楞伽阿跋多羅寶經註解 2.39

2.39

此言六因所生之法非二種生相，但是凡夫妄想之所分別，故言不漸次生、不俱生也。所以者何下，徵結不生所以。若一切法頓生者，則能作之因、所作之法無有分別，求其因相不可得也。若一切法漸生者，求其體相亦不可得，故喻云「如不生子安有父名」。

This says that dharmas produced by the six causes do not have the two kinds of characteristics of arising; they are merely discriminated by the false imagination of ordinary beings, hence it is said that they 'neither arise gradually nor arise simultaneously.' From 'Why is this?' onward questions and concludes the reason for non-arising. If all dharmas were to arise instantaneously, then between the cause that is able to produce and the dharma that is produced there would be no distinction, and in seeking the characteristic of the cause it would be unfindable. If all dharmas were to arise gradually, in seeking their essential characteristic it would also be unfindable; hence the simile says: 'Just as with an unbegotten son, how could there be a father's name?'