“Evametaṃ, gahapati, evametaṃ, gahapati! Āturo hāyaṃ, gahapati, kāyo aṇḍabhūto pariyonaddho. Yo hi, gahapati, imaṃ kāyaṃ pariharanto muhuttampi ārogyaṃ paṭijāneyya, kimaññatra bālyā? Tasmātiha te, gahapati, evaṃ sikkhitabbaṃ — ‘āturakāyassa me sato cittaṃ anāturaṃ bhavissatī’ti. Evañhi te, gahapati, sikkhitabban”ti…

“Kathañca, gahapati, āturakāyo hi kho hoti no ca āturacitto? Idha, gahapati, sutavā ariyasāvako ariyānaṃ dassāvī ariyadhammassa kovido ariyadhamme suvinīto sappurisānaṃ dassāvī sappurisadhammassa kovido sappurisadhamme suvinīto na rūpaṃ attato samanupassati, na rūpavantaṃ vā attānaṃ; na attani vā rūpaṃ, na rūpasmiṃ vā attānaṃ. ‘ahaṃ rūpaṃ, mama rūpan’ti na pariyuṭṭhaṭṭhāyī hoti. Tassa ‘ahaṃ rūpaṃ, mama rūpan’ti apariyuṭṭhaṭṭhāyino, taṃ rūpaṃ vipariṇamati aññathā hoti. Tassa rūpavipariṇāmaññathābhāvā nuppajjanti sokaparidevadukkhadomanassupāyāsā.”

[Buddha:]

So it is, Gahapati, so it is! I’d say your body has become overwhelmed by disease. Who, carrying around a body like that, would think for a second that it was healthy, other than out of foolishness? Therefore, Gahapati, you should practice this way: “Though afflicted in body, mindful, my heart will dwell unafflicted.” That’s the way you should train yourself, Gahapati…

[Sariputta:]

How, Gahapati, can one be afflicted in body but unafflicted in heart? It happens when a learned disciple of the noble ones, one with noble vision, practicing the noble Dhamma, well versed in the teachings of the noble ones, a well-trained righteous one, one with the vision of the righteous, practicing the Dhamma of the righteous, well versed in the teachings of the righteous, does not perceive the body as the self, does not see the self has having bodily form, does not locate the self in the body, does not locate the self in bodily form. [The thought]“This is my body, I have a body” does not arise. Because “This is my body, I have a body” does not arise, when that body changes for the worse, the fact that it is changing for the worse does not cause grief, sobbing, suffering, sadness, and despair.

Nakulapitu Sutta, SN 22.1