Poṭṭhapāda asks various questions reagrding the nature of Saññā.

Now, lord, does perception arise first, and knowledge after; or does knowledge arise first, and perception after; or do perception & knowledge arise simultaneously?

Potthapada, perception arises first, and knowledge after. And the arising of knowledge comes from the arising of perception. One discerns, 'It's in dependence on this that my knowledge has arisen.' Through this line of reasoning one can realize how perception arises first, and knowledge after, and how the arising of knowledge comes from the arising of perception.