因明入正理論疏 2.137

2.137

如聲論者對佛法者立聲常宗，心、心所法所量度性為因，空等常法為同品，瓶等無常為異品。故釋共義。同、異品中此因皆遍，二共有故，名為不定。

For instance, a Śabda theorist sets up the thesis that sound is permanent against a Buddhist, using being an object of cognition by the mind and mental factors as the reason, taking permanent phenomena such as space as the sapakṣa, and impermanent phenomena such as jars as the vipakṣa. Hence it explains the meaning of common: this reason pervades both sapakṣa and vipakṣa, and because it is common to both, it is named indeterminate.