Question on Phassa

In Ñanavira's book Notes on Dhamma: > Phassa is included in nāma since nāma, in specifying saññā, necessarily specifies the pair of āyatanāni ('bases') and kind of viññāna involved (e.g. perception of sourness specifies tongue, tastes, and tongue-consciousness), whereas rūpa does not (inertia or behaviour does not specify its mode of appearance, visual, auditory, and so on): nāma, in other words, entails (but does not include) viññāna, whereas rūpa is simply 'discovered' by viññāna. I don't follow his reasoning here... why is it that Phassa is included in nama instead of rupa because nāma 'entails (but does not include) viññāna, whereas rūpa is simply 'discovered' by viññāna.'? Also why is sañña given some kind of precedence in its inclusion within the category ahead of Phassa?