PV. III.285-6:tasmād bhūtām abhūtaṃ vā yad yad evābhibhāvyate. bhāvanāpariniṣpattau tat sphuṭākalpadhīphalam.
But I am not absolutely sure of how could something non-existing be seen in a non-conceptual (akalpa) way. Does not the very fact that it does not exist imply its being of conceptual nature? Luckily enough, V. Eltsch.'s translation adds that it just seems to be non-conceptual.