[565] What is that form (rūpa) which is the form base (rūpāyatana)?
That form which, derived from the four primary elements (mahābhūta), is the appearance of color (vaṇṇanibhā) … this is also the form element (rūpadhātu). This is that form which is the form base.
What is that form which is not the form base?
The eye base (cakkhāyatana) … edible food (kabaḷīkāro āhāro). This is that form which is not the form base.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.