因明入正理論疏 1.44

1.44

俱不成者，合二為之。要有二種，互相差別不相離性，方得成宗。彼二並非，何成宗義？是故加之。

The unestablished in both combines the two. There must be two factors whose mutual qualification forms an inseparable nature before a proposition can be constituted. If both of them are non-existent, how could the meaning of the proposition be established? Therefore it is added.