I know this is a big topic, but I have a specific question.
In Bikkhu Nanamoli's translation of Buddhaghosha's Visuddhimagga (The Path of Purification), three widely cited passages (XIV.141, XIV.96, 98) say about Mindfulness that it has "the characteristic not wobbling" or "the function of preventing wobbling".
That's always confused me because it sounds more like Concentration. In fact, III.4 says: "Concentration has non-distraction as its characteristic. Its function is to eliminate distraction. It is manifested as non-wavering."
Not knowing any Pali, I may be missing some subtleties here, but this makes is sound like Mindfulness and Concentration share the aspect of "non-wobbling", "non-distraction", "non-wavering" which all seem in English to be the same or very similar. Or at least "non-wobbling" and "non-wavering" seem like synonyms in English -- what are the Pali words here? Maybe "non-distraction" is something else.
Anyway, why would the Buddha ascribe such a crucial characteristic/function to two supposedly distinct elements of the Eightfold Path? Or am I being too pedantic (and modern?) to expect that kind of crisp orthogonality. In any case, my gut interpretation says that these are about Concentration, not Mindfulness. Or maybe it's saying that Mindfulness is never present without some measure of Concentration?
To make matters a bit worse, I.141 also mentions "non-wavering". That's the chapter on Sila, but I can't tell what the referent of "non-wavering" is.