In Kesa Kudoku Dogen writes the following:

This being so, we should make [the ka??ya] properly, according to the method
for making the ka??yathat has been authentically transmitted by the Buddhist
patriarchs. This alone is the authentic tradition, and so it has long been expe-
rienced and recognized by all common and sacred beings, human beings and
gods, and dragons and spirits. Having been born to meet the spread of this
Dharma, if we cover our body with the ka??yaonly once, receiving it and
retaining it for just a k???aor a muh?rta,31that [experience] will surely serve
as a talisman to protect us32in the realization of the supreme state of bodhi.
When we dye the body and mind with a single phrase or a single verse, it
becomes a seed of everlasting brightness which ?nally leads us to the supreme
state of bodhi.When we dye the body and mind with one real dharma or one
good deed, it may be also like this. Mental images arise and vanish instanta-
neously; they are without an abode. The physical body also arises and van-
ishes instantaneously; it too is without an abode. Nevertheless, the merit that
we practice always has its time of ripening and shedding. The ka??ya,simi-
larly, is beyond elaboration and beyond non-elaboration, it is beyond having
an abode and beyond having no abode: it is that which “buddhas alone, together
with buddhas, perfectly realize.”(Nishijima Cross)
Let's see what is the kesa made of. Rags. Just rags.

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