[633] Which form (rūpa) is internal and not edible food (kabaḷīkāra āhāra)?
The eye sense base (cakkhāyatana) … the body sense base (kāyāyatana) — this is the form that is internal and not edible food.
Which form is external and edible food?
Boiled rice, sour gruel, parched flour, fish, meat, milk, curds, ghee, fresh butter, oil, honey, molasses; or whatever other form there is, in this or that country, which for these or those beings is to be taken into the mouth, chewed by the teeth, swallowed down the throat, and sustains the belly — the nutritive essence (ojā) by which beings sustain themselves — this is the form that is external and edible food.
Which form is external and not edible food?
The form sense base (rūpāyatana) … the impermanence (aniccatā) of form — this is the form that is external and not edible food.
Thus is the grouping of form by the threefold method.
The Exposition of the Triads (Tikaniddesa).
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.