpose, or at least one central purpose, of
satipatthana is to get into jhana. There are several suttas in the
Satipatthana samyutta that emphasize this aspect of satipatthana,
using these kinds of terms.
As the examples from the commentary show, the Theravada tradition
did not have one clear idea of what the phrase meant. This shows
that either the word itself is very vague (in which case further
research is of dubious value), or that they had forgotten the
meaning.
Further evidence for the latter position comes from the Chinese, who
translated ekaayana differently in almost every occurence, and
clearly did not understand the term. In some cases they rendered it
as if from a different Indic original. For example, yi ru dao (=
eka+aayatana magga) or yi chu dao (= eka+yaana magga).
(If Stephen is reading this, perhaps he might give us some examples
from the Tibetan?)
One plausible explanation for this situation is that the term
originated in a context that was known to the Buddha and
contemporaries, but unknown to later writers, in other words, the
contemporary religious scene in 2500BC.
Our best evidence for that comes from the early Upanishads. The
earliest, most important, and largest Upanishad is the
Brihadarannyaka. The most important sage therein is Yajnavalkya, and
the most important dialogue, repeated twice, is the account of his
conversation with his wife Maitreyi on hi going forth. This dialogue
positively bristles with Buddhist parallels, one of which was noted
by Bhikkhu Bodhi in the CDB. Here is a translation (after
Radhakrishnan) of the climax of this dialogue (BU 2.4, 4.5):
11. `As the ocean is the convergence (ekāyana) of all waters, as the
skin is the convergence of all touches, as the nose is the
convergence of all smells, as the tongue is the convergence of all
tastes, as the eye is the convergence of all forms, as the ear is
the convergence of all sounds, as the mind is the convergence of all
thoughts (samkalpa), as the heart is the convergence of all
realizations (vidya), as the hands are the convergence of all
actions, as the genitals are the convergence of all pleasure, as the
anus is the convergence of all excretion, as the feet are the
convergence of all movements, as speech is the convergence of all
Vedas.
12. `Just as a lump of salt thrown in water becomes dissolved in the
water and there would not be any to be seized by the hand, but
wherever you might take it up it is salty; so too this great being,
infinite, with nothing beyond it, is a sheer mass of cognition
(vijñāna-ghana eva). Having arisen out of these elements, one
perishes back into them. When departed, there is no more perception
(samjñā). This is what I say, my dear.' So said Yājñavalkya.
13. Then said Maitreyī: `In this, indeed, you have confused me,
Venerable Sir, by saying that "when departed, there is no more
perception".' Then Yājñavalkya said: `Certainly I am not saying
anything confusing. This is enough for cognizance.'
14. `For where there is duality, as it were, there one smells
another, one sees another, one hears another, one addresses another,
one thinks of another, one cognizes another. But where All has
become identical with Self, then by what and whom should one smell
see hear speak think by what and whom should one cognize? That by
which all this is cognized, by what is that to be cognized? By what,
my dear, should one cognize the cognizer?'
It can hardly be clearer than this passage that 'ekaayana' is
referring to the return to the place of convergence, where all comes
together as one. It was precisely the term that was used, with such
overwhelming emphasis, by the Brahmanical tradition to express this
insight.
It should not need saying that the Buddha, of course, did not agree
that this was the 'Self'; but this Buddhist insight is on the plane
of wisdom, not of samaadhi. As far as samaadhi goes, i think the
Buddha was using the term in exactly the same sense.
Further evidence for the Brahmanical implications of ekaayana are
given to us on a platter in the canon. The phrase is actually put
into the mouth of Brahma, in the first discourse of the Satipatthana
Samyutta! The textual situation here is complex, but i have
concluded, after comparing with several versions of this discourse
available in Chinese, that this is actually the primary context of
the term ekaayana in the Suttas. A similar context also occurs in a
sutta of the Anuruddha Samyutta in Chinese, where Anuruddha
specifically says that the practice leads to one-pointedness of mind.
So to conclude, i think ekaayana was one of the many terms adopted
by the Buddha from the contemporary meditation context. It was used
by him in a similar way to the prevailing usage, and was usurped
from the Brahmans by having Brahma himself come down and praise it
as a Buddhist idea. The later traditions were not so familiar with
the Upanishads, etc., and so a wide variety of explanations grew up.
These were intended, not to convert Brahmans, but to fit ekaayana
within the wider Buddhist context. This is, in fact, quite
necessary, otherwise the term would become redundant when speaking
to a non-Brahmanical audience. (Perhaps the traditions even did this
knowing they were adapting the meaning). But the disadvantage is
that, with a welter of possibilities available, the term becomes
available for a new interpretation, one that is more narrowly
polemical.
Well, as this is a note, not a thesis, i'd better stop there!