Anapanasati, fourth tetrad, no 1.

The Visuddhimagga (VIII, 237) states about the fourth tetrad,

ŒThis tetrad deals only with pure insight while the previous three deal with
serenity and insight.¹
As regards the words of the fourth tetrad, ³(XIII) I shall breathe in...
breathe out contemplating impermanence², the Visuddhimagga (VIII, 234)
states:

Π... Impermanence is the rise and fall and change in those same khandhas,
or it is their non-existence after having been; the meaning is, it is the
break-up of produced khandhas through their momentary dissolution since they
do not remain in the same mode. Contemplation of impermanence is
contemplation of materiality, etc., as ³impermanent² in virtue of that
impermanence...¹

As regards the clause: ³(XIV) I shall breathe in... breathe out
contemplating fading away², the Visuddhimagga states that there are two
kinds of fading away, namely: ³fading away as destruction² which is the
³momentary dissolution of formations² (conditioned realities) and ³absolute
fading away² which is nibbåna. The text (Visuddhimagga VIII, 235) states:

Œ... Contemplation of fading away is insight and it is the path, which occur
as the seeing of these two. It is when he possesses this twofold
contemplation that it can be understood of him ³He trains thus, I shall
breathe in... shall breathe out contemplating fading away.² ¹

The same method of explanation is applied to the clause ³contemplating
cessation² (XV). And with regard to the clause (XVI) ³contemplating
relinquishment², the Visuddhimagga states:

³relinquishment is of two kinds too, that is to say, relinquishment as
giving up, and relinquishment as entering into.²

³Giving up² is the giving up of defilements, and ³entering into² is the
entering into nibbåna, the Visuddhimagga explains. We read:
<For insight is called both relinquishment as giving up, and relinquishment
as entering into since (firstly) through substitution of opposite qualities
it gives up defilements with their aggregate-producing kamma formations, and
(secondly), through seeing the wretchedness of what is formed (Ch XXI,18).
Also the Path is called both relinquishment as giving up, and relinquishment
as entering into, since it gives up defilements with their
aggregate-producing kamma formations by cutting them off, and it enters into
nibbana by making it its object...>
*****
Nina.