大般涅槃經義記 7.55

7.55

「生不可說」牒問第五，「以生無」者生相無常，一滅去者更不重來故曰生無，是故不可定說為生。

"Arising cannot be spoken of" cites the question regarding the fifth. "Because arising is nonexistent" means that the mark of arising is impermanent; once it has ceased, it does not recur; therefore it is said that arising is nonexistent. For this reason, it cannot be definitively spoken of as arising.