We read about the benefits of Anapanasati in the Visuddhimagga. We read
Vis. VIII, 239: <Also its great beneficialness should be understood as the
root condition for the perfecting of clear vision and deliverance for this
has been said by the Blessed One: 'Bhikkhus, mindfulness of breathing, when
developed and much practised, perfects the four foundations of mindfulness.
The four foundations of mindfulness, when developed and much practised,
perfect the seven enlightenment factors. The seven enlightenment factors,
when developed and much practised, perfect clear vision and deliverance'
(M.III, 82).>

The Enlightenment Factors.
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We read in the Anapanasati Sutta, (in the translation by Ven. Nyanatiloka,
but abridged):
<1. On whatever occasion, bhikkhus, a bhikkhu abides contemplating the body
in the body, ardent, clearly comprehending, mindful, having put away
covetousness and grief regarding the world- on that occasion, unremitting
mindfulness is established in him...on that occasion the mindfulness
enlightenment factor is aroused in him, and he develops it, and by
development it comes to perfection in him.
2. Abiding thus mindful, he investigates, examines that state with
understanding, and embarks upon a scrutiny (of it)... on that occasion the
investigation-of-states (dhamma vivaya) enlightenment is aroused in him, and
he develops it, and by development it comes to perfection in him...
3.On whatever occasion, bhikkhus, a bhikkhu who investigates, examines that
state (dhamma) with understanding, and embarks upon a scrutiny (of it),
tireless energy is aroused... on that occasion the energy enlightenment
factor is aroused in him, and he develops it, and by development it comes to
perfection in him...
4. On whatever occasion, bhikkhus, a bhikkhu who has aroused energy,
unworldly (niramisa, not involved with the senses) rapture arises...
³The body and mind of one whose mind is held in rapture, becomes
tranquillized.²
5.On whatever occasion, bhikkhus, the body and mind of a bhikkhu who is held
in rapture, become tranquillized- on that occasion the tranquillity
enlightenment factor is aroused in him...
³The mind of one who is tranquillized in body and blissful becomes
concentrated.²
6. On whatever occasion, bhikkhus, mind of a bhikkhu who is tranquillized in
body and blissful becomes concentrated- on that occasion the concentration
enlightenment factor is aroused in him...
³He becomes one who looks on with complete equanimity on the mind thus
concentrated.²
7. On whatever occasion, bhikkhus, a bhikkhu becomes one who looks on with
complete equanimity on the mind thus concentrated- on that occasion the
equanimity enlightenment factor is aroused in him, and he develops it, and
by development it comes to perfection in him.>
The same is stated with regard to the other three applications of
mindfulness.
<Thus developed, bhikkhus, thuse repeatedly practised, do the four
foundations of mindfulness fulfil the seven enlightenment factors>

The Co. to this sutta (ven. Nyanatiloka):...<The mindfulness in regard to
the body, in the bhikkhu who lays hold of the body in the fourteen ways (of
contemplating the body, given in the satipatthana sutta) thus, is
mindfulness enlightenment factor; the knowledge associated with that is the
investigation-of-states (dhammas) enlightenment factor; the bodily and
mental energy associated with that is the energy enlightenment factor. (and
so on in like manner with) rapture and tranquillity; one-pointedness is the
concentration enlightenment factor; it is the quality of equipoise called
absence of lagging behind or of overrunning on the part of the aforesaid six
enlightenment factors, that constututes the equanimity enlightenment factor
...>
The Co. the uses a simile of a charioteer and horses which are advancing
evenly, not overrunning nor holding back. Evenso is equanimity. We then
read:
<Up to this point what has been expounded? What are expounded are the seven
enlightenment factors of the insight of a single conscious moment,
characterized by various essentials (lakkhana). >

The enlightenment factors are included in the fourth Application of
Mindfulness, contemplating dhammas in dhammas. They should not be taken for
self. We read in the Co to the Satipatthana Sutta (tr. by ven. Soma) about
the conditions for the enlightenment factors, and among them is <an
abundance of right reflection>. We read about right reflection in the
section on the hindrance of covetousness:
<Right reflection is expedient reflection; reflection going on the right
track. It is reflection that considers the facts of impermanence, suffering
(dukkha), soullessness and of impurity, according to reality.>
This is not merely thinking, it is deeply considering and contemplating with
mindfulness of the object that appears and right understanding of its
characteristic.
We read more about this kind of reflection in the <Kindred Sayings>, V,
Mahå-vagga, Kindred Sayings on the Limbs of Wisdom, Ch IV, § 8, Restraint
and Hindrance. The enlightenment factors are translated here as Limbs of
Wisdom. We read:
... At the time, monks, when the Ariyan disciple makes the Norm (Dhamma) his
object, gives attention to it, with all his mind considers it, with ready
ear listens to the Norm,- at such time the five hindrances exist not in him,
at such time these seven limbs of wisdom by cultivation go to fulfilment.>
It all begins with listening, considering, and then there are conditions for
mindfulness and direct understanding of whatever reality appears.
There should be equanimity, evenmindedness and impartiality towards the
object that appears. No matter whether the object is greatly disturbing, it
can be object of mindfulness. It is conditioned and it has no owner.
Nina.